Imported Upstream version 1.1.0upstream/1.1.0

53 files changed, 21062 insertions, 0 deletions

diff --git a/src/mscorlib/src/System/Collections/ArrayList.cs b/src/mscorlib/src/System/Collections/ArrayList.cs
new file mode 100644
index 0000000000..94f4dc74e8
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/ArrayList.cs
@@ -0,0 +1,2635 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** 
+** 
+**
+**
+** Purpose: Implements a dynamically sized List as an array,
+**          and provides many convenience methods for treating
+**          an array as an IList.
+**
+** 
+===========================================================*/
+namespace System.Collections {
+    using System;
+    using System.Runtime;
+    using System.Security;
+    using System.Security.Permissions;
+    using System.Diagnostics;
+    using System.Runtime.CompilerServices;
+    using System.Runtime.Serialization;
+    using System.Diagnostics.CodeAnalysis;
+    using System.Diagnostics.Contracts;
+
+    // Implements a variable-size List that uses an array of objects to store the
+    // elements. A ArrayList has a capacity, which is the allocated length
+    // of the internal array. As elements are added to a ArrayList, the capacity
+    // of the ArrayList is automatically increased as required by reallocating the
+    // internal array.
+    // 
+#if FEATURE_CORECLR
+    [FriendAccessAllowed]
+#endif
+    [DebuggerTypeProxy(typeof(System.Collections.ArrayList.ArrayListDebugView))]   
+    [DebuggerDisplay("Count = {Count}")]
+    [Serializable]
+    [System.Runtime.InteropServices.ComVisible(true)]
+    public class ArrayList : IList, ICloneable
+    {
+        private Object[] _items;
+        [ContractPublicPropertyName("Count")]
+        private int _size;
+        private int _version;
+        [NonSerialized]
+        private Object _syncRoot;
+        
+        private const int _defaultCapacity = 4;
+        private static readonly Object[] emptyArray = EmptyArray<Object>.Value; 
+    
+        // Note: this constructor is a bogus constructor that does nothing
+        // and is for use only with SyncArrayList.
+        internal ArrayList( bool trash )
+        {
+        }
+
+        // Constructs a ArrayList. The list is initially empty and has a capacity
+        // of zero. Upon adding the first element to the list the capacity is
+        // increased to _defaultCapacity, and then increased in multiples of two as required.
+        public ArrayList() {
+            _items = emptyArray;  
+        }
+    
+        // Constructs a ArrayList with a given initial capacity. The list is
+        // initially empty, but will have room for the given number of elements
+        // before any reallocations are required.
+        // 
+         public ArrayList(int capacity) {
+             if (capacity < 0) throw new ArgumentOutOfRangeException("capacity", Environment.GetResourceString("ArgumentOutOfRange_MustBeNonNegNum", "capacity"));
+             Contract.EndContractBlock();
+
+             if (capacity == 0)
+                 _items = emptyArray;
+             else
+                 _items = new Object[capacity];
+        }
+    
+        // Constructs a ArrayList, copying the contents of the given collection. The
+        // size and capacity of the new list will both be equal to the size of the
+        // given collection.
+        // 
+        public ArrayList(ICollection c) {
+            if (c==null)
+                throw new ArgumentNullException("c", Environment.GetResourceString("ArgumentNull_Collection"));
+            Contract.EndContractBlock();
+
+            int count = c.Count;
+            if (count == 0)
+            {
+                _items = emptyArray;
+            }
+            else {
+                _items = new Object[count];
+                AddRange(c);
+            }
+        }
+    
+        // Gets and sets the capacity of this list.  The capacity is the size of
+        // the internal array used to hold items.  When set, the internal 
+        // array of the list is reallocated to the given capacity.
+        // 
+         public virtual int Capacity {
+            get {
+                Contract.Ensures(Contract.Result<int>() >= Count);
+                return _items.Length;
+            }
+            set {
+                if (value < _size) {
+                    throw new ArgumentOutOfRangeException("value", Environment.GetResourceString("ArgumentOutOfRange_SmallCapacity"));
+                }
+                Contract.Ensures(Capacity >= 0);
+                Contract.EndContractBlock();
+                // We don't want to update the version number when we change the capacity.
+                // Some existing applications have dependency on this.
+                if (value != _items.Length) {
+                    if (value > 0) {
+                        Object[] newItems = new Object[value];
+                        if (_size > 0) { 
+                            Array.Copy(_items, 0, newItems, 0, _size);
+                        }
+                        _items = newItems;
+                    }
+                    else {
+                        _items = new Object[_defaultCapacity];
+                    }
+                }            
+            }
+        }
+
+        // Read-only property describing how many elements are in the List.
+        public virtual int Count {
+            get {
+                Contract.Ensures(Contract.Result<int>() >= 0);
+                return _size;
+            }
+        }
+
+        public virtual bool IsFixedSize {
+            get { return false; }
+        }
+
+            
+        // Is this ArrayList read-only?
+        public virtual bool IsReadOnly {
+            get { return false; }
+        }
+
+        // Is this ArrayList synchronized (thread-safe)?
+        public virtual bool IsSynchronized {
+            get { return false; }
+        }
+    
+        // Synchronization root for this object.
+        public virtual Object SyncRoot {
+            get { 
+                if( _syncRoot == null) {
+                    System.Threading.Interlocked.CompareExchange<Object>(ref _syncRoot, new Object(), null);    
+                }
+                return _syncRoot; 
+            }
+        }
+    
+        // Sets or Gets the element at the given index.
+        // 
+        public virtual Object this[int index] {
+            get {
+                if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+                return _items[index];
+            }
+            set {
+                if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+                _items[index] = value;
+                _version++;
+            }
+        }
+    
+        // Creates a ArrayList wrapper for a particular IList.  This does not
+        // copy the contents of the IList, but only wraps the ILIst.  So any
+        // changes to the underlying list will affect the ArrayList.  This would
+        // be useful if you want to Reverse a subrange of an IList, or want to
+        // use a generic BinarySearch or Sort method without implementing one yourself.
+        // However, since these methods are generic, the performance may not be
+        // nearly as good for some operations as they would be on the IList itself.
+        //
+        public static ArrayList Adapter(IList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+            return new IListWrapper(list);
+        }
+        
+        // Adds the given object to the end of this list. The size of the list is
+        // increased by one. If required, the capacity of the list is doubled
+        // before adding the new element.
+        //
+        public virtual int Add(Object value) {
+            Contract.Ensures(Contract.Result<int>() >= 0);
+            if (_size == _items.Length) EnsureCapacity(_size + 1);
+            _items[_size] = value;
+            _version++;
+            return _size++;
+        }
+    
+        // Adds the elements of the given collection to the end of this list. If
+        // required, the capacity of the list is increased to twice the previous
+        // capacity or the new size, whichever is larger.
+        //
+        public virtual void AddRange(ICollection c) {
+            InsertRange(_size, c);
+        }
+    
+        // Searches a section of the list for a given element using a binary search
+        // algorithm. Elements of the list are compared to the search value using
+        // the given IComparer interface. If comparer is null, elements of
+        // the list are compared to the search value using the IComparable
+        // interface, which in that case must be implemented by all elements of the
+        // list and the given search value. This method assumes that the given
+        // section of the list is already sorted; if this is not the case, the
+        // result will be incorrect.
+        //
+        // The method returns the index of the given value in the list. If the
+        // list does not contain the given value, the method returns a negative
+        // integer. The bitwise complement operator (~) can be applied to a
+        // negative result to produce the index of the first element (if any) that
+        // is larger than the given search value. This is also the index at which
+        // the search value should be inserted into the list in order for the list
+        // to remain sorted.
+        // 
+        // The method uses the Array.BinarySearch method to perform the
+        // search.
+        // 
+        public virtual int BinarySearch(int index, int count, Object value, IComparer comparer) {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.Ensures(Contract.Result<int>() < index + count);
+            Contract.EndContractBlock();
+    
+            return Array.BinarySearch((Array)_items, index, count, value, comparer);
+        }
+    
+        public virtual int BinarySearch(Object value)
+        {
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return BinarySearch(0, Count, value, null);
+        }
+
+        public virtual int BinarySearch(Object value, IComparer comparer)
+        {
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return BinarySearch(0, Count, value, comparer);
+        }
+
+    
+        // Clears the contents of ArrayList.
+        public virtual void Clear() {
+            if (_size > 0)
+            {
+            Array.Clear(_items, 0, _size); // Don't need to doc this but we clear the elements so that the gc can reclaim the references.
+            _size = 0;
+            }
+            _version++;
+        }
+    
+        // Clones this ArrayList, doing a shallow copy.  (A copy is made of all
+        // Object references in the ArrayList, but the Objects pointed to 
+        // are not cloned).
+        public virtual Object Clone()
+        {
+            Contract.Ensures(Contract.Result<Object>() != null);
+            ArrayList la = new ArrayList(_size);
+            la._size = _size;
+            la._version = _version;
+            Array.Copy(_items, 0, la._items, 0, _size);
+            return la;
+        }
+    
+    
+        // Contains returns true if the specified element is in the ArrayList.
+        // It does a linear, O(n) search.  Equality is determined by calling
+        // item.Equals().
+        //
+        public virtual bool Contains(Object item) {
+            if (item==null) {
+                for(int i=0; i<_size; i++)
+                    if (_items[i]==null)
+                        return true;
+                return false;
+            }
+            else {
+                for(int i=0; i<_size; i++)
+                    if ( (_items[i] != null) && (_items[i].Equals(item)) )
+                        return true;
+                return false;
+            }
+        }
+    
+        // Copies this ArrayList into array, which must be of a 
+        // compatible array type.  
+        //
+        public virtual void CopyTo(Array array) {
+            CopyTo(array, 0);
+        }
+
+        // Copies this ArrayList into array, which must be of a 
+        // compatible array type.  
+        //
+        public virtual void CopyTo(Array array, int arrayIndex) {
+            if ((array != null) && (array.Rank != 1))
+                throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+            Contract.EndContractBlock();
+            // Delegate rest of error checking to Array.Copy.
+            Array.Copy(_items, 0, array, arrayIndex, _size);
+        }
+    
+        // Copies a section of this list to the given array at the given index.
+        // 
+        // The method uses the Array.Copy method to copy the elements.
+        // 
+        public virtual void CopyTo(int index, Array array, int arrayIndex, int count) {
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            if ((array != null) && (array.Rank != 1))
+                throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+            Contract.EndContractBlock();
+            // Delegate rest of error checking to Array.Copy.
+            Array.Copy(_items, index, array, arrayIndex, count);
+        }
+
+        // Ensures that the capacity of this list is at least the given minimum
+        // value. If the currect capacity of the list is less than min, the
+        // capacity is increased to twice the current capacity or to min,
+        // whichever is larger.
+        private void EnsureCapacity(int min) {
+            if (_items.Length < min) {
+                int newCapacity = _items.Length == 0? _defaultCapacity: _items.Length * 2;
+                // Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
+                // Note that this check works even when _items.Length overflowed thanks to the (uint) cast
+                if ((uint)newCapacity > Array.MaxArrayLength) newCapacity = Array.MaxArrayLength;
+                if (newCapacity < min) newCapacity = min;
+                Capacity = newCapacity;
+            }
+        }
+    
+        // Returns a list wrapper that is fixed at the current size.  Operations
+        // that add or remove items will fail, however, replacing items is allowed.
+        //
+        public static IList FixedSize(IList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<IList>() != null);
+            Contract.EndContractBlock();
+            return new FixedSizeList(list);
+        }
+
+        // Returns a list wrapper that is fixed at the current size.  Operations
+        // that add or remove items will fail, however, replacing items is allowed.
+        //
+        public static ArrayList FixedSize(ArrayList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+            return new FixedSizeArrayList(list);
+        }
+    
+        // Returns an enumerator for this list with the given
+        // permission for removal of elements. If modifications made to the list 
+        // while an enumeration is in progress, the MoveNext and 
+        // GetObject methods of the enumerator will throw an exception.
+        //
+        public virtual IEnumerator GetEnumerator() {
+            Contract.Ensures(Contract.Result<IEnumerator>() != null);
+            return new ArrayListEnumeratorSimple(this);
+        }
+    
+        // Returns an enumerator for a section of this list with the given
+        // permission for removal of elements. If modifications made to the list 
+        // while an enumeration is in progress, the MoveNext and 
+        // GetObject methods of the enumerator will throw an exception.
+        //
+        public virtual IEnumerator GetEnumerator(int index, int count) {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.Ensures(Contract.Result<IEnumerator>() != null);
+            Contract.EndContractBlock();
+    
+            return new ArrayListEnumerator(this, index, count);
+        }
+    
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards from beginning to end.
+        // The elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public virtual int IndexOf(Object value) {
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return Array.IndexOf((Array)_items, value, 0, _size);
+        }
+    
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards, starting at index
+        // startIndex and ending at count number of elements. The
+        // elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public virtual int IndexOf(Object value, int startIndex) {
+            if (startIndex > _size)
+                throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+            return Array.IndexOf((Array)_items, value, startIndex, _size - startIndex);
+        }
+
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards, starting at index
+        // startIndex and upto count number of elements. The
+        // elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public virtual int IndexOf(Object value, int startIndex, int count) {
+            if (startIndex > _size)
+                throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            if (count <0 || startIndex > _size - count) throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+            return Array.IndexOf((Array)_items, value, startIndex, count);
+        }
+    
+        // Inserts an element into this list at a given index. The size of the list
+        // is increased by one. If required, the capacity of the list is doubled
+        // before inserting the new element.
+        // 
+        public virtual void Insert(int index, Object value) {
+            // Note that insertions at the end are legal.
+            if (index < 0 || index > _size) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_ArrayListInsert"));
+            //Contract.Ensures(Count == Contract.OldValue(Count) + 1);
+            Contract.EndContractBlock();
+
+            if (_size == _items.Length) EnsureCapacity(_size + 1);
+            if (index < _size) {
+                Array.Copy(_items, index, _items, index + 1, _size - index);
+            }
+            _items[index] = value;
+            _size++;
+            _version++;
+        }
+    
+        // Inserts the elements of the given collection at a given index. If
+        // required, the capacity of the list is increased to twice the previous
+        // capacity or the new size, whichever is larger.  Ranges may be added
+        // to the end of the list by setting index to the ArrayList's size.
+        //
+        public virtual void InsertRange(int index, ICollection c) {
+            if (c==null)
+                throw new ArgumentNullException("c", Environment.GetResourceString("ArgumentNull_Collection"));
+            if (index < 0 || index > _size) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            //Contract.Ensures(Count == Contract.OldValue(Count) + c.Count);
+            Contract.EndContractBlock();
+
+            int count = c.Count;
+            if (count > 0) {
+                EnsureCapacity(_size + count);                
+                // shift existing items
+                if (index < _size) {
+                    Array.Copy(_items, index, _items, index + count, _size - index);
+                }
+
+                Object[] itemsToInsert = new Object[count];
+                c.CopyTo(itemsToInsert, 0);
+                itemsToInsert.CopyTo(_items, index);
+                _size += count;
+                _version++;
+            }
+        }
+    
+        // Returns the index of the last occurrence of a given value in a range of
+        // this list. The list is searched backwards, starting at the end 
+        // and ending at the first element in the list. The elements of the list 
+        // are compared to the given value using the Object.Equals method.
+        // 
+        // This method uses the Array.LastIndexOf method to perform the
+        // search.
+        // 
+        public virtual int LastIndexOf(Object value)
+        {
+            Contract.Ensures(Contract.Result<int>() < _size);
+            return LastIndexOf(value, _size - 1, _size);
+        }
+
+        // Returns the index of the last occurrence of a given value in a range of
+        // this list. The list is searched backwards, starting at index
+        // startIndex and ending at the first element in the list. The 
+        // elements of the list are compared to the given value using the 
+        // Object.Equals method.
+        // 
+        // This method uses the Array.LastIndexOf method to perform the
+        // search.
+        // 
+        public virtual int LastIndexOf(Object value, int startIndex)
+        {
+            if (startIndex >= _size)
+                throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+            return LastIndexOf(value, startIndex, startIndex + 1);
+        }
+
+        // Returns the index of the last occurrence of a given value in a range of
+        // this list. The list is searched backwards, starting at index
+        // startIndex and upto count elements. The elements of
+        // the list are compared to the given value using the Object.Equals
+        // method.
+        // 
+        // This method uses the Array.LastIndexOf method to perform the
+        // search.
+        // 
+        public virtual int LastIndexOf(Object value, int startIndex, int count) {
+            if (Count != 0 && (startIndex < 0 || count < 0))
+                throw new ArgumentOutOfRangeException((startIndex<0 ? "startIndex" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+
+            if (_size == 0)  // Special case for an empty list
+                return -1;
+
+            if (startIndex >= _size || count > startIndex + 1) 
+                throw new ArgumentOutOfRangeException((startIndex>=_size ? "startIndex" : "count"), Environment.GetResourceString("ArgumentOutOfRange_BiggerThanCollection"));
+
+            return Array.LastIndexOf((Array)_items, value, startIndex, count);
+        }
+    
+        // Returns a read-only IList wrapper for the given IList.
+        //
+#if FEATURE_CORECLR
+        [FriendAccessAllowed]
+#endif
+        public static IList ReadOnly(IList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<IList>() != null);
+            Contract.EndContractBlock();
+            return new ReadOnlyList(list);
+        }
+
+        // Returns a read-only ArrayList wrapper for the given ArrayList.
+        //
+        public static ArrayList ReadOnly(ArrayList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+            return new ReadOnlyArrayList(list);
+        }
+    
+        // Removes the element at the given index. The size of the list is
+        // decreased by one.
+        // 
+        public virtual void Remove(Object obj) {
+            Contract.Ensures(Count >= 0);
+
+            int index = IndexOf(obj);
+            BCLDebug.Correctness(index >= 0 || !(obj is Int32), "You passed an Int32 to Remove that wasn't in the ArrayList." + Environment.NewLine + "Did you mean RemoveAt?  int: "+obj+"  Count: "+Count);
+            if (index >=0) 
+                RemoveAt(index);
+        }
+    
+        // Removes the element at the given index. The size of the list is
+        // decreased by one.
+        // 
+        public virtual void RemoveAt(int index) {
+            if (index < 0 || index >= _size) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            Contract.Ensures(Count >= 0);
+            //Contract.Ensures(Count == Contract.OldValue(Count) - 1);
+            Contract.EndContractBlock();
+
+            _size--;
+            if (index < _size) {
+                Array.Copy(_items, index + 1, _items, index, _size - index);
+            }
+            _items[_size] = null;
+            _version++;
+        }
+    
+        // Removes a range of elements from this list.
+        // 
+        public virtual void RemoveRange(int index, int count) {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.Ensures(Count >= 0);
+            //Contract.Ensures(Count == Contract.OldValue(Count) - count);
+            Contract.EndContractBlock();
+    
+            if (count > 0) {
+                int i = _size;
+                _size -= count;
+                if (index < _size) {
+                    Array.Copy(_items, index + count, _items, index, _size - index);
+                }
+                while (i > _size) _items[--i] = null;
+                _version++;
+            }
+        }
+    
+        // Returns an IList that contains count copies of value.
+        //
+        public static ArrayList Repeat(Object value, int count) {
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count",Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+
+            ArrayList list = new ArrayList((count>_defaultCapacity)?count:_defaultCapacity);
+            for(int i=0; i<count; i++)
+                list.Add(value);
+            return list;
+        }
+
+        // Reverses the elements in this list.
+        public virtual void Reverse() {
+            Reverse(0, Count);
+        }
+    
+        // Reverses the elements in a range of this list. Following a call to this
+        // method, an element in the range given by index and count
+        // which was previously located at index i will now be located at
+        // index index + (index + count - i - 1).
+        // 
+        // This method uses the Array.Reverse method to reverse the
+        // elements.
+        // 
+        public virtual void Reverse(int index, int count) {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.EndContractBlock();
+            Array.Reverse(_items, index, count);
+            _version++;
+        }
+    
+        // Sets the elements starting at the given index to the elements of the
+        // given collection.
+        //
+        public virtual void SetRange(int index, ICollection c) {
+            if (c==null) throw new ArgumentNullException("c", Environment.GetResourceString("ArgumentNull_Collection"));
+            Contract.EndContractBlock();
+            int count = c.Count;
+            if (index < 0 || index > _size - count) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            
+            if (count > 0) {
+                c.CopyTo(_items, index);
+                _version++;
+            }
+        }
+    
+        public virtual ArrayList GetRange(int index, int count) {
+            if (index < 0 || count < 0)
+                throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+            return new Range(this,index, count);
+        }
+
+        // Sorts the elements in this list.  Uses the default comparer and 
+        // Array.Sort.
+        public virtual void Sort()
+        {
+            Sort(0, Count, Comparer.Default);
+        }
+
+        // Sorts the elements in this list.  Uses Array.Sort with the
+        // provided comparer.
+        public virtual void Sort(IComparer comparer)
+        {
+            Sort(0, Count, comparer);
+        }
+
+        // Sorts the elements in a section of this list. The sort compares the
+        // elements to each other using the given IComparer interface. If
+        // comparer is null, the elements are compared to each other using
+        // the IComparable interface, which in that case must be implemented by all
+        // elements of the list.
+        // 
+        // This method uses the Array.Sort method to sort the elements.
+        // 
+        public virtual void Sort(int index, int count, IComparer comparer) {
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            if (_size - index < count)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.EndContractBlock();
+            
+            Array.Sort(_items, index, count, comparer);
+            _version++;
+        }
+    
+        // Returns a thread-safe wrapper around an IList.
+        //
+        [HostProtection(Synchronization=true)]
+        public static IList Synchronized(IList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<IList>() != null);
+            Contract.EndContractBlock();
+            return new SyncIList(list);
+        }
+    
+        // Returns a thread-safe wrapper around a ArrayList.
+        //
+        [HostProtection(Synchronization=true)]
+        public static ArrayList Synchronized(ArrayList list) {
+            if (list==null)
+                throw new ArgumentNullException("list");
+            Contract.Ensures(Contract.Result<ArrayList>() != null);
+            Contract.EndContractBlock();
+            return new SyncArrayList(list);
+        }
+    
+        // ToArray returns a new Object array containing the contents of the ArrayList.
+        // This requires copying the ArrayList, which is an O(n) operation.
+        public virtual Object[] ToArray() {
+            Contract.Ensures(Contract.Result<Object[]>() != null);
+
+            Object[] array = new Object[_size];
+            Array.Copy(_items, 0, array, 0, _size);
+            return array;
+        }
+    
+        // ToArray returns a new array of a particular type containing the contents 
+        // of the ArrayList.  This requires copying the ArrayList and potentially
+        // downcasting all elements.  This copy may fail and is an O(n) operation.
+        // Internally, this implementation calls Array.Copy.
+        //
+        [SecuritySafeCritical]
+        public virtual Array ToArray(Type type) {
+            if (type==null)
+                throw new ArgumentNullException("type");
+            Contract.Ensures(Contract.Result<Array>() != null);
+            Contract.EndContractBlock();
+            Array array = Array.UnsafeCreateInstance(type, _size);
+            Array.Copy(_items, 0, array, 0, _size);
+            return array;
+        }
+    
+        // Sets the capacity of this list to the size of the list. This method can
+        // be used to minimize a list's memory overhead once it is known that no
+        // new elements will be added to the list. To completely clear a list and
+        // release all memory referenced by the list, execute the following
+        // statements:
+        // 
+        // list.Clear();
+        // list.TrimToSize();
+        // 
+        public virtual void TrimToSize() {
+            Capacity = _size;
+        }
+    
+    
+        // This class wraps an IList, exposing it as a ArrayList
+        // Note this requires reimplementing half of ArrayList...
+        [Serializable]
+        private class IListWrapper : ArrayList
+        {
+            private IList _list;
+            
+            internal IListWrapper(IList list) {
+                _list = list;
+                _version = 0; // list doesn't not contain a version number
+            }
+    
+             public override int Capacity {
+                get { return _list.Count; }
+                set {
+                    if (value < Count) throw new ArgumentOutOfRangeException("value", Environment.GetResourceString("ArgumentOutOfRange_SmallCapacity"));
+                    Contract.EndContractBlock();
+                }
+            }
+    
+            public override int Count { 
+                get { return _list.Count; }
+            }
+    
+            public override bool IsReadOnly { 
+                get { return _list.IsReadOnly; }
+            }
+
+            public override bool IsFixedSize {
+                get { return _list.IsFixedSize; }
+            }
+
+        
+            public override bool IsSynchronized { 
+                get { return _list.IsSynchronized; }
+            }
+            
+             public override Object this[int index] {
+                get {
+                    return _list[index];
+                }
+                set {
+                    _list[index] = value;
+                    _version++;
+                }
+            }
+    
+            public override Object SyncRoot {
+                get { return _list.SyncRoot; }
+            }
+            
+            public override int Add(Object obj) {
+                int i = _list.Add(obj);
+                _version++;
+                return i;
+            }
+        
+            public override void AddRange(ICollection c) {
+                InsertRange(Count, c);
+            }
+    
+            // Other overloads with automatically work
+            public override int BinarySearch(int index, int count, Object value, IComparer comparer) 
+            {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (this.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+                if (comparer == null)
+                    comparer = Comparer.Default;
+                
+                int lo = index;
+                int hi = index + count - 1;
+                int mid;
+                while (lo <= hi) {
+                    mid = (lo+hi)/2;
+                    int r = comparer.Compare(value, _list[mid]);
+                    if (r == 0)
+                        return mid;
+                    if (r < 0)
+                        hi = mid-1;
+                    else 
+                        lo = mid+1;
+                }
+                // return bitwise complement of the first element greater than value.
+                // Since hi is less than lo now, ~lo is the correct item.
+                return ~lo;
+            }
+    
+            public override void Clear() {
+                // If _list is an array, it will support Clear method.
+                // We shouldn't allow clear operation on a FixedSized ArrayList
+                if(_list.IsFixedSize) {
+                    throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+                }
+
+                _list.Clear();
+                _version++;
+            }
+    
+            public override Object Clone() {
+                // This does not do a shallow copy of _list into a ArrayList!
+                // This clones the IListWrapper, creating another wrapper class!
+                return new IListWrapper(_list);
+            }
+    
+            public override bool Contains(Object obj) {
+                return _list.Contains(obj);
+            }
+    
+            public override void CopyTo(Array array, int index) {
+                _list.CopyTo(array, index);
+            }
+    
+            public override void CopyTo(int index, Array array, int arrayIndex, int count) {
+                if (array==null)
+                    throw new ArgumentNullException("array");
+                if (index < 0 || arrayIndex < 0)
+                    throw new ArgumentOutOfRangeException((index < 0) ? "index" : "arrayIndex", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if( count < 0)
+                    throw new ArgumentOutOfRangeException( "count" , Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));                 
+                if (array.Length - arrayIndex < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                if (array.Rank != 1)
+                    throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+                Contract.EndContractBlock();
+
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                
+                for(int i=index; i<index+count; i++)
+                    array.SetValue(_list[i], arrayIndex++);
+            }
+    
+            public override IEnumerator GetEnumerator() {
+                return _list.GetEnumerator();
+            }
+    
+            public override IEnumerator GetEnumerator(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                Contract.EndContractBlock();
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+    
+                return new IListWrapperEnumWrapper(this, index, count);
+            }
+    
+            public override int IndexOf(Object value) {
+                return _list.IndexOf(value);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex) {
+                return IndexOf(value, startIndex, _list.Count - startIndex);
+            }
+    
+            public override int IndexOf(Object value, int startIndex, int count) {
+                if (startIndex < 0 || startIndex > this.Count) throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                if (count < 0 || startIndex > this.Count - count) throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
+                Contract.EndContractBlock();
+
+                int endIndex = startIndex + count;
+                if (value == null) {
+                    for(int i=startIndex; i<endIndex; i++)
+                        if (_list[i] == null)
+                            return i;
+                    return -1;
+                } else {
+                    for(int i=startIndex; i<endIndex; i++)
+                        if (_list[i] != null && _list[i].Equals(value))
+                            return i;
+                    return -1;
+                }
+            }
+    
+            public override void Insert(int index, Object obj) {
+                _list.Insert(index, obj);
+                _version++;
+            }
+    
+            public override void InsertRange(int index, ICollection c) {
+                if (c==null)
+                    throw new ArgumentNullException("c", Environment.GetResourceString("ArgumentNull_Collection"));
+                if (index < 0 || index > this.Count) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+
+                if( c.Count > 0) {
+                    ArrayList al = _list as ArrayList;
+                    if( al != null) { 
+                        // We need to special case ArrayList. 
+                        // When c is a range of _list, we need to handle this in a special way.
+                        // See ArrayList.InsertRange for details.
+                        al.InsertRange(index, c);    
+                    }
+                    else {
+                        IEnumerator en = c.GetEnumerator();
+                        while(en.MoveNext()) {
+                            _list.Insert(index++, en.Current);
+                        }                   
+                    }
+                    _version++;
+                }
+            }
+    
+            public override int LastIndexOf(Object value) {
+                 return LastIndexOf(value,_list.Count - 1, _list.Count);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex) {
+                return LastIndexOf(value, startIndex, startIndex + 1);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex, int count) {
+                if (_list.Count == 0)
+                    return -1;
+   
+                if (startIndex < 0 || startIndex >= _list.Count) throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                if (count < 0 || count > startIndex + 1) throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
+
+                int endIndex = startIndex - count + 1;
+                if (value == null) {
+                    for(int i=startIndex; i >= endIndex; i--)
+                        if (_list[i] == null)
+                            return i;
+                    return -1;
+                } else {
+                    for(int i=startIndex; i >= endIndex; i--)
+                        if (_list[i] != null && _list[i].Equals(value))
+                            return i;
+                    return -1;
+                }
+            }
+    
+            public override void Remove(Object value) {
+                int index = IndexOf(value);                
+                if (index >=0) 
+                    RemoveAt(index);
+            }
+    
+            public override void RemoveAt(int index) {
+                _list.RemoveAt(index);
+                _version++;
+            }
+    
+            public override void RemoveRange(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                Contract.EndContractBlock();
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                
+                if( count > 0)    // be consistent with ArrayList
+                    _version++;
+
+                while(count > 0) {
+                    _list.RemoveAt(index);
+                    count--;
+                }
+            }
+    
+            public override void Reverse(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                Contract.EndContractBlock();
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+    
+                int i = index;
+                int j = index + count - 1;
+                while (i < j)
+                {
+                    Object tmp = _list[i];
+                    _list[i++] = _list[j];
+                    _list[j--] = tmp;
+                }
+                _version++;
+            }
+    
+            public override void SetRange(int index, ICollection c) {
+                if (c==null) {
+                    throw new ArgumentNullException("c", Environment.GetResourceString("ArgumentNull_Collection"));
+                }
+                Contract.EndContractBlock();
+
+                if (index < 0 || index > _list.Count - c.Count) {
+                    throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));            
+                }
+                   
+                if( c.Count > 0) {                                     
+                    IEnumerator en = c.GetEnumerator();
+                    while(en.MoveNext()) {
+                        _list[index++] = en.Current;
+                    }
+                    _version++;
+                }
+            }
+    
+            public override ArrayList GetRange(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                Contract.EndContractBlock();
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                return new Range(this,index, count);
+            }
+
+            public override void Sort(int index, int count, IComparer comparer) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                Contract.EndContractBlock();
+                if (_list.Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                
+                Object [] array = new Object[count];
+                CopyTo(index, array, 0, count);
+                Array.Sort(array, 0, count, comparer);
+                for(int i=0; i<count; i++)
+                    _list[i+index] = array[i];
+
+                _version++;
+            }
+
+
+            public override Object[] ToArray() {
+                Object[] array = new Object[Count];
+                _list.CopyTo(array, 0);
+                return array;
+            }
+
+            [SecuritySafeCritical]
+            public override Array ToArray(Type type)
+            {
+                if (type==null)
+                    throw new ArgumentNullException("type");
+                Contract.EndContractBlock();
+                Array array = Array.UnsafeCreateInstance(type, _list.Count);
+                _list.CopyTo(array, 0);
+                return array;
+            }
+
+            public override void TrimToSize()
+            {
+                // Can't really do much here...
+            }
+    
+            // This is the enumerator for an IList that's been wrapped in another
+            // class that implements all of ArrayList's methods.
+            [Serializable]
+            private sealed class IListWrapperEnumWrapper : IEnumerator, ICloneable
+            {
+                private IEnumerator _en;
+                private int _remaining;
+                private int _initialStartIndex;   // for reset
+                private int _initialCount;        // for reset
+                private bool _firstCall;       // firstCall to MoveNext
+    
+                private IListWrapperEnumWrapper()
+                {
+                }
+
+                internal IListWrapperEnumWrapper(IListWrapper listWrapper, int startIndex, int count) 
+                {
+                    _en = listWrapper.GetEnumerator();
+                    _initialStartIndex = startIndex;
+                    _initialCount = count;
+                    while(startIndex-- > 0 && _en.MoveNext());
+                    _remaining = count;
+                    _firstCall = true;
+                }
+
+                public Object Clone() {
+                    // We must clone the underlying enumerator, I think.
+                    IListWrapperEnumWrapper clone = new IListWrapperEnumWrapper();
+                    clone._en = (IEnumerator) ((ICloneable)_en).Clone();
+                    clone._initialStartIndex = _initialStartIndex;
+                    clone._initialCount = _initialCount;
+                    clone._remaining = _remaining;
+                    clone._firstCall = _firstCall;
+                    return clone;
+                }
+
+                public bool MoveNext() {
+                    if (_firstCall) {
+                        _firstCall = false;
+                        return _remaining-- > 0 && _en.MoveNext();
+                    }
+                    if (_remaining < 0)
+                        return false;
+                    bool r = _en.MoveNext();
+                    return r && _remaining-- > 0;
+                }
+                
+                public Object Current {
+                    get {
+                        if (_firstCall)
+                            throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumNotStarted));
+                        if (_remaining < 0)
+                            throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumEnded));
+                        return _en.Current;
+                    }
+                }
+    
+                public void Reset() {
+                    _en.Reset();
+                    int startIndex = _initialStartIndex;
+                    while(startIndex-- > 0 && _en.MoveNext());
+                    _remaining = _initialCount;
+                    _firstCall = true;
+                }
+            }
+        }
+    
+    
+        [Serializable]
+        private class SyncArrayList : ArrayList
+        {
+            private ArrayList _list;
+            private Object _root;
+
+            internal SyncArrayList(ArrayList list)
+                : base( false )
+            {
+                _list = list;
+                _root = list.SyncRoot;
+            }
+    
+            public override int Capacity {
+                get {
+                    lock(_root) {
+                        return _list.Capacity;
+                    }
+                }
+                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+                set {
+                    lock(_root) {
+                        _list.Capacity = value;
+                    }
+                }
+            }
+    
+            public override int Count { 
+                get { lock(_root) { return _list.Count; } }
+            }
+    
+            public override bool IsReadOnly {
+                get { return _list.IsReadOnly; }
+            }
+
+            public override bool IsFixedSize {
+                get { return _list.IsFixedSize; }
+            }
+
+                    
+            public override bool IsSynchronized { 
+                get { return true; }
+            }
+            
+             public override Object this[int index] {
+                get {
+                    lock(_root) {
+                        return _list[index];
+                    }
+                }
+                set {
+                    lock(_root) {
+                        _list[index] = value;
+                    }
+                }
+            }
+    
+            public override Object SyncRoot {
+                get { return _root; }
+            }
+    
+            public override int Add(Object value) {
+                lock(_root) {
+                    return _list.Add(value);
+                }
+            }
+    
+            public override void AddRange(ICollection c) {
+                lock(_root) {
+                    _list.AddRange(c);
+                }
+            }
+
+            public override int BinarySearch(Object value) {
+                lock(_root) {
+                    return _list.BinarySearch(value);
+                }
+            }
+
+            public override int BinarySearch(Object value, IComparer comparer) {
+                lock(_root) {
+                    return _list.BinarySearch(value, comparer);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int BinarySearch(int index, int count, Object value, IComparer comparer) {
+                lock(_root) {
+                    return _list.BinarySearch(index, count, value, comparer);
+                }
+            }
+
+            public override void Clear() {
+                lock(_root) {
+                    _list.Clear();
+                }
+            }
+
+            public override Object Clone() {
+                lock(_root) {
+                    return new SyncArrayList((ArrayList)_list.Clone());
+                }
+            }
+    
+            public override bool Contains(Object item) {
+                lock(_root) {
+                    return _list.Contains(item);
+                }
+            }
+    
+            public override void CopyTo(Array array) {
+                lock(_root) {
+                    _list.CopyTo(array);
+                }
+            }
+
+            public override void CopyTo(Array array, int index) {
+                lock(_root) {
+                    _list.CopyTo(array, index);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void CopyTo(int index, Array array, int arrayIndex, int count) {
+                lock(_root) {
+                    _list.CopyTo(index, array, arrayIndex, count);
+                }
+            }
+    
+            public override IEnumerator GetEnumerator() {
+                lock(_root) {
+                    return _list.GetEnumerator();
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override IEnumerator GetEnumerator(int index, int count) {
+                lock(_root) {
+                    return _list.GetEnumerator(index, count);
+                }
+            }
+    
+            public override int IndexOf(Object value) {
+                lock(_root) {
+                    return _list.IndexOf(value);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex) {
+                lock(_root) {
+                    return _list.IndexOf(value, startIndex);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex, int count) {
+                lock(_root) {
+                    return _list.IndexOf(value, startIndex, count);
+                }
+            }
+    
+            public override void Insert(int index, Object value) {
+                lock(_root) {
+                    _list.Insert(index, value);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void InsertRange(int index, ICollection c) {
+                lock(_root) {
+                    _list.InsertRange(index, c);
+                }
+            }
+    
+            public override int LastIndexOf(Object value) {
+                lock(_root) {
+                    return _list.LastIndexOf(value);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex) {
+                lock(_root) {
+                    return _list.LastIndexOf(value, startIndex);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex, int count) {
+                lock(_root) {
+                    return _list.LastIndexOf(value, startIndex, count);
+                }
+            }
+    
+            public override void Remove(Object value) {
+                lock(_root) {
+                    _list.Remove(value);
+                }
+            }
+    
+            public override void RemoveAt(int index) {
+                lock(_root) {
+                    _list.RemoveAt(index);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void RemoveRange(int index, int count) {
+                lock(_root) {
+                    _list.RemoveRange(index, count);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Reverse(int index, int count) {
+                lock(_root) {
+                    _list.Reverse(index, count);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void SetRange(int index, ICollection c) {
+                lock(_root) {
+                    _list.SetRange(index, c);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override ArrayList GetRange(int index, int count) {
+                lock(_root) {
+                    return _list.GetRange(index, count);
+                }
+            }
+
+            public override void Sort() {
+                lock(_root) {
+                    _list.Sort();
+                }
+            }
+    
+            public override void Sort(IComparer comparer) {
+                lock(_root) {
+                    _list.Sort(comparer);
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Sort(int index, int count, IComparer comparer) {
+                lock(_root) {
+                    _list.Sort(index, count, comparer);
+                }
+            }
+    
+            public override Object[] ToArray() {
+                lock(_root) {
+                    return _list.ToArray();
+                }
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override Array ToArray(Type type) {
+                lock(_root) {
+                    return _list.ToArray(type);
+                }
+            }
+    
+            public override void TrimToSize() {
+                lock(_root) {
+                    _list.TrimToSize();
+                }
+            }
+        }
+    
+    
+        [Serializable]
+        private class SyncIList : IList
+        {
+            private IList _list;
+            private Object _root;
+    
+            internal SyncIList(IList list) {
+                _list = list;
+                _root = list.SyncRoot;
+            }
+    
+            public virtual int Count { 
+                get { lock(_root) { return _list.Count; } }
+            }
+    
+            public virtual bool IsReadOnly {
+                get { return _list.IsReadOnly; }
+            }
+
+            public virtual bool IsFixedSize {
+                get { return _list.IsFixedSize; }
+            }
+
+            
+            public virtual bool IsSynchronized { 
+                get { return true; }
+            }
+            
+             public virtual Object this[int index] {
+                get {
+                    lock(_root) {
+                        return _list[index];
+                    }
+                }
+                set {
+                    lock(_root) {
+                        _list[index] = value;
+                    }
+                }
+            }
+    
+            public virtual Object SyncRoot {
+                get { return _root; }
+            }
+    
+            public virtual int Add(Object value) {
+                lock(_root) {
+                    return _list.Add(value);
+                }
+            }
+                 
+    
+            public virtual void Clear() {
+                lock(_root) {
+                    _list.Clear();
+                }
+            }
+    
+            public virtual bool Contains(Object item) {
+                lock(_root) {
+                    return _list.Contains(item);
+                }
+            }
+    
+            public virtual void CopyTo(Array array, int index) {
+                lock(_root) {
+                    _list.CopyTo(array, index);
+                }
+            }
+    
+            public virtual IEnumerator GetEnumerator() {
+                lock(_root) {
+                    return _list.GetEnumerator();
+                }
+            }
+    
+            public virtual int IndexOf(Object value) {
+                lock(_root) {
+                    return _list.IndexOf(value);
+                }
+            }
+    
+            public virtual void Insert(int index, Object value) {
+                lock(_root) {
+                    _list.Insert(index, value);
+                }
+            }
+    
+            public virtual void Remove(Object value) {
+                lock(_root) {
+                    _list.Remove(value);
+                }
+            }
+    
+            public virtual void RemoveAt(int index) {
+                lock(_root) {
+                    _list.RemoveAt(index);
+                }
+            }
+        }
+    
+        [Serializable]
+        private class FixedSizeList : IList
+        {
+            private IList _list;
+    
+            internal FixedSizeList(IList l) {
+                _list = l;
+            }
+    
+            public virtual int Count { 
+                get { return _list.Count; }
+            }
+    
+            public virtual bool IsReadOnly {
+                get { return _list.IsReadOnly; }
+            }
+
+            public virtual bool IsFixedSize {
+                get { return true; }
+            }
+
+            public virtual bool IsSynchronized { 
+                get { return _list.IsSynchronized; }
+            }
+            
+             public virtual Object this[int index] {
+                get {
+                    return _list[index];
+                }
+                set {
+                    _list[index] = value;
+                }
+            }
+    
+            public virtual Object SyncRoot {
+                get { return _list.SyncRoot; }
+            }
+            
+            public virtual int Add(Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public virtual void Clear() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public virtual bool Contains(Object obj) {
+                return _list.Contains(obj);
+            }
+    
+            public virtual void CopyTo(Array array, int index) {
+                _list.CopyTo(array, index);
+            }
+    
+            public virtual IEnumerator GetEnumerator() {
+                return _list.GetEnumerator();
+            }
+    
+            public virtual int IndexOf(Object value) {
+                return _list.IndexOf(value);
+            }
+    
+            public virtual void Insert(int index, Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public virtual void Remove(Object value) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public virtual void RemoveAt(int index) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+        }
+
+        [Serializable]
+        private class FixedSizeArrayList : ArrayList
+        {
+            private ArrayList _list;
+    
+            internal FixedSizeArrayList(ArrayList l) {
+                _list = l;
+                _version = _list._version;
+            }
+    
+            public override int Count { 
+                get { return _list.Count; }
+            }
+    
+            public override bool IsReadOnly {
+                get { return _list.IsReadOnly; }
+            }
+
+            public override bool IsFixedSize {
+                get { return true; }
+            }
+
+            public override bool IsSynchronized { 
+                get { return _list.IsSynchronized; }
+            }
+            
+             public override Object this[int index] {
+                get {
+                    return _list[index];
+                }
+                set {
+                    _list[index] = value;
+                    _version = _list._version;
+                }
+            }
+    
+            public override Object SyncRoot {
+                get { return _list.SyncRoot; }
+            }
+            
+            public override int Add(Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public override void AddRange(ICollection c) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int BinarySearch(int index, int count, Object value, IComparer comparer) {
+                return _list.BinarySearch(index, count, value, comparer);
+            }
+
+            public override int Capacity {
+                get { return _list.Capacity; }
+                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+                set { throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection")); }
+            }
+
+            public override void Clear() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public override Object Clone() {
+                FixedSizeArrayList arrayList = new FixedSizeArrayList(_list);
+                arrayList._list = (ArrayList)_list.Clone();
+                return arrayList;
+            }
+
+            public override bool Contains(Object obj) {
+                return _list.Contains(obj);
+            }
+    
+            public override void CopyTo(Array array, int index) {
+                _list.CopyTo(array, index);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void CopyTo(int index, Array array, int arrayIndex, int count) {
+                _list.CopyTo(index, array, arrayIndex, count);
+            }
+
+            public override IEnumerator GetEnumerator() {
+                return _list.GetEnumerator();
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override IEnumerator GetEnumerator(int index, int count) {
+                return _list.GetEnumerator(index, count);
+            }
+
+            public override int IndexOf(Object value) {
+                return _list.IndexOf(value);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex) {
+                return _list.IndexOf(value, startIndex);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex, int count) {
+                return _list.IndexOf(value, startIndex, count);
+            }
+    
+            public override void Insert(int index, Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void InsertRange(int index, ICollection c) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+
+            public override int LastIndexOf(Object value) {
+                return _list.LastIndexOf(value);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex) {
+                return _list.LastIndexOf(value, startIndex);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex, int count) {
+                return _list.LastIndexOf(value, startIndex, count);
+            }
+
+            public override void Remove(Object value) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+    
+            public override void RemoveAt(int index) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void RemoveRange(int index, int count) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void SetRange(int index, ICollection c) {
+                _list.SetRange(index, c);
+                _version = _list._version;
+            }
+
+            public override ArrayList GetRange(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                return new Range(this,index, count);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Reverse(int index, int count) {
+                _list.Reverse(index, count);
+                _version = _list._version;
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Sort(int index, int count, IComparer comparer) {
+                _list.Sort(index, count, comparer);
+                _version = _list._version;
+            }
+
+            public override Object[] ToArray() {
+                return _list.ToArray();
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override Array ToArray(Type type) {
+                return _list.ToArray(type);
+            }
+    
+            public override void TrimToSize() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_FixedSizeCollection"));
+            }
+        }
+    
+        [Serializable]
+        private class ReadOnlyList : IList
+        {
+            private IList _list;
+    
+            internal ReadOnlyList(IList l) {
+                _list = l;
+            }
+    
+            public virtual int Count { 
+                get { return _list.Count; }
+            }
+    
+            public virtual bool IsReadOnly {
+                get { return true; }
+            }
+
+            public virtual bool IsFixedSize {
+                get { return true; }
+            }
+
+            public virtual bool IsSynchronized { 
+                get { return _list.IsSynchronized; }
+            }
+            
+             public virtual Object this[int index] {
+                get {
+                    return _list[index];
+                }
+                set {
+                    throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+                }
+            }
+    
+            public virtual Object SyncRoot {
+                get { return _list.SyncRoot; }
+            }
+            
+            public virtual int Add(Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public virtual void Clear() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public virtual bool Contains(Object obj) {
+                return _list.Contains(obj);
+            }
+            
+            public virtual void CopyTo(Array array, int index) {
+                _list.CopyTo(array, index);
+            }
+    
+            public virtual IEnumerator GetEnumerator() {
+                return _list.GetEnumerator();
+            }
+    
+            public virtual int IndexOf(Object value) {
+                return _list.IndexOf(value);
+            }
+    
+            public virtual void Insert(int index, Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            public virtual void Remove(Object value) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public virtual void RemoveAt(int index) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+        }
+
+        [Serializable]
+        private class ReadOnlyArrayList : ArrayList
+        {
+            private ArrayList _list;
+    
+            internal ReadOnlyArrayList(ArrayList l) {
+                _list = l;
+            }
+    
+            public override int Count { 
+                get { return _list.Count; }
+            }
+    
+            public override bool IsReadOnly {
+                get { return true; }
+            }
+
+            public override bool IsFixedSize {
+                get { return true; }
+            }
+
+            public override bool IsSynchronized { 
+                get { return _list.IsSynchronized; }
+            }
+            
+             public override Object this[int index] {
+                get {
+                    return _list[index];
+                }
+                set {
+                    throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+                }
+            }
+    
+            public override Object SyncRoot {
+                get { return _list.SyncRoot; }
+            }
+            
+            public override int Add(Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public override void AddRange(ICollection c) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int BinarySearch(int index, int count, Object value, IComparer comparer) {
+                return _list.BinarySearch(index, count, value, comparer);
+            }
+
+
+            public override int Capacity {
+                get { return _list.Capacity; }
+                [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+                set { throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection")); }
+            }
+
+            public override void Clear() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            public override Object Clone() {
+                ReadOnlyArrayList arrayList = new ReadOnlyArrayList(_list);
+                arrayList._list = (ArrayList)_list.Clone();
+                return arrayList;
+            }
+    
+            public override bool Contains(Object obj) {
+                return _list.Contains(obj);
+            }
+    
+            public override void CopyTo(Array array, int index) {
+                _list.CopyTo(array, index);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void CopyTo(int index, Array array, int arrayIndex, int count) {
+                _list.CopyTo(index, array, arrayIndex, count);
+            }
+
+            public override IEnumerator GetEnumerator() {
+                return _list.GetEnumerator();
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override IEnumerator GetEnumerator(int index, int count) {
+                return _list.GetEnumerator(index, count);
+            }
+
+            public override int IndexOf(Object value) {
+                return _list.IndexOf(value);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex) {
+                return _list.IndexOf(value, startIndex);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int IndexOf(Object value, int startIndex, int count) {
+                return _list.IndexOf(value, startIndex, count);
+            }
+    
+            public override void Insert(int index, Object obj) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void InsertRange(int index, ICollection c) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            public override int LastIndexOf(Object value) {
+                return _list.LastIndexOf(value);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex) {
+                return _list.LastIndexOf(value, startIndex);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex, int count) {
+                return _list.LastIndexOf(value, startIndex, count);
+            }
+
+            public override void Remove(Object value) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public override void RemoveAt(int index) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void RemoveRange(int index, int count) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void SetRange(int index, ICollection c) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+    
+            public override ArrayList GetRange(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (Count - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                return new Range(this,index, count);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Reverse(int index, int count) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void Sort(int index, int count, IComparer comparer) {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+
+            public override Object[] ToArray() {
+                return _list.ToArray();
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override Array ToArray(Type type) {
+                return _list.ToArray(type);
+            }
+    
+            public override void TrimToSize() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_ReadOnlyCollection"));
+            }
+        }
+
+    
+        // Implements an enumerator for a ArrayList. The enumerator uses the
+        // internal version number of the list to ensure that no modifications are
+        // made to the list while an enumeration is in progress.
+        [Serializable]
+        private sealed class ArrayListEnumerator : IEnumerator, ICloneable
+        {
+            private ArrayList list;
+            private int index;
+            private int endIndex;       // Where to stop.
+            private int version;
+            private Object currentElement;
+            private int startIndex;     // Save this for Reset.
+    
+            internal ArrayListEnumerator(ArrayList list, int index, int count) {
+                this.list = list;
+                startIndex = index;
+                this.index = index - 1;
+                endIndex = this.index + count;  // last valid index
+                version = list._version;
+                currentElement = null;
+            }
+
+            public Object Clone() {
+                return MemberwiseClone();
+            }
+    
+            public bool MoveNext() {
+                if (version != list._version) throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                if (index < endIndex) {
+                    currentElement = list[++index];
+                    return true;
+                }
+                else {
+                    index = endIndex + 1;
+                }
+                
+                return false;
+            }
+    
+            public Object Current {
+                get {
+                    if (index < startIndex) 
+                        throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumNotStarted));
+                    else if (index > endIndex) {
+                        throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumEnded));
+                    }
+                    return currentElement;
+                }
+            }
+       
+            public void Reset() {
+                if (version != list._version) throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                index = startIndex - 1;                
+            }
+        }
+
+        // Implementation of a generic list subrange. An instance of this class
+        // is returned by the default implementation of List.GetRange.
+        [Serializable]
+        private class Range: ArrayList
+        {
+            private ArrayList _baseList;
+            private int _baseIndex;
+            [ContractPublicPropertyName("Count")]
+            private int _baseSize;
+            private int _baseVersion;
+                        
+            internal Range(ArrayList list, int index, int count) : base(false) {
+                _baseList = list;
+                _baseIndex = index;
+                _baseSize = count;
+                _baseVersion = list._version;
+                // we also need to update _version field to make Range of Range work
+                _version = list._version;                
+            }
+
+            private void InternalUpdateRange()
+            {
+                if (_baseVersion != _baseList._version)
+                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_UnderlyingArrayListChanged"));
+            }
+
+            private void InternalUpdateVersion() {
+                _baseVersion++;
+                _version++;                
+            }
+            
+            public override int Add(Object value) {
+                InternalUpdateRange();
+                _baseList.Insert(_baseIndex + _baseSize, value);
+                InternalUpdateVersion();
+                return _baseSize++;
+            }
+
+            public override void AddRange(ICollection c) {
+                if( c ==  null ) {
+                    throw new ArgumentNullException("c");
+                }
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                int count = c.Count;
+                if( count > 0) {
+                    _baseList.InsertRange(_baseIndex + _baseSize, c);
+                    InternalUpdateVersion();
+                    _baseSize += count;
+                }
+            }
+
+            // Other overloads with automatically work 
+            public override int BinarySearch(int index, int count, Object value, IComparer comparer) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+                InternalUpdateRange();
+
+                int i = _baseList.BinarySearch(_baseIndex + index, count, value, comparer);
+                if (i >= 0) return i - _baseIndex;
+                return i + _baseIndex;
+            }
+
+            public override int Capacity {
+                get {
+                    return _baseList.Capacity;
+                }
+             
+                set {
+                    if (value < Count) throw new ArgumentOutOfRangeException("value", Environment.GetResourceString("ArgumentOutOfRange_SmallCapacity"));
+                    Contract.EndContractBlock();
+                }
+            }
+    
+
+            public override void Clear() {
+                InternalUpdateRange();
+                if (_baseSize != 0)
+                {
+                    _baseList.RemoveRange(_baseIndex, _baseSize);
+                    InternalUpdateVersion();
+                    _baseSize = 0;
+                }
+            }
+
+            public override Object Clone() {
+                InternalUpdateRange();
+                Range arrayList = new Range(_baseList,_baseIndex,_baseSize);
+                arrayList._baseList = (ArrayList)_baseList.Clone();
+                return arrayList;
+            }
+
+            public override bool Contains(Object item) {
+              InternalUpdateRange();
+              if (item==null) {
+                    for(int i=0; i<_baseSize; i++)
+                        if (_baseList[_baseIndex + i]==null)
+                            return true;
+                    return false;
+                }
+                else {
+                    for(int i=0; i<_baseSize; i++)
+                        if (_baseList[_baseIndex + i] != null && _baseList[_baseIndex + i].Equals(item))
+                            return true;
+                    return false;
+                }
+            }
+    
+            public override void CopyTo(Array array, int index) {
+                if (array==null)
+                    throw new ArgumentNullException("array");
+                if (array.Rank != 1)
+                    throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+                if (index < 0)
+                    throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (array.Length - index < _baseSize)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.CopyTo(_baseIndex, array, index, _baseSize);
+            }
+
+            public override void CopyTo(int index, Array array, int arrayIndex, int count) {
+                if (array==null)
+                    throw new ArgumentNullException("array");
+                if (array.Rank != 1)
+                    throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (array.Length - arrayIndex < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.CopyTo(_baseIndex + index, array, arrayIndex, count);
+            }
+
+            public override int Count {
+                get {
+                 InternalUpdateRange();
+                 return _baseSize; 
+                }
+            }
+
+            public override bool IsReadOnly {
+                get { return _baseList.IsReadOnly; }
+            }
+
+            public override bool IsFixedSize {
+                get { return _baseList.IsFixedSize; }
+            }
+
+            public override bool IsSynchronized {
+                get { return _baseList.IsSynchronized; }
+            }
+                                
+            public override IEnumerator GetEnumerator() {
+                return GetEnumerator(0,_baseSize);
+            }
+
+            public override IEnumerator GetEnumerator(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                return _baseList.GetEnumerator(_baseIndex + index, count);
+            }
+
+            public override ArrayList GetRange(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                return new Range(this, index, count);
+            }
+
+            public override Object SyncRoot {
+                get {
+                    return _baseList.SyncRoot;
+                }
+            }
+        
+
+            public override int IndexOf(Object value) {
+                InternalUpdateRange();
+                int i = _baseList.IndexOf(value, _baseIndex, _baseSize);
+                if (i >= 0) return i - _baseIndex;
+                return -1;
+            }
+
+            public override int IndexOf(Object value, int startIndex) {
+                if (startIndex < 0)
+                    throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (startIndex > _baseSize)
+                    throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                int i = _baseList.IndexOf(value, _baseIndex + startIndex, _baseSize - startIndex);
+                if (i >= 0) return i - _baseIndex;
+                return -1;
+            }
+            
+            public override int IndexOf(Object value, int startIndex, int count) {
+                if (startIndex < 0 || startIndex > _baseSize)
+                    throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                    
+                if (count < 0 || (startIndex > _baseSize - count))
+                    throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                int i = _baseList.IndexOf(value, _baseIndex + startIndex, count);
+                if (i >= 0) return i - _baseIndex;
+                return -1;
+            }
+
+            public override void Insert(int index, Object value) {
+                if (index < 0 || index > _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.Insert(_baseIndex + index, value);
+                InternalUpdateVersion();
+                _baseSize++;
+            }
+
+            public override void InsertRange(int index, ICollection c) {
+                if (index < 0 || index > _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                if( c == null) {
+                    throw new ArgumentNullException("c");
+                }
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();                
+                int count = c.Count;
+                if( count > 0) {
+                    _baseList.InsertRange(_baseIndex + index, c);
+                    _baseSize += count;                    
+                    InternalUpdateVersion();
+                }
+            }
+
+            public override int LastIndexOf(Object value) {
+                InternalUpdateRange();
+                int i = _baseList.LastIndexOf(value, _baseIndex + _baseSize - 1, _baseSize);
+                if (i >= 0) return i - _baseIndex;
+                return -1;
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex) {
+                return LastIndexOf(value, startIndex, startIndex + 1);
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override int LastIndexOf(Object value, int startIndex, int count) {
+                InternalUpdateRange();
+                if (_baseSize == 0)
+                    return -1;
+   
+                if (startIndex >= _baseSize)
+                    throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                if (startIndex < 0)
+                    throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                
+                int i = _baseList.LastIndexOf(value, _baseIndex + startIndex, count);
+                if (i >= 0) return i - _baseIndex;
+                return -1;
+            }
+
+            // Don't need to override Remove
+
+            public override void RemoveAt(int index) {
+                if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.RemoveAt(_baseIndex + index);
+                InternalUpdateVersion();
+                _baseSize--;
+            }
+
+            public override void RemoveRange(int index, int count) {            
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                // No need to call _bastList.RemoveRange if count is 0.
+                // In addition, _baseList won't change the vresion number if count is 0. 
+                if( count > 0) {
+                    _baseList.RemoveRange(_baseIndex + index, count);
+                    InternalUpdateVersion();
+                    _baseSize -= count;
+                }
+            }
+
+            public override void Reverse(int index, int count) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.Reverse(_baseIndex + index, count);
+                InternalUpdateVersion();
+            }
+
+            [SuppressMessage("Microsoft.Contracts", "CC1055")]  // Skip extra error checking to avoid *potential* AppCompat problems.
+            public override void SetRange(int index, ICollection c) {
+                InternalUpdateRange();
+                if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                _baseList.SetRange(_baseIndex + index, c);
+                if( c.Count > 0) {
+                    InternalUpdateVersion();
+                }
+            }
+
+            public override void Sort(int index, int count, IComparer comparer) {
+                if (index < 0 || count < 0)
+                    throw new ArgumentOutOfRangeException((index<0 ? "index" : "count"), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                if (_baseSize - index < count)
+                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                _baseList.Sort(_baseIndex + index, count, comparer);
+                InternalUpdateVersion();
+            }
+
+            public override Object this[int index] {
+                get {
+                    InternalUpdateRange();
+                    if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                    return _baseList[_baseIndex + index];
+                }
+                set {
+                    InternalUpdateRange();
+                    if (index < 0 || index >= _baseSize) throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                    _baseList[_baseIndex + index] = value;
+                    InternalUpdateVersion();
+                }
+            }
+
+            public override Object[] ToArray() {
+                InternalUpdateRange();
+                Object[] array = new Object[_baseSize];
+                Array.Copy(_baseList._items, _baseIndex, array, 0, _baseSize);
+                return array;
+            }
+
+            [SecuritySafeCritical]
+            public override Array ToArray(Type type) {
+                if (type==null)
+                    throw new ArgumentNullException("type");
+                Contract.EndContractBlock();
+
+                InternalUpdateRange();
+                Array array = Array.UnsafeCreateInstance(type, _baseSize);
+                _baseList.CopyTo(_baseIndex, array, 0, _baseSize);
+                return array;
+            }
+
+            public override void TrimToSize() {
+                throw new NotSupportedException(Environment.GetResourceString("NotSupported_RangeCollection"));
+            }
+        }
+
+        [Serializable]
+        private sealed class ArrayListEnumeratorSimple : IEnumerator, ICloneable {
+            private ArrayList list;
+            private int index;
+            private int version;
+            private Object currentElement;
+            [NonSerialized]
+            private bool isArrayList;
+            // this object is used to indicate enumeration has not started or has terminated
+            static Object dummyObject = new Object();  
+                            
+            internal ArrayListEnumeratorSimple(ArrayList list) {
+                this.list = list;
+                this.index = -1;
+                version = list._version;
+                isArrayList = (list.GetType() == typeof(ArrayList));
+                currentElement = dummyObject;                
+            }
+            
+            public Object Clone() {
+                return MemberwiseClone();
+            }
+    
+            public bool MoveNext() {
+                if (version != list._version) {
+                    throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                }
+
+                if( isArrayList) {  // avoid calling virtual methods if we are operating on ArrayList to improve performance
+                    if (index < list._size - 1) {
+                        currentElement = list._items[++index];
+                        return true;
+                    }
+                    else {
+                        currentElement = dummyObject;
+                        index =list._size;
+                        return false;
+                    }                    
+                }
+                else {                    
+                    if (index < list.Count - 1) {
+                        currentElement = list[++index];
+                        return true;
+                    }
+                    else {
+                        index = list.Count;
+                        currentElement = dummyObject;
+                        return false;
+                    }
+                }
+            }
+    
+            public Object Current {
+                get {
+                    object temp = currentElement;
+                    if(dummyObject == temp) { // check if enumeration has not started or has terminated
+                        if (index == -1) {
+                            throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumNotStarted));
+                        }
+                        else {                    
+                            throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumEnded));                        
+                        }
+                    }
+
+                    return temp;
+                }
+            }
+    
+            public void Reset() {
+                if (version != list._version) {
+                    throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                }    
+                
+                currentElement = dummyObject;
+                index = -1;
+            }
+        }
+
+        internal class ArrayListDebugView {
+            private ArrayList arrayList; 
+        
+            public ArrayListDebugView( ArrayList arrayList) {
+                if( arrayList == null)
+                    throw new ArgumentNullException("arrayList");
+
+                this.arrayList = arrayList;
+            }
+
+            [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+            public Object[] Items { 
+                get {
+                    return arrayList.ToArray();
+                }
+            }
+        }
+    }    
+}
diff --git a/src/mscorlib/src/System/Collections/BitArray.cs b/src/mscorlib/src/System/Collections/BitArray.cs
new file mode 100644
index 0000000000..2f565f83af
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/BitArray.cs
@@ -0,0 +1,524 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*=============================================================================
+**
+**
+** 
+**
+**
+** Purpose: The BitArray class manages a compact array of bit values.
+**
+**
+=============================================================================*/
+namespace System.Collections {
+
+    using System;
+    using System.Security.Permissions;
+    using System.Diagnostics.Contracts;
+    // A vector of bits.  Use this to store bits efficiently, without having to do bit 
+    // shifting yourself.
+[System.Runtime.InteropServices.ComVisible(true)]
+    [Serializable()] public sealed class BitArray : ICollection, ICloneable {
+        private BitArray() {
+        }
+
+        /*=========================================================================
+        ** Allocates space to hold length bit values. All of the values in the bit
+        ** array are set to false.
+        **
+        ** Exceptions: ArgumentException if length < 0.
+        =========================================================================*/
+        public BitArray(int length) 
+            : this(length, false) {
+        }
+    
+        /*=========================================================================
+        ** Allocates space to hold length bit values. All of the values in the bit
+        ** array are set to defaultValue.
+        **
+        ** Exceptions: ArgumentOutOfRangeException if length < 0.
+        =========================================================================*/
+        public BitArray(int length, bool defaultValue) {
+            if (length < 0) {
+                throw new ArgumentOutOfRangeException(nameof(length), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+            }
+            Contract.EndContractBlock();
+    
+            m_array = new int[GetArrayLength(length, BitsPerInt32)];
+            m_length = length;
+    
+            int fillValue = defaultValue ? unchecked(((int)0xffffffff)) : 0;
+            for (int i = 0; i < m_array.Length; i++) {
+                m_array[i] = fillValue;
+            }
+
+            _version = 0;
+        }
+    
+        /*=========================================================================
+        ** Allocates space to hold the bit values in bytes. bytes[0] represents
+        ** bits 0 - 7, bytes[1] represents bits 8 - 15, etc. The LSB of each byte
+        ** represents the lowest index value; bytes[0] & 1 represents bit 0,
+        ** bytes[0] & 2 represents bit 1, bytes[0] & 4 represents bit 2, etc.
+        **
+        ** Exceptions: ArgumentException if bytes == null.
+        =========================================================================*/
+        public BitArray(byte[] bytes) {
+            if (bytes == null) {
+                throw new ArgumentNullException(nameof(bytes));
+            }
+            Contract.EndContractBlock();
+            // this value is chosen to prevent overflow when computing m_length.
+            // m_length is of type int32 and is exposed as a property, so 
+            // type of m_length can't be changed to accommodate.
+            if (bytes.Length > Int32.MaxValue / BitsPerByte) {
+                throw new ArgumentException(Environment.GetResourceString("Argument_ArrayTooLarge", BitsPerByte), nameof(bytes));
+            }
+    
+            m_array = new int[GetArrayLength(bytes.Length, BytesPerInt32)];
+            m_length = bytes.Length * BitsPerByte;
+    
+            int i = 0;
+            int j = 0;
+            while (bytes.Length - j >= 4) {
+                m_array[i++] = (bytes[j] & 0xff) |
+                              ((bytes[j + 1] & 0xff) << 8) |
+                              ((bytes[j + 2] & 0xff) << 16) |
+                              ((bytes[j + 3] & 0xff) << 24);
+                j += 4;
+            }
+    
+            Contract.Assert(bytes.Length - j >= 0, "BitArray byteLength problem");
+            Contract.Assert(bytes.Length - j < 4, "BitArray byteLength problem #2");
+    
+            switch (bytes.Length - j) {
+                case 3:
+                    m_array[i] = ((bytes[j + 2] & 0xff) << 16);
+                    goto case 2;
+                    // fall through
+                case 2:
+                    m_array[i] |= ((bytes[j + 1] & 0xff) << 8);
+                    goto case 1;
+                    // fall through
+                case 1:
+                    m_array[i] |= (bytes[j] & 0xff);
+            break;
+            }
+
+            _version = 0;
+        }
+
+        public BitArray(bool[] values) {
+            if (values == null) {
+                throw new ArgumentNullException(nameof(values));
+            }
+            Contract.EndContractBlock();
+    
+            m_array = new int[GetArrayLength(values.Length, BitsPerInt32)];
+            m_length = values.Length;
+    
+            for (int i = 0;i<values.Length;i++) {
+                if (values[i])
+                    m_array[i/32] |= (1 << (i%32));
+            }
+
+            _version = 0;
+
+        }
+    
+        /*=========================================================================
+        ** Allocates space to hold the bit values in values. values[0] represents
+        ** bits 0 - 31, values[1] represents bits 32 - 63, etc. The LSB of each
+        ** integer represents the lowest index value; values[0] & 1 represents bit
+        ** 0, values[0] & 2 represents bit 1, values[0] & 4 represents bit 2, etc.
+        **
+        ** Exceptions: ArgumentException if values == null.
+        =========================================================================*/
+        public BitArray(int[] values) {
+            if (values == null) {
+                throw new ArgumentNullException(nameof(values));
+            }
+            Contract.EndContractBlock();
+            // this value is chosen to prevent overflow when computing m_length
+            if (values.Length > Int32.MaxValue / BitsPerInt32) {
+                throw new ArgumentException(Environment.GetResourceString("Argument_ArrayTooLarge", BitsPerInt32), nameof(values));
+            }
+    
+            m_array = new int[values.Length];
+            m_length = values.Length * BitsPerInt32;
+    
+            Array.Copy(values, m_array, values.Length);
+
+            _version = 0;
+        }
+    
+        /*=========================================================================
+        ** Allocates a new BitArray with the same length and bit values as bits.
+        **
+        ** Exceptions: ArgumentException if bits == null.
+        =========================================================================*/
+        public BitArray(BitArray bits) {
+            if (bits == null) {
+                throw new ArgumentNullException(nameof(bits));
+            }
+            Contract.EndContractBlock();
+    
+            int arrayLength = GetArrayLength(bits.m_length, BitsPerInt32);
+            m_array = new int[arrayLength];
+            m_length = bits.m_length;
+    
+            Array.Copy(bits.m_array, m_array, arrayLength);
+
+            _version = bits._version;
+        }
+
+        public bool this[int index] {
+                get {
+                    return Get(index);
+                }
+                set {
+                    Set(index,value);
+                }
+         }
+    
+        /*=========================================================================
+        ** Returns the bit value at position index.
+        **
+        ** Exceptions: ArgumentOutOfRangeException if index < 0 or
+        **             index >= GetLength().
+        =========================================================================*/
+        public bool Get(int index) {
+            if (index < 0 || index >= Length) {
+                throw new ArgumentOutOfRangeException(nameof(index), Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            }
+            Contract.EndContractBlock();
+    
+            return (m_array[index / 32] & (1 << (index % 32))) != 0;
+        }
+    
+        /*=========================================================================
+        ** Sets the bit value at position index to value.
+        **
+        ** Exceptions: ArgumentOutOfRangeException if index < 0 or
+        **             index >= GetLength().
+        =========================================================================*/
+        public void Set(int index, bool value) {
+            if (index < 0 || index >= Length) {
+                throw new ArgumentOutOfRangeException(nameof(index), Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            }
+            Contract.EndContractBlock();
+    
+            if (value) {
+                m_array[index / 32] |= (1 << (index % 32));
+            } else {
+                m_array[index / 32] &= ~(1 << (index % 32));
+            }
+
+            _version++;
+        }
+    
+        /*=========================================================================
+        ** Sets all the bit values to value.
+        =========================================================================*/
+        public void SetAll(bool value) {
+            int fillValue = value ? unchecked(((int)0xffffffff)) : 0;
+            int ints = GetArrayLength(m_length, BitsPerInt32);
+            for (int i = 0; i < ints; i++) {
+                m_array[i] = fillValue;
+            }
+
+            _version++;
+        }
+    
+        /*=========================================================================
+        ** Returns a reference to the current instance ANDed with value.
+        **
+        ** Exceptions: ArgumentException if value == null or
+        **             value.Length != this.Length.
+        =========================================================================*/
+        public BitArray And(BitArray value) {
+            if (value==null)
+                throw new ArgumentNullException(nameof(value));
+            if (Length != value.Length)
+                throw new ArgumentException(Environment.GetResourceString("Arg_ArrayLengthsDiffer"));
+            Contract.EndContractBlock();
+    
+            int ints = GetArrayLength(m_length, BitsPerInt32);
+            for (int i = 0; i < ints; i++) {
+                m_array[i] &= value.m_array[i];
+            }
+    
+            _version++;
+            return this;
+        }
+    
+        /*=========================================================================
+        ** Returns a reference to the current instance ORed with value.
+        **
+        ** Exceptions: ArgumentException if value == null or
+        **             value.Length != this.Length.
+        =========================================================================*/
+        public BitArray Or(BitArray value) {
+            if (value==null)
+                throw new ArgumentNullException(nameof(value));
+            if (Length != value.Length)
+                throw new ArgumentException(Environment.GetResourceString("Arg_ArrayLengthsDiffer"));
+            Contract.EndContractBlock();
+    
+            int ints = GetArrayLength(m_length, BitsPerInt32);
+            for (int i = 0; i < ints; i++) {
+                m_array[i] |= value.m_array[i];
+            }
+    
+            _version++;
+            return this;
+        }
+    
+        /*=========================================================================
+        ** Returns a reference to the current instance XORed with value.
+        **
+        ** Exceptions: ArgumentException if value == null or
+        **             value.Length != this.Length.
+        =========================================================================*/
+        public BitArray Xor(BitArray value) {
+            if (value==null)
+                throw new ArgumentNullException(nameof(value));
+            if (Length != value.Length)
+                throw new ArgumentException(Environment.GetResourceString("Arg_ArrayLengthsDiffer"));
+            Contract.EndContractBlock();
+    
+            int ints = GetArrayLength(m_length, BitsPerInt32);
+            for (int i = 0; i < ints; i++) {
+                m_array[i] ^= value.m_array[i];
+            }
+            
+            _version++;
+            return this;
+        }
+    
+        /*=========================================================================
+        ** Inverts all the bit values. On/true bit values are converted to
+        ** off/false. Off/false bit values are turned on/true. The current instance
+        ** is updated and returned.
+        =========================================================================*/
+        public BitArray Not() {
+            int ints = GetArrayLength(m_length, BitsPerInt32);
+            for (int i = 0; i < ints; i++) {
+                m_array[i] = ~m_array[i];
+            }
+    
+            _version++;
+            return this;
+        }
+
+        public int Length {
+            get {
+                Contract.Ensures(Contract.Result<int>() >= 0);
+                return m_length;
+            }
+            set {
+                if (value < 0) {
+                    throw new ArgumentOutOfRangeException(nameof(value), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+                }
+                Contract.EndContractBlock();
+
+                int newints = GetArrayLength(value, BitsPerInt32);
+                if (newints > m_array.Length || newints + _ShrinkThreshold < m_array.Length) {
+                    // grow or shrink (if wasting more than _ShrinkThreshold ints)
+                    int[] newarray = new int[newints];
+                    Array.Copy(m_array, newarray, newints > m_array.Length ? m_array.Length : newints);
+                    m_array = newarray;
+                }
+                
+                if (value > m_length) {
+                    // clear high bit values in the last int
+                    int last = GetArrayLength(m_length, BitsPerInt32) - 1;
+                    int bits = m_length % 32;
+                    if (bits > 0) {
+                        m_array[last] &= (1 << bits) - 1;
+                    }
+                    
+                    // clear remaining int values
+                    Array.Clear(m_array, last + 1, newints - last - 1);
+                }
+                
+                m_length = value;
+                _version++;
+            }
+        }
+
+        // ICollection implementation
+        public void CopyTo(Array array, int index)
+        {
+            if (array == null)
+                throw new ArgumentNullException(nameof(array));
+
+            if (index < 0)
+                throw new ArgumentOutOfRangeException(nameof(index), Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+
+            if (array.Rank != 1)
+                throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"), nameof(array));
+                
+            Contract.EndContractBlock();
+
+            if (array is int[])
+            {
+                Array.Copy(m_array, 0, array, index, GetArrayLength(m_length, BitsPerInt32));
+            }
+            else if (array is byte[])
+            {
+                int arrayLength = GetArrayLength(m_length, BitsPerByte);
+                if ((array.Length - index) < arrayLength)
+                     throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+
+                byte [] b = (byte[])array;
+                for (int i = 0; i < arrayLength; i++)
+                    b[index + i] = (byte)((m_array[i/4] >> ((i%4)*8)) & 0x000000FF); // Shift to bring the required byte to LSB, then mask
+            }
+            else if (array is bool[])
+            {
+                if (array.Length - index < m_length)
+                     throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+
+                bool [] b = (bool[])array;
+                for (int i = 0;i<m_length;i++)
+                    b[index + i] = ((m_array[i/32] >> (i%32)) & 0x00000001) != 0;
+            }
+            else
+                throw new ArgumentException(Environment.GetResourceString("Arg_BitArrayTypeUnsupported"), nameof(array));
+        }
+        
+        public int Count
+        { 
+            get
+            {
+                Contract.Ensures(Contract.Result<int>() >= 0);
+
+                return m_length;
+            }
+        }
+
+        public Object Clone()
+        {
+            Contract.Ensures(Contract.Result<Object>() != null);
+            Contract.Ensures(((BitArray)Contract.Result<Object>()).Length == this.Length);
+
+            return new BitArray(this);
+        }
+        
+        public Object SyncRoot
+        {
+             get
+             {
+                if( _syncRoot == null) {
+                    System.Threading.Interlocked.CompareExchange<Object>(ref _syncRoot, new Object(), null);    
+                }
+                return _syncRoot; 
+             }
+        }
+    
+        public bool IsReadOnly 
+        { 
+            get
+            {
+                return false;
+            }
+        }
+
+        public bool IsSynchronized
+        { 
+            get
+            {
+                return false;
+            }
+        }
+
+        public IEnumerator GetEnumerator()
+        {
+            return new BitArrayEnumeratorSimple(this);
+        }
+
+        // XPerY=n means that n Xs can be stored in 1 Y. 
+        private const int BitsPerInt32 = 32;
+        private const int BytesPerInt32 = 4;
+        private const int BitsPerByte = 8;
+
+        /// <summary>
+        /// Used for conversion between different representations of bit array. 
+        /// Returns (n+(div-1))/div, rearranged to avoid arithmetic overflow. 
+        /// For example, in the bit to int case, the straightforward calc would 
+        /// be (n+31)/32, but that would cause overflow. So instead it's 
+        /// rearranged to ((n-1)/32) + 1, with special casing for 0.
+        /// 
+        /// Usage:
+        /// GetArrayLength(77, BitsPerInt32): returns how many ints must be 
+        /// allocated to store 77 bits.
+        /// </summary>
+        /// <param name="n"></param>
+        /// <param name="div">use a conversion constant, e.g. BytesPerInt32 to get
+        /// how many ints are required to store n bytes</param>
+        /// <returns></returns>
+        private static int GetArrayLength(int n, int div) {
+            Contract.Assert(div > 0, "GetArrayLength: div arg must be greater than 0");
+            return n > 0 ? (((n - 1) / div) + 1) : 0;
+        }
+
+        [Serializable]
+        private class BitArrayEnumeratorSimple : IEnumerator, ICloneable
+        {
+            private BitArray bitarray;
+            private int index;
+            private int version;
+            private bool currentElement;
+               
+            internal BitArrayEnumeratorSimple(BitArray bitarray) {
+                this.bitarray = bitarray;
+                this.index = -1;
+                version = bitarray._version;
+            }
+
+            public Object Clone() {
+                return MemberwiseClone();
+            }
+                
+            public virtual bool MoveNext() {
+                if (version != bitarray._version) throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                if (index < (bitarray.Count-1)) {
+                    index++;
+                    currentElement = bitarray.Get(index);
+                    return true;
+                }
+                else
+                    index = bitarray.Count;
+                
+                return false;
+            }
+    
+            public virtual Object Current {
+                get {
+                    if (index == -1)
+                        throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumNotStarted));
+                    if (index >= bitarray.Count)
+                        throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumEnded)); 
+                    return currentElement;
+                }
+            }
+    
+            public void Reset() {
+                if (version != bitarray._version) throw new InvalidOperationException(Environment.GetResourceString(ResId.InvalidOperation_EnumFailedVersion));
+                index = -1;
+            }
+        }
+    
+        private int[] m_array;
+        private int m_length;
+        private int _version;
+        [NonSerialized]
+        private Object _syncRoot;
+    
+        private const int _ShrinkThreshold = 256;
+    }
+
+}
diff --git a/src/mscorlib/src/System/Collections/CollectionBase.cs b/src/mscorlib/src/System/Collections/CollectionBase.cs
new file mode 100644
index 0000000000..1bb08af27a
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/CollectionBase.cs
@@ -0,0 +1,215 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+//------------------------------------------------------------------------------
+//------------------------------------------------------------------------------
+// 
+
+namespace System.Collections {
+    using System;
+    using System.Diagnostics.Contracts;
+
+    // Useful base class for typed read/write collections where items derive from object
+    [Serializable]
+[System.Runtime.InteropServices.ComVisible(true)]
+    public abstract class CollectionBase : IList {
+        ArrayList list;
+
+        protected CollectionBase() {
+            list = new ArrayList();
+        }
+        
+        protected CollectionBase(int capacity) {
+            list = new ArrayList(capacity);
+        }
+
+
+        protected ArrayList InnerList { 
+            get { 
+                if (list == null)
+                    list = new ArrayList();
+                return list;
+            }
+        }
+
+        protected IList List {
+            get { return (IList)this; }
+        }
+
+        [System.Runtime.InteropServices.ComVisible(false)]        
+        public int Capacity {
+            get {
+                return InnerList.Capacity;
+            }
+            set {
+                InnerList.Capacity = value;
+            }
+        }
+
+
+        public int Count {
+            get {
+                return list == null ? 0 : list.Count;
+            }
+        }
+
+        public void Clear() {
+            OnClear();
+            InnerList.Clear();
+            OnClearComplete();
+        }
+
+        public void RemoveAt(int index) {
+            if (index < 0 || index >= Count)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            Contract.EndContractBlock();
+            Object temp = InnerList[index];
+            OnValidate(temp);
+            OnRemove(index, temp);
+            InnerList.RemoveAt(index);
+            try {
+                OnRemoveComplete(index, temp);
+            }
+            catch {
+                InnerList.Insert(index, temp);
+                throw;
+            }
+
+        }
+
+        bool IList.IsReadOnly {
+            get { return InnerList.IsReadOnly; }
+        }
+
+        bool IList.IsFixedSize {
+            get { return InnerList.IsFixedSize; }
+        }
+
+        bool ICollection.IsSynchronized {
+            get { return InnerList.IsSynchronized; }
+        }
+
+        Object ICollection.SyncRoot {
+            get { return InnerList.SyncRoot; }
+        }
+
+        void ICollection.CopyTo(Array array, int index) {
+            InnerList.CopyTo(array, index);
+        }
+
+        Object IList.this[int index] {
+            get { 
+                if (index < 0 || index >= Count)
+                    throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+                return InnerList[index]; 
+            }
+            set { 
+                if (index < 0 || index >= Count)
+                    throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+                Contract.EndContractBlock();
+                OnValidate(value);
+                Object temp = InnerList[index];
+                OnSet(index, temp, value); 
+                InnerList[index] = value; 
+                try {
+                    OnSetComplete(index, temp, value);
+                }
+                catch {
+                    InnerList[index] = temp; 
+                    throw;
+                }
+            }
+        }
+
+        bool IList.Contains(Object value) {
+            return InnerList.Contains(value);
+        }
+
+        int IList.Add(Object value) {
+            OnValidate(value);
+            OnInsert(InnerList.Count, value);
+            int index = InnerList.Add(value);
+            try {
+                OnInsertComplete(index, value);
+            }
+            catch {
+                InnerList.RemoveAt(index);
+                throw;
+            }
+            return index;
+        }
+
+       
+        void IList.Remove(Object value) {
+            OnValidate(value);
+            int index = InnerList.IndexOf(value);
+            if (index < 0) throw new ArgumentException(Environment.GetResourceString("Arg_RemoveArgNotFound"));
+            OnRemove(index, value);
+            InnerList.RemoveAt(index);
+            try{
+                OnRemoveComplete(index, value);
+            }
+            catch {
+                InnerList.Insert(index, value);
+                throw;
+            }
+        }
+
+        int IList.IndexOf(Object value) {
+            return InnerList.IndexOf(value);
+        }
+
+        void IList.Insert(int index, Object value) {
+            if (index < 0 || index > Count)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            Contract.EndContractBlock();
+            OnValidate(value);
+            OnInsert(index, value);
+            InnerList.Insert(index, value);
+            try {
+                OnInsertComplete(index, value);
+            }
+            catch {
+                InnerList.RemoveAt(index);
+                throw;
+            }
+        }
+
+        public IEnumerator GetEnumerator() {
+            return InnerList.GetEnumerator();
+        }
+
+        protected virtual void OnSet(int index, Object oldValue, Object newValue) { 
+        }
+
+        protected virtual void OnInsert(int index, Object value) { 
+        }
+
+        protected virtual void OnClear() { 
+        }
+
+        protected virtual void OnRemove(int index, Object value) { 
+        }
+
+        protected virtual void OnValidate(Object value) { 
+            if (value == null) throw new ArgumentNullException("value");
+            Contract.EndContractBlock();
+        }
+
+        protected virtual void OnSetComplete(int index, Object oldValue, Object newValue) { 
+        }
+
+        protected virtual void OnInsertComplete(int index, Object value) { 
+        }
+
+        protected virtual void OnClearComplete() { 
+        }
+
+        protected virtual void OnRemoveComplete(int index, Object value) { 
+        }
+    
+    }
+
+}
diff --git a/src/mscorlib/src/System/Collections/Comparer.cs b/src/mscorlib/src/System/Collections/Comparer.cs
new file mode 100644
index 0000000000..11e26252a8
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Comparer.cs
@@ -0,0 +1,98 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** 
+** 
+**
+**
+** Purpose: Default IComparer implementation.
+**
+** 
+===========================================================*/
+namespace System.Collections {
+    
+    using System;
+    using System.Globalization;
+    using System.Runtime.Serialization;
+    using System.Security.Permissions;
+    using System.Diagnostics.Contracts;
+
+    [Serializable]
+    [System.Runtime.InteropServices.ComVisible(true)]
+    public sealed class Comparer : IComparer , ISerializable
+    {
+        private CompareInfo m_compareInfo;   
+        public static readonly Comparer Default = new Comparer(CultureInfo.CurrentCulture);
+        public static readonly Comparer DefaultInvariant = new Comparer(CultureInfo.InvariantCulture);
+        
+        private const String CompareInfoName = "CompareInfo";
+
+        private Comparer() {
+            m_compareInfo = null;
+        }
+
+        public Comparer(CultureInfo culture) {
+            if (culture==null) {
+                throw new ArgumentNullException("culture");
+            }
+            Contract.EndContractBlock();
+            m_compareInfo = culture.CompareInfo;
+        }
+        
+        private Comparer(SerializationInfo info, StreamingContext context) {            
+            m_compareInfo = null;
+            SerializationInfoEnumerator enumerator = info.GetEnumerator();                        
+            while( enumerator.MoveNext()) {
+                switch( enumerator.Name) {
+                    case CompareInfoName:
+                        m_compareInfo = (CompareInfo) info.GetValue(CompareInfoName, typeof(CompareInfo));
+                        break;
+                }
+            }
+        }
+    
+        // Compares two Objects by calling CompareTo.  If a == 
+        // b,0 is returned.  If a implements 
+        // IComparable, a.CompareTo(b) is returned.  If a 
+        // doesn't implement IComparable and b does, 
+        // -(b.CompareTo(a)) is returned, otherwise an 
+        // exception is thrown.
+        // 
+        public int Compare(Object a, Object b) {
+            if (a == b) return 0;
+            if (a == null) return -1;
+            if (b == null) return 1;
+            if (m_compareInfo != null) {
+                String sa = a as String;
+                String sb = b as String;
+                if (sa != null && sb != null)
+                    return m_compareInfo.Compare(sa, sb);
+            }
+
+            IComparable ia = a as IComparable;
+            if (ia != null)
+                return ia.CompareTo(b);
+
+            IComparable ib = b as IComparable;
+            if (ib != null)
+                return -ib.CompareTo(a);
+
+            throw new ArgumentException(Environment.GetResourceString("Argument_ImplementIComparable"));
+        }
+
+        [System.Security.SecurityCritical]  // auto-generated_required
+        public void GetObjectData(SerializationInfo info, StreamingContext context) {
+            if (info==null) {
+                throw new ArgumentNullException("info");
+            }
+            Contract.EndContractBlock();
+
+            if( m_compareInfo != null) {
+                info.AddValue(CompareInfoName, m_compareInfo);
+            }
+        }        
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/CompatibleComparer.cs b/src/mscorlib/src/System/Collections/CompatibleComparer.cs
new file mode 100644
index 0000000000..85e6c3f0f3
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/CompatibleComparer.cs
@@ -0,0 +1,67 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// 
+
+using System.Diagnostics.Contracts;
+
+namespace System.Collections {
+
+    [Serializable]
+    internal class CompatibleComparer: IEqualityComparer {
+        IComparer _comparer;
+#pragma warning disable 618
+        IHashCodeProvider _hcp;
+
+        internal CompatibleComparer(IComparer comparer, IHashCodeProvider hashCodeProvider) {
+            _comparer = comparer;
+            _hcp = hashCodeProvider;
+        }
+#pragma warning restore 618
+
+        public int Compare(Object a, Object b) {
+            if (a == b) return 0;
+            if (a == null) return -1;
+            if (b == null) return 1;
+            if (_comparer != null)
+                return _comparer.Compare(a,b);
+            IComparable ia = a as IComparable;
+            if (ia != null)
+                return ia.CompareTo(b);
+
+            throw new ArgumentException(Environment.GetResourceString("Argument_ImplementIComparable"));
+        }
+
+        public new bool Equals(Object a, Object b) {
+            return Compare(a, b) == 0;                
+        }
+
+        public int GetHashCode(Object obj) {
+            if( obj == null) {
+                throw new ArgumentNullException("obj");
+            }
+            Contract.EndContractBlock();
+
+            if (_hcp != null)
+                return _hcp.GetHashCode(obj);
+            return obj.GetHashCode();
+        }
+
+        // These are helpers for the Hashtable to query the IKeyComparer infrastructure.
+        internal IComparer Comparer {
+            get {
+                return _comparer;
+            }
+        }
+
+        // These are helpers for the Hashtable to query the IKeyComparer infrastructure.
+#pragma warning disable 618
+        internal IHashCodeProvider HashCodeProvider {
+            get {
+                return _hcp;
+            }
+        }
+#pragma warning restore 618
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs
new file mode 100644
index 0000000000..d805dc8be7
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentDictionary.cs
@@ -0,0 +1,2095 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+//
+/*============================================================
+**
+**
+**
+** Purpose: A scalable dictionary for concurrent access
+**
+**
+===========================================================*/
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
+using System.Diagnostics.Contracts;
+using System.Runtime.InteropServices;
+using System.Runtime.Serialization;
+using System.Text;
+using System.Threading;
+using System.Security;
+using System.Security.Permissions;
+
+namespace System.Collections.Concurrent
+{
+
+    /// <summary>
+    /// Represents a thread-safe collection of keys and values.
+    /// </summary>
+    /// <typeparam name="TKey">The type of the keys in the dictionary.</typeparam>
+    /// <typeparam name="TValue">The type of the values in the dictionary.</typeparam>
+    /// <remarks>
+    /// All public and protected members of <see cref="ConcurrentDictionary{TKey,TValue}"/> are thread-safe and may be used
+    /// concurrently from multiple threads.
+    /// </remarks>
+#if !FEATURE_CORECLR
+    [Serializable]
+#endif
+    [ComVisible(false)]
+    [DebuggerTypeProxy(typeof(Mscorlib_DictionaryDebugView<,>))]
+    [DebuggerDisplay("Count = {Count}")]
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+    public class ConcurrentDictionary<TKey, TValue> : IDictionary<TKey, TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue>
+    {
+        /// <summary>
+        /// Tables that hold the internal state of the ConcurrentDictionary
+        ///
+        /// Wrapping the three tables in a single object allows us to atomically
+        /// replace all tables at once.
+        /// </summary>
+        private class Tables
+        {
+            internal readonly Node[] m_buckets; // A singly-linked list for each bucket.
+            internal readonly object[] m_locks; // A set of locks, each guarding a section of the table.
+            internal volatile int[] m_countPerLock; // The number of elements guarded by each lock.
+            internal readonly IEqualityComparer<TKey> m_comparer; // Key equality comparer
+
+            internal Tables(Node[] buckets, object[] locks, int[] countPerLock, IEqualityComparer<TKey> comparer)
+            {
+                m_buckets = buckets;
+                m_locks = locks;
+                m_countPerLock = countPerLock;
+                m_comparer = comparer;
+            }
+        }
+#if !FEATURE_CORECLR
+        [NonSerialized]
+#endif
+        private volatile Tables m_tables; // Internal tables of the dictionary       
+        // NOTE: this is only used for compat reasons to serialize the comparer.
+        // This should not be accessed from anywhere else outside of the serialization methods.
+        internal IEqualityComparer<TKey> m_comparer; 
+#if !FEATURE_CORECLR
+        [NonSerialized]
+#endif
+        private readonly bool m_growLockArray; // Whether to dynamically increase the size of the striped lock
+
+        // How many times we resized becaused of collisions. 
+        // This is used to make sure we don't resize the dictionary because of multi-threaded Add() calls
+        // that generate collisions. Whenever a GrowTable() should be the only place that changes this
+#if !FEATURE_CORECLR
+        // The field should be have been marked as NonSerialized but because we shipped it without that attribute in 4.5.1.
+        // we can't add it back without breaking compat. To maximize compat we are going to keep the OptionalField attribute 
+        // This will prevent cases where the field was not serialized.
+        [OptionalField]
+#endif
+        private int m_keyRehashCount;
+
+#if !FEATURE_CORECLR
+        [NonSerialized]
+#endif
+        private int m_budget; // The maximum number of elements per lock before a resize operation is triggered
+
+#if !FEATURE_CORECLR // These fields are not used in CoreCLR
+        private KeyValuePair<TKey, TValue>[] m_serializationArray; // Used for custom serialization
+
+        private int m_serializationConcurrencyLevel; // used to save the concurrency level in serialization
+
+        private int m_serializationCapacity; // used to save the capacity in serialization
+#endif
+        // The default concurrency level is DEFAULT_CONCURRENCY_MULTIPLIER * #CPUs. The higher the
+        // DEFAULT_CONCURRENCY_MULTIPLIER, the more concurrent writes can take place without interference
+        // and blocking, but also the more expensive operations that require all locks become (e.g. table
+        // resizing, ToArray, Count, etc). According to brief benchmarks that we ran, 4 seems like a good
+        // compromise.
+        private const int DEFAULT_CONCURRENCY_MULTIPLIER = 4;
+
+        // The default capacity, i.e. the initial # of buckets. When choosing this value, we are making
+        // a trade-off between the size of a very small dictionary, and the number of resizes when
+        // constructing a large dictionary. Also, the capacity should not be divisible by a small prime.
+        private const int DEFAULT_CAPACITY = 31;
+
+        // The maximum size of the striped lock that will not be exceeded when locks are automatically
+        // added as the dictionary grows. However, the user is allowed to exceed this limit by passing
+        // a concurrency level larger than MAX_LOCK_NUMBER into the constructor.
+        private const int MAX_LOCK_NUMBER = 1024;
+
+        // Whether TValue is a type that can be written atomically (i.e., with no danger of torn reads)
+        private static readonly bool s_isValueWriteAtomic = IsValueWriteAtomic();
+
+
+        /// <summary>
+        /// Determines whether type TValue can be written atomically
+        /// </summary>
+        private static bool IsValueWriteAtomic()
+        {
+            Type valueType = typeof(TValue);
+
+            //
+            // Section 12.6.6 of ECMA CLI explains which types can be read and written atomically without
+            // the risk of tearing.
+            //
+            // See http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-335.pdf
+            //
+            bool isAtomic =
+                (valueType.IsClass)
+                || valueType == typeof(Boolean)
+                || valueType == typeof(Char)
+                || valueType == typeof(Byte)
+                || valueType == typeof(SByte)
+                || valueType == typeof(Int16)
+                || valueType == typeof(UInt16)
+                || valueType == typeof(Int32)
+                || valueType == typeof(UInt32)
+                || valueType == typeof(Single);
+
+            if (!isAtomic && IntPtr.Size == 8)
+            {
+                isAtomic |= valueType == typeof(Double) || valueType == typeof(Int64);
+            }
+
+            return isAtomic;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that is empty, has the default concurrency level, has the default initial capacity, and
+        /// uses the default comparer for the key type.
+        /// </summary>
+        public ConcurrentDictionary() : this(DefaultConcurrencyLevel, DEFAULT_CAPACITY, true, EqualityComparer<TKey>.Default) { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that is empty, has the specified concurrency level and capacity, and uses the default
+        /// comparer for the key type.
+        /// </summary>
+        /// <param name="concurrencyLevel">The estimated number of threads that will update the
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/> concurrently.</param>
+        /// <param name="capacity">The initial number of elements that the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// can contain.</param>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"><paramref name="concurrencyLevel"/> is
+        /// less than 1.</exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> <paramref name="capacity"/> is less than
+        /// 0.</exception>
+        public ConcurrentDictionary(int concurrencyLevel, int capacity) : this(concurrencyLevel, capacity, false, EqualityComparer<TKey>.Default) { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that contains elements copied from the specified <see
+        /// cref="T:System.Collections.IEnumerable{KeyValuePair{TKey,TValue}}"/>, has the default concurrency
+        /// level, has the default initial capacity, and uses the default comparer for the key type.
+        /// </summary>
+        /// <param name="collection">The <see
+        /// cref="T:System.Collections.IEnumerable{KeyValuePair{TKey,TValue}}"/> whose elements are copied to
+        /// the new
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/>.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="collection"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentException"><paramref name="collection"/> contains one or more
+        /// duplicate keys.</exception>
+        public ConcurrentDictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection) : this(collection, EqualityComparer<TKey>.Default) { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that is empty, has the specified concurrency level and capacity, and uses the specified
+        /// <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>.
+        /// </summary>
+        /// <param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>
+        /// implementation to use when comparing keys.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="comparer"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        public ConcurrentDictionary(IEqualityComparer<TKey> comparer) : this(DefaultConcurrencyLevel, DEFAULT_CAPACITY, true, comparer) { }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that contains elements copied from the specified <see
+        /// cref="T:System.Collections.IEnumerable"/>, has the default concurrency level, has the default
+        /// initial capacity, and uses the specified
+        /// <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>.
+        /// </summary>
+        /// <param name="collection">The <see
+        /// cref="T:System.Collections.IEnumerable{KeyValuePair{TKey,TValue}}"/> whose elements are copied to
+        /// the new
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/>.</param>
+        /// <param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>
+        /// implementation to use when comparing keys.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="collection"/> is a null reference
+        /// (Nothing in Visual Basic). -or-
+        /// <paramref name="comparer"/> is a null reference (Nothing in Visual Basic).
+        /// </exception>
+        public ConcurrentDictionary(IEnumerable<KeyValuePair<TKey, TValue>> collection, IEqualityComparer<TKey> comparer)
+            : this(comparer)
+        {
+            if (collection == null) throw new ArgumentNullException("collection");
+
+            InitializeFromCollection(collection);
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentDictionary{TKey,TValue}"/> 
+        /// class that contains elements copied from the specified <see cref="T:System.Collections.IEnumerable"/>, 
+        /// has the specified concurrency level, has the specified initial capacity, and uses the specified 
+        /// <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>.
+        /// </summary>
+        /// <param name="concurrencyLevel">The estimated number of threads that will update the 
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/> concurrently.</param>
+        /// <param name="collection">The <see cref="T:System.Collections.IEnumerable{KeyValuePair{TKey,TValue}}"/> whose elements are copied to the new 
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/>.</param>
+        /// <param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/> implementation to use 
+        /// when comparing keys.</param>
+        /// <exception cref="T:System.ArgumentNullException">
+        /// <paramref name="collection"/> is a null reference (Nothing in Visual Basic).
+        /// -or-
+        /// <paramref name="comparer"/> is a null reference (Nothing in Visual Basic).
+        /// </exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException">
+        /// <paramref name="concurrencyLevel"/> is less than 1.
+        /// </exception>
+        /// <exception cref="T:System.ArgumentException"><paramref name="collection"/> contains one or more duplicate keys.</exception>
+        public ConcurrentDictionary(
+            int concurrencyLevel, IEnumerable<KeyValuePair<TKey, TValue>> collection, IEqualityComparer<TKey> comparer)
+            : this(concurrencyLevel, DEFAULT_CAPACITY, false, comparer)
+        {
+            if (collection == null) throw new ArgumentNullException("collection");
+            if (comparer == null) throw new ArgumentNullException("comparer");
+
+            InitializeFromCollection(collection);
+        }
+
+        private void InitializeFromCollection(IEnumerable<KeyValuePair<TKey, TValue>> collection)
+        {
+            TValue dummy;
+            foreach (KeyValuePair<TKey, TValue> pair in collection)
+            {
+                if (pair.Key == null) throw new ArgumentNullException("key");
+
+                if (!TryAddInternal(pair.Key, pair.Value, false, false, out dummy))
+                {
+                    throw new ArgumentException(GetResource("ConcurrentDictionary_SourceContainsDuplicateKeys"));
+                }
+            }
+
+            if (m_budget == 0)
+            {
+                m_budget = m_tables.m_buckets.Length / m_tables.m_locks.Length;
+            }
+
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// class that is empty, has the specified concurrency level, has the specified initial capacity, and
+        /// uses the specified <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>.
+        /// </summary>
+        /// <param name="concurrencyLevel">The estimated number of threads that will update the
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/> concurrently.</param>
+        /// <param name="capacity">The initial number of elements that the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// can contain.</param>
+        /// <param name="comparer">The <see cref="T:System.Collections.Generic.IEqualityComparer{TKey}"/>
+        /// implementation to use when comparing keys.</param>
+        /// <exception cref="T:System.ArgumentOutOfRangeException">
+        /// <paramref name="concurrencyLevel"/> is less than 1. -or-
+        /// <paramref name="capacity"/> is less than 0.
+        /// </exception>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="comparer"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        public ConcurrentDictionary(int concurrencyLevel, int capacity, IEqualityComparer<TKey> comparer)
+            : this(concurrencyLevel, capacity, false, comparer)
+        {
+        }
+
+        internal ConcurrentDictionary(int concurrencyLevel, int capacity, bool growLockArray, IEqualityComparer<TKey> comparer)
+        {
+            if (concurrencyLevel < 1)
+            {
+                throw new ArgumentOutOfRangeException("concurrencyLevel", GetResource("ConcurrentDictionary_ConcurrencyLevelMustBePositive"));
+            }
+            if (capacity < 0)
+            {
+                throw new ArgumentOutOfRangeException("capacity", GetResource("ConcurrentDictionary_CapacityMustNotBeNegative"));
+            }
+            if (comparer == null) throw new ArgumentNullException("comparer");
+
+            // The capacity should be at least as large as the concurrency level. Otherwise, we would have locks that don't guard
+            // any buckets.
+            if (capacity < concurrencyLevel)
+            {
+                capacity = concurrencyLevel;
+            }
+
+            object[] locks = new object[concurrencyLevel];
+            for (int i = 0; i < locks.Length; i++)
+            {
+                locks[i] = new object();
+            }
+
+            int[] countPerLock = new int[locks.Length];
+            Node[] buckets = new Node[capacity];
+            m_tables = new Tables(buckets, locks, countPerLock, comparer);
+
+            m_growLockArray = growLockArray;
+            m_budget = buckets.Length / locks.Length;
+        }
+
+
+        /// <summary>
+        /// Attempts to add the specified key and value to the <see cref="ConcurrentDictionary{TKey,
+        /// TValue}"/>.
+        /// </summary>
+        /// <param name="key">The key of the element to add.</param>
+        /// <param name="value">The value of the element to add. The value can be a null reference (Nothing
+        /// in Visual Basic) for reference types.</param>
+        /// <returns>true if the key/value pair was added to the <see cref="ConcurrentDictionary{TKey,
+        /// TValue}"/>
+        /// successfully; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The <see cref="ConcurrentDictionary{TKey, TValue}"/>
+        /// contains too many elements.</exception>
+        public bool TryAdd(TKey key, TValue value)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+            TValue dummy;
+            return TryAddInternal(key, value, false, true, out dummy);
+        }
+
+        /// <summary>
+        /// Determines whether the <see cref="ConcurrentDictionary{TKey, TValue}"/> contains the specified
+        /// key.
+        /// </summary>
+        /// <param name="key">The key to locate in the <see cref="ConcurrentDictionary{TKey,
+        /// TValue}"/>.</param>
+        /// <returns>true if the <see cref="ConcurrentDictionary{TKey, TValue}"/> contains an element with
+        /// the specified key; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        public bool ContainsKey(TKey key)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            TValue throwAwayValue;
+            return TryGetValue(key, out throwAwayValue);
+        }
+
+        /// <summary>
+        /// Attempts to remove and return the the value with the specified key from the
+        /// <see cref="ConcurrentDictionary{TKey, TValue}"/>.
+        /// </summary>
+        /// <param name="key">The key of the element to remove and return.</param>
+        /// <param name="value">When this method returns, <paramref name="value"/> contains the object removed from the
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/> or the default value of <typeparamref
+        /// name="TValue"/>
+        /// if the operation failed.</param>
+        /// <returns>true if an object was removed successfully; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        public bool TryRemove(TKey key, out TValue value)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            return TryRemoveInternal(key, out value, false, default(TValue));
+        }
+
+        /// <summary>
+        /// Removes the specified key from the dictionary if it exists and returns its associated value.
+        /// If matchValue flag is set, the key will be removed only if is associated with a particular
+        /// value.
+        /// </summary>
+        /// <param name="key">The key to search for and remove if it exists.</param>
+        /// <param name="value">The variable into which the removed value, if found, is stored.</param>
+        /// <param name="matchValue">Whether removal of the key is conditional on its value.</param>
+        /// <param name="oldValue">The conditional value to compare against if <paramref name="matchValue"/> is true</param>
+        /// <returns></returns>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
+        private bool TryRemoveInternal(TKey key, out TValue value, bool matchValue, TValue oldValue)
+        {
+            while (true)
+            {
+                Tables tables = m_tables;
+
+                IEqualityComparer<TKey> comparer = tables.m_comparer;
+
+                int bucketNo, lockNo;
+                GetBucketAndLockNo(comparer.GetHashCode(key), out bucketNo, out lockNo, tables.m_buckets.Length, tables.m_locks.Length);
+
+                lock (tables.m_locks[lockNo])
+                {
+                    // If the table just got resized, we may not be holding the right lock, and must retry.
+                    // This should be a rare occurence.
+                    if (tables != m_tables)
+                    {
+                        continue;
+                    }
+
+                    Node prev = null;
+                    for (Node curr = tables.m_buckets[bucketNo]; curr != null; curr = curr.m_next)
+                    {
+                        Assert((prev == null && curr == tables.m_buckets[bucketNo]) || prev.m_next == curr);
+
+                        if (comparer.Equals(curr.m_key, key))
+                        {
+                            if (matchValue)
+                            {
+                                bool valuesMatch = EqualityComparer<TValue>.Default.Equals(oldValue, curr.m_value);
+                                if (!valuesMatch)
+                                {
+                                    value = default(TValue);
+                                    return false;
+                                }
+                            }
+
+                            if (prev == null)
+                            {
+                                Volatile.Write<Node>(ref tables.m_buckets[bucketNo], curr.m_next);
+                            }
+                            else
+                            {
+                                prev.m_next = curr.m_next;
+                            }
+
+                            value = curr.m_value;
+                            tables.m_countPerLock[lockNo]--;
+                            return true;
+                        }
+                        prev = curr;
+                    }
+                }
+
+                value = default(TValue);
+                return false;
+            }
+        }
+
+        /// <summary>
+        /// Attempts to get the value associated with the specified key from the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <param name="key">The key of the value to get.</param>
+        /// <param name="value">When this method returns, <paramref name="value"/> contains the object from
+        /// the
+        /// <see cref="ConcurrentDictionary{TKey,TValue}"/> with the specified key or the default value of
+        /// <typeparamref name="TValue"/>, if the operation failed.</param>
+        /// <returns>true if the key was found in the <see cref="ConcurrentDictionary{TKey,TValue}"/>;
+        /// otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
+        public bool TryGetValue(TKey key, out TValue value)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            int bucketNo, lockNoUnused;
+
+            // We must capture the m_buckets field in a local variable. It is set to a new table on each table resize.
+            Tables tables = m_tables;
+            IEqualityComparer<TKey> comparer = tables.m_comparer;
+            GetBucketAndLockNo(comparer.GetHashCode(key), out bucketNo, out lockNoUnused, tables.m_buckets.Length, tables.m_locks.Length);
+
+            // We can get away w/out a lock here.
+            // The Volatile.Read ensures that the load of the fields of 'n' doesn't move before the load from buckets[i].
+            Node n = Volatile.Read<Node>(ref tables.m_buckets[bucketNo]);
+
+            while (n != null)
+            {
+                if (comparer.Equals(n.m_key, key))
+                {
+                    value = n.m_value;
+                    return true;
+                }
+                n = n.m_next;
+            }
+
+            value = default(TValue);
+            return false;
+        }
+
+        /// <summary>
+        /// Compares the existing value for the specified key with a specified value, and if they're equal,
+        /// updates the key with a third value.
+        /// </summary>
+        /// <param name="key">The key whose value is compared with <paramref name="comparisonValue"/> and
+        /// possibly replaced.</param>
+        /// <param name="newValue">The value that replaces the value of the element with <paramref
+        /// name="key"/> if the comparison results in equality.</param>
+        /// <param name="comparisonValue">The value that is compared to the value of the element with
+        /// <paramref name="key"/>.</param>
+        /// <returns>true if the value with <paramref name="key"/> was equal to <paramref
+        /// name="comparisonValue"/> and replaced with <paramref name="newValue"/>; otherwise,
+        /// false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null
+        /// reference.</exception>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
+        public bool TryUpdate(TKey key, TValue newValue, TValue comparisonValue)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            IEqualityComparer<TValue> valueComparer = EqualityComparer<TValue>.Default;
+
+            while (true)
+            {
+                int bucketNo;
+                int lockNo;
+                int hashcode;
+
+                Tables tables = m_tables;
+                IEqualityComparer<TKey> comparer = tables.m_comparer;
+
+                hashcode = comparer.GetHashCode(key);
+                GetBucketAndLockNo(hashcode, out bucketNo, out lockNo, tables.m_buckets.Length, tables.m_locks.Length);
+
+                lock (tables.m_locks[lockNo])
+                {
+                    // If the table just got resized, we may not be holding the right lock, and must retry.
+                    // This should be a rare occurence.
+                    if (tables != m_tables)
+                    {
+                        continue;
+                    }
+
+                    // Try to find this key in the bucket
+                    Node prev = null;
+                    for (Node node = tables.m_buckets[bucketNo]; node != null; node = node.m_next)
+                    {
+                        Assert((prev == null && node == tables.m_buckets[bucketNo]) || prev.m_next == node);
+                        if (comparer.Equals(node.m_key, key))
+                        {
+                            if (valueComparer.Equals(node.m_value, comparisonValue))
+                            {
+                                if (s_isValueWriteAtomic)
+                                {
+                                    node.m_value = newValue;
+                                }
+                                else
+                                {
+                                    Node newNode = new Node(node.m_key, newValue, hashcode, node.m_next);
+
+                                    if (prev == null)
+                                    {
+                                        tables.m_buckets[bucketNo] = newNode;
+                                    }
+                                    else
+                                    {
+                                        prev.m_next = newNode;
+                                    }
+                                }
+
+                                return true;
+                            }
+
+                            return false;
+                        }
+
+                        prev = node;
+                    }
+
+                    //didn't find the key
+                    return false;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Removes all keys and values from the <see cref="ConcurrentDictionary{TKey,TValue}"/>.
+        /// </summary>
+        public void Clear()
+        {
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+
+                Tables newTables = new Tables(new Node[DEFAULT_CAPACITY], m_tables.m_locks, new int[m_tables.m_countPerLock.Length], m_tables.m_comparer);
+                m_tables = newTables;
+                m_budget = Math.Max(1, newTables.m_buckets.Length / newTables.m_locks.Length);
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Copies the elements of the <see cref="T:System.Collections.Generic.ICollection"/> to an array of
+        /// type <see cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/>, starting at the
+        /// specified array index.
+        /// </summary>
+        /// <param name="array">The one-dimensional array of type <see
+        /// cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/>
+        /// that is the destination of the <see
+        /// cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/> elements copied from the <see
+        /// cref="T:System.Collections.ICollection"/>. The array must have zero-based indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="array"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// 0.</exception>
+        /// <exception cref="T:System.ArgumentException"><paramref name="index"/> is equal to or greater than
+        /// the length of the <paramref name="array"/>. -or- The number of elements in the source <see
+        /// cref="T:System.Collections.ICollection"/>
+        /// is greater than the available space from <paramref name="index"/> to the end of the destination
+        /// <paramref name="array"/>.</exception>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+        void ICollection<KeyValuePair<TKey, TValue>>.CopyTo(KeyValuePair<TKey, TValue>[] array, int index)
+        {
+            if (array == null) throw new ArgumentNullException("array");
+            if (index < 0) throw new ArgumentOutOfRangeException("index", GetResource("ConcurrentDictionary_IndexIsNegative"));
+
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+
+                int count = 0;
+
+                for (int i = 0; i < m_tables.m_locks.Length && count >= 0; i++)
+                {
+                    count += m_tables.m_countPerLock[i];
+                }
+
+                if (array.Length - count < index || count < 0) //"count" itself or "count + index" can overflow
+                {
+                    throw new ArgumentException(GetResource("ConcurrentDictionary_ArrayNotLargeEnough"));
+                }
+
+                CopyToPairs(array, index);
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Copies the key and value pairs stored in the <see cref="ConcurrentDictionary{TKey,TValue}"/> to a
+        /// new array.
+        /// </summary>
+        /// <returns>A new array containing a snapshot of key and value pairs copied from the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.</returns>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+        public KeyValuePair<TKey, TValue>[] ToArray()
+        {
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+                int count = 0;
+                checked
+                {
+                    for (int i = 0; i < m_tables.m_locks.Length; i++)
+                    {
+                        count += m_tables.m_countPerLock[i];
+                    }
+                }
+
+                KeyValuePair<TKey, TValue>[] array = new KeyValuePair<TKey, TValue>[count];
+
+                CopyToPairs(array, 0);
+                return array;
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Copy dictionary contents to an array - shared implementation between ToArray and CopyTo.
+        /// 
+        /// Important: the caller must hold all locks in m_locks before calling CopyToPairs.
+        /// </summary>
+        private void CopyToPairs(KeyValuePair<TKey, TValue>[] array, int index)
+        {
+            Node[] buckets = m_tables.m_buckets;
+            for (int i = 0; i < buckets.Length; i++)
+            {
+                for (Node current = buckets[i]; current != null; current = current.m_next)
+                {
+                    array[index] = new KeyValuePair<TKey, TValue>(current.m_key, current.m_value);
+                    index++; //this should never flow, CopyToPairs is only called when there's no overflow risk
+                }
+            }
+        }
+
+        /// <summary>
+        /// Copy dictionary contents to an array - shared implementation between ToArray and CopyTo.
+        /// 
+        /// Important: the caller must hold all locks in m_locks before calling CopyToEntries.
+        /// </summary>
+        private void CopyToEntries(DictionaryEntry[] array, int index)
+        {
+            Node[] buckets = m_tables.m_buckets;
+            for (int i = 0; i < buckets.Length; i++)
+            {
+                for (Node current = buckets[i]; current != null; current = current.m_next)
+                {
+                    array[index] = new DictionaryEntry(current.m_key, current.m_value);
+                    index++;  //this should never flow, CopyToEntries is only called when there's no overflow risk
+                }
+            }
+        }
+
+        /// <summary>
+        /// Copy dictionary contents to an array - shared implementation between ToArray and CopyTo.
+        /// 
+        /// Important: the caller must hold all locks in m_locks before calling CopyToObjects.
+        /// </summary>
+        private void CopyToObjects(object[] array, int index)
+        {
+            Node[] buckets = m_tables.m_buckets;
+            for (int i = 0; i < buckets.Length; i++)
+            {
+                for (Node current = buckets[i]; current != null; current = current.m_next)
+                {
+                    array[index] = new KeyValuePair<TKey, TValue>(current.m_key, current.m_value);
+                    index++; //this should never flow, CopyToObjects is only called when there's no overflow risk
+                }
+            }
+        }
+
+        /// <summary>Returns an enumerator that iterates through the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.</summary>
+        /// <returns>An enumerator for the <see cref="ConcurrentDictionary{TKey,TValue}"/>.</returns>
+        /// <remarks>
+        /// The enumerator returned from the dictionary is safe to use concurrently with
+        /// reads and writes to the dictionary, however it does not represent a moment-in-time snapshot
+        /// of the dictionary.  The contents exposed through the enumerator may contain modifications
+        /// made to the dictionary after <see cref="GetEnumerator"/> was called.
+        /// </remarks>
+        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
+        {
+            Node[] buckets = m_tables.m_buckets;
+
+            for (int i = 0; i < buckets.Length; i++)
+            {
+                // The Volatile.Read ensures that the load of the fields of 'current' doesn't move before the load from buckets[i].
+                Node current = Volatile.Read<Node>(ref buckets[i]);
+
+                while (current != null)
+                {
+                    yield return new KeyValuePair<TKey, TValue>(current.m_key, current.m_value);
+                    current = current.m_next;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Shared internal implementation for inserts and updates.
+        /// If key exists, we always return false; and if updateIfExists == true we force update with value;
+        /// If key doesn't exist, we always add value and return true;
+        /// </summary>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
+        private bool TryAddInternal(TKey key, TValue value, bool updateIfExists, bool acquireLock, out TValue resultingValue)
+        {
+            while (true)
+            {
+                int bucketNo, lockNo;
+                int hashcode;
+
+                Tables tables = m_tables;
+                IEqualityComparer<TKey> comparer = tables.m_comparer;
+                hashcode = comparer.GetHashCode(key);
+                GetBucketAndLockNo(hashcode, out bucketNo, out lockNo, tables.m_buckets.Length, tables.m_locks.Length);
+
+                bool resizeDesired = false;
+                bool lockTaken = false;
+#if FEATURE_RANDOMIZED_STRING_HASHING
+#if !FEATURE_CORECLR                
+                bool resizeDueToCollisions = false;
+#endif // !FEATURE_CORECLR
+#endif
+
+                try
+                {
+                    if (acquireLock)
+                        Monitor.Enter(tables.m_locks[lockNo], ref lockTaken);
+
+                    // If the table just got resized, we may not be holding the right lock, and must retry.
+                    // This should be a rare occurence.
+                    if (tables != m_tables)
+                    {
+                        continue;
+                    }
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+#if !FEATURE_CORECLR
+                    int collisionCount = 0;
+#endif // !FEATURE_CORECLR
+#endif
+
+                    // Try to find this key in the bucket
+                    Node prev = null;
+                    for (Node node = tables.m_buckets[bucketNo]; node != null; node = node.m_next)
+                    {
+                        Assert((prev == null && node == tables.m_buckets[bucketNo]) || prev.m_next == node);
+                        if (comparer.Equals(node.m_key, key))
+                        {
+                            // The key was found in the dictionary. If updates are allowed, update the value for that key.
+                            // We need to create a new node for the update, in order to support TValue types that cannot
+                            // be written atomically, since lock-free reads may be happening concurrently.
+                            if (updateIfExists)
+                            {
+                                if (s_isValueWriteAtomic)
+                                {
+                                    node.m_value = value;
+                                }
+                                else
+                                {
+                                    Node newNode = new Node(node.m_key, value, hashcode, node.m_next);
+                                    if (prev == null)
+                                    {
+                                        tables.m_buckets[bucketNo] = newNode;
+                                    }
+                                    else
+                                    {
+                                        prev.m_next = newNode;
+                                    }
+                                }
+                                resultingValue = value;
+                            }
+                            else
+                            {
+                                resultingValue = node.m_value;
+                            }
+                            return false;
+                        }
+                        prev = node;
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+#if !FEATURE_CORECLR
+                        collisionCount++;
+#endif // !FEATURE_CORECLR
+#endif
+                    }
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+#if !FEATURE_CORECLR
+                    if(collisionCount > HashHelpers.HashCollisionThreshold && HashHelpers.IsWellKnownEqualityComparer(comparer)) 
+                    {
+                        resizeDesired = true;
+                        resizeDueToCollisions = true;
+                    }
+#endif // !FEATURE_CORECLR
+#endif
+
+                    // The key was not found in the bucket. Insert the key-value pair.
+                    Volatile.Write<Node>(ref tables.m_buckets[bucketNo], new Node(key, value, hashcode, tables.m_buckets[bucketNo]));
+                    checked
+                    {
+                        tables.m_countPerLock[lockNo]++;
+                    }
+
+                    //
+                    // If the number of elements guarded by this lock has exceeded the budget, resize the bucket table.
+                    // It is also possible that GrowTable will increase the budget but won't resize the bucket table.
+                    // That happens if the bucket table is found to be poorly utilized due to a bad hash function.
+                    //
+                    if (tables.m_countPerLock[lockNo] > m_budget)
+                    {
+                        resizeDesired = true;
+                    }
+                }
+                finally
+                {
+                    if (lockTaken)
+                        Monitor.Exit(tables.m_locks[lockNo]);
+                }
+
+                //
+                // The fact that we got here means that we just performed an insertion. If necessary, we will grow the table.
+                //
+                // Concurrency notes:
+                // - Notice that we are not holding any locks at when calling GrowTable. This is necessary to prevent deadlocks.
+                // - As a result, it is possible that GrowTable will be called unnecessarily. But, GrowTable will obtain lock 0
+                //   and then verify that the table we passed to it as the argument is still the current table.
+                //
+                if (resizeDesired)
+                {
+#if FEATURE_RANDOMIZED_STRING_HASHING
+#if !FEATURE_CORECLR
+                    if (resizeDueToCollisions)
+                    {
+                        GrowTable(tables, (IEqualityComparer<TKey>)HashHelpers.GetRandomizedEqualityComparer(comparer), true, m_keyRehashCount);
+                    }
+                    else
+#endif // !FEATURE_CORECLR
+                    {
+                        GrowTable(tables, tables.m_comparer, false, m_keyRehashCount);
+                    }
+#else
+                    GrowTable(tables, tables.m_comparer, false, m_keyRehashCount);
+#endif
+                }
+
+                resultingValue = value;
+                return true;
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the value associated with the specified key.
+        /// </summary>
+        /// <param name="key">The key of the value to get or set.</param>
+        /// <value>The value associated with the specified key. If the specified key is not found, a get
+        /// operation throws a
+        /// <see cref="T:Sytem.Collections.Generic.KeyNotFoundException"/>, and a set operation creates a new
+        /// element with the specified key.</value>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.Collections.Generic.KeyNotFoundException">The property is retrieved and
+        /// <paramref name="key"/>
+        /// does not exist in the collection.</exception>
+        public TValue this[TKey key]
+        {
+            get
+            {
+                TValue value;
+                if (!TryGetValue(key, out value))
+                {
+                    throw new KeyNotFoundException();
+                }
+                return value;
+            }
+            set
+            {
+                if (key == null) throw new ArgumentNullException("key");
+                TValue dummy;
+                TryAddInternal(key, value, true, true, out dummy);
+            }
+        }
+
+        /// <summary>
+        /// Gets the number of key/value pairs contained in the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <value>The number of key/value paris contained in the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.</value>
+        /// <remarks>Count has snapshot semantics and represents the number of items in the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>
+        /// at the moment when Count was accessed.</remarks>
+        public int Count
+        {
+            [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+            get
+            {
+                int count = 0;
+
+                int acquiredLocks = 0;
+                try
+                {
+                    // Acquire all locks
+                    AcquireAllLocks(ref acquiredLocks);
+
+                    // Compute the count, we allow overflow
+                    for (int i = 0; i < m_tables.m_countPerLock.Length; i++)
+                    {
+                        count += m_tables.m_countPerLock[i];
+                    }
+
+                }
+                finally
+                {
+                    // Release locks that have been acquired earlier
+                    ReleaseLocks(0, acquiredLocks);
+                }
+
+                return count;
+            }
+        }
+
+        /// <summary>
+        /// Adds a key/value pair to the <see cref="ConcurrentDictionary{TKey,TValue}"/> 
+        /// if the key does not already exist.
+        /// </summary>
+        /// <param name="key">The key of the element to add.</param>
+        /// <param name="valueFactory">The function used to generate a value for the key</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="valueFactory"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <returns>The value for the key.  This will be either the existing value for the key if the
+        /// key is already in the dictionary, or the new value for the key as returned by valueFactory
+        /// if the key was not in the dictionary.</returns>
+        public TValue GetOrAdd(TKey key, Func<TKey, TValue> valueFactory)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+            if (valueFactory == null) throw new ArgumentNullException("valueFactory");
+
+            TValue resultingValue;
+            if (TryGetValue(key, out resultingValue))
+            {
+                return resultingValue;
+            }
+            TryAddInternal(key, valueFactory(key), false, true, out resultingValue);
+            return resultingValue;
+        }
+
+        /// <summary>
+        /// Adds a key/value pair to the <see cref="ConcurrentDictionary{TKey,TValue}"/> 
+        /// if the key does not already exist.
+        /// </summary>
+        /// <param name="key">The key of the element to add.</param>
+        /// <param name="value">the value to be added, if the key does not already exist</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <returns>The value for the key.  This will be either the existing value for the key if the 
+        /// key is already in the dictionary, or the new value if the key was not in the dictionary.</returns>
+        public TValue GetOrAdd(TKey key, TValue value)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            TValue resultingValue;
+            TryAddInternal(key, value, false, true, out resultingValue);
+            return resultingValue;
+        }
+
+        /// <summary>
+        /// Adds a key/value pair to the <see cref="ConcurrentDictionary{TKey,TValue}"/> if the key does not already 
+        /// exist, or updates a key/value pair in the <see cref="ConcurrentDictionary{TKey,TValue}"/> if the key 
+        /// already exists.
+        /// </summary>
+        /// <param name="key">The key to be added or whose value should be updated</param>
+        /// <param name="addValueFactory">The function used to generate a value for an absent key</param>
+        /// <param name="updateValueFactory">The function used to generate a new value for an existing key
+        /// based on the key's existing value</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="addValueFactory"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="updateValueFactory"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <returns>The new value for the key.  This will be either be the result of addValueFactory (if the key was 
+        /// absent) or the result of updateValueFactory (if the key was present).</returns>
+        public TValue AddOrUpdate(TKey key, Func<TKey, TValue> addValueFactory, Func<TKey, TValue, TValue> updateValueFactory)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+            if (addValueFactory == null) throw new ArgumentNullException("addValueFactory");
+            if (updateValueFactory == null) throw new ArgumentNullException("updateValueFactory");
+
+            TValue newValue, resultingValue;
+            while (true)
+            {
+                TValue oldValue;
+                if (TryGetValue(key, out oldValue))
+                //key exists, try to update
+                {
+                    newValue = updateValueFactory(key, oldValue);
+                    if (TryUpdate(key, newValue, oldValue))
+                    {
+                        return newValue;
+                    }
+                }
+                else //try add
+                {
+                    newValue = addValueFactory(key);
+                    if (TryAddInternal(key, newValue, false, true, out resultingValue))
+                    {
+                        return resultingValue;
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Adds a key/value pair to the <see cref="ConcurrentDictionary{TKey,TValue}"/> if the key does not already 
+        /// exist, or updates a key/value pair in the <see cref="ConcurrentDictionary{TKey,TValue}"/> if the key 
+        /// already exists.
+        /// </summary>
+        /// <param name="key">The key to be added or whose value should be updated</param>
+        /// <param name="addValue">The value to be added for an absent key</param>
+        /// <param name="updateValueFactory">The function used to generate a new value for an existing key based on 
+        /// the key's existing value</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="updateValueFactory"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <returns>The new value for the key.  This will be either be the result of addValueFactory (if the key was 
+        /// absent) or the result of updateValueFactory (if the key was present).</returns>
+        public TValue AddOrUpdate(TKey key, TValue addValue, Func<TKey, TValue, TValue> updateValueFactory)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+            if (updateValueFactory == null) throw new ArgumentNullException("updateValueFactory");
+            TValue newValue, resultingValue;
+            while (true)
+            {
+                TValue oldValue;
+                if (TryGetValue(key, out oldValue))
+                //key exists, try to update
+                {
+                    newValue = updateValueFactory(key, oldValue);
+                    if (TryUpdate(key, newValue, oldValue))
+                    {
+                        return newValue;
+                    }
+                }
+                else //try add
+                {
+                    if (TryAddInternal(key, addValue, false, true, out resultingValue))
+                    {
+                        return resultingValue;
+                    }
+                }
+            }
+        }
+
+
+
+        /// <summary>
+        /// Gets a value that indicates whether the <see cref="ConcurrentDictionary{TKey,TValue}"/> is empty.
+        /// </summary>
+        /// <value>true if the <see cref="ConcurrentDictionary{TKey,TValue}"/> is empty; otherwise,
+        /// false.</value>
+        public bool IsEmpty
+        {
+            [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+            get
+            {
+                int acquiredLocks = 0;
+                try
+                {
+                    // Acquire all locks
+                    AcquireAllLocks(ref acquiredLocks);
+
+                    for (int i = 0; i < m_tables.m_countPerLock.Length; i++)
+                    {
+                        if (m_tables.m_countPerLock[i] != 0)
+                        {
+                            return false;
+                        }
+                    }
+                }
+                finally
+                {
+                    // Release locks that have been acquired earlier
+                    ReleaseLocks(0, acquiredLocks);
+                }
+
+                return true;
+            }
+        }
+
+        #region IDictionary<TKey,TValue> members
+
+        /// <summary>
+        /// Adds the specified key and value to the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <param name="key">The object to use as the key of the element to add.</param>
+        /// <param name="value">The object to use as the value of the element to add.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <exception cref="T:System.ArgumentException">
+        /// An element with the same key already exists in the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.</exception>
+        void IDictionary<TKey, TValue>.Add(TKey key, TValue value)
+        {
+            if (!TryAdd(key, value))
+            {
+                throw new ArgumentException(GetResource("ConcurrentDictionary_KeyAlreadyExisted"));
+            }
+        }
+
+        /// <summary>
+        /// Removes the element with the specified key from the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <param name="key">The key of the element to remove.</param>
+        /// <returns>true if the element is successfully remove; otherwise false. This method also returns
+        /// false if
+        /// <paramref name="key"/> was not found in the original <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.
+        /// </returns>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        bool IDictionary<TKey, TValue>.Remove(TKey key)
+        {
+            TValue throwAwayValue;
+            return TryRemove(key, out throwAwayValue);
+        }
+
+        /// <summary>
+        /// Gets a collection containing the keys in the <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.Generic.ICollection{TKey}"/> containing the keys in the
+        /// <see cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.</value>
+        public ICollection<TKey> Keys
+        {
+            get { return GetKeys(); }
+        }
+
+        /// <summary>
+        /// Gets an <see cref="T:System.Collections.Generic.IEnumerable{TKey}"/> containing the keys of
+        /// the <see cref="T:System.Collections.Generic.IReadOnlyDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.Generic.IEnumerable{TKey}"/> containing the keys of
+        /// the <see cref="T:System.Collections.Generic.IReadOnlyDictionary{TKey,TValue}"/>.</value>
+        IEnumerable<TKey> IReadOnlyDictionary<TKey, TValue>.Keys
+        {
+            get { return GetKeys(); }
+        }
+
+        /// <summary>
+        /// Gets a collection containing the values in the <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.Generic.ICollection{TValue}"/> containing the values in
+        /// the
+        /// <see cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.</value>
+        public ICollection<TValue> Values
+        {
+            get { return GetValues(); }
+        }
+
+        /// <summary>
+        /// Gets an <see cref="T:System.Collections.Generic.IEnumerable{TValue}"/> containing the values
+        /// in the <see cref="T:System.Collections.Generic.IReadOnlyDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.Generic.IEnumerable{TValue}"/> containing the
+        /// values in the <see cref="T:System.Collections.Generic.IReadOnlyDictionary{TKey,TValue}"/>.</value>
+        IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values
+        {
+            get { return GetValues(); }
+        }
+        #endregion
+
+        #region ICollection<KeyValuePair<TKey,TValue>> Members
+
+        /// <summary>
+        /// Adds the specified value to the <see cref="T:System.Collections.Generic.ICollection{TValue}"/>
+        /// with the specified key.
+        /// </summary>
+        /// <param name="keyValuePair">The <see cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/>
+        /// structure representing the key and value to add to the <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.</param>
+        /// <exception cref="T:System.ArgumentNullException">The <paramref name="keyValuePair"/> of <paramref
+        /// name="keyValuePair"/> is null.</exception>
+        /// <exception cref="T:System.OverflowException">The <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>
+        /// contains too many elements.</exception>
+        /// <exception cref="T:System.ArgumentException">An element with the same key already exists in the
+        /// <see cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/></exception>
+        void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> keyValuePair)
+        {
+            ((IDictionary<TKey, TValue>)this).Add(keyValuePair.Key, keyValuePair.Value);
+        }
+
+        /// <summary>
+        /// Determines whether the <see cref="T:System.Collections.Generic.ICollection{TKey,TValue}"/>
+        /// contains a specific key and value.
+        /// </summary>
+        /// <param name="keyValuePair">The <see cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/>
+        /// structure to locate in the <see
+        /// cref="T:System.Collections.Generic.ICollection{TValue}"/>.</param>
+        /// <returns>true if the <paramref name="keyValuePair"/> is found in the <see
+        /// cref="T:System.Collections.Generic.ICollection{TKey,TValue}"/>; otherwise, false.</returns>
+        bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue> keyValuePair)
+        {
+            TValue value;
+            if (!TryGetValue(keyValuePair.Key, out value))
+            {
+                return false;
+            }
+            return EqualityComparer<TValue>.Default.Equals(value, keyValuePair.Value);
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the dictionary is read-only.
+        /// </summary>
+        /// <value>true if the <see cref="T:System.Collections.Generic.ICollection{TKey,TValue}"/> is
+        /// read-only; otherwise, false. For <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>, this property always returns
+        /// false.</value>
+        bool ICollection<KeyValuePair<TKey, TValue>>.IsReadOnly
+        {
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Removes a key and value from the dictionary.
+        /// </summary>
+        /// <param name="keyValuePair">The <see
+        /// cref="T:System.Collections.Generic.KeyValuePair{TKey,TValue}"/>
+        /// structure representing the key and value to remove from the <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.</param>
+        /// <returns>true if the key and value represented by <paramref name="keyValuePair"/> is successfully
+        /// found and removed; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException">The Key property of <paramref
+        /// name="keyValuePair"/> is a null reference (Nothing in Visual Basic).</exception>
+        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> keyValuePair)
+        {
+            if (keyValuePair.Key == null) throw new ArgumentNullException(GetResource("ConcurrentDictionary_ItemKeyIsNull"));
+
+            TValue throwAwayValue;
+            return TryRemoveInternal(keyValuePair.Key, out throwAwayValue, true, keyValuePair.Value);
+        }
+
+        #endregion
+
+        #region IEnumerable Members
+
+        /// <summary>Returns an enumerator that iterates through the <see
+        /// cref="ConcurrentDictionary{TKey,TValue}"/>.</summary>
+        /// <returns>An enumerator for the <see cref="ConcurrentDictionary{TKey,TValue}"/>.</returns>
+        /// <remarks>
+        /// The enumerator returned from the dictionary is safe to use concurrently with
+        /// reads and writes to the dictionary, however it does not represent a moment-in-time snapshot
+        /// of the dictionary.  The contents exposed through the enumerator may contain modifications
+        /// made to the dictionary after <see cref="GetEnumerator"/> was called.
+        /// </remarks>
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((ConcurrentDictionary<TKey, TValue>)this).GetEnumerator();
+        }
+
+        #endregion
+
+        #region IDictionary Members
+
+        /// <summary>
+        /// Adds the specified key and value to the dictionary.
+        /// </summary>
+        /// <param name="key">The object to use as the key.</param>
+        /// <param name="value">The object to use as the value.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.OverflowException">The dictionary contains too many
+        /// elements.</exception>
+        /// <exception cref="T:System.ArgumentException">
+        /// <paramref name="key"/> is of a type that is not assignable to the key type <typeparamref
+        /// name="TKey"/> of the <see cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>. -or-
+        /// <paramref name="value"/> is of a type that is not assignable to <typeparamref name="TValue"/>,
+        /// the type of values in the <see cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.
+        /// -or- A value with the same key already exists in the <see
+        /// cref="T:System.Collections.Generic.Dictionary{TKey,TValue}"/>.
+        /// </exception>
+        void IDictionary.Add(object key, object value)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+            if (!(key is TKey)) throw new ArgumentException(GetResource("ConcurrentDictionary_TypeOfKeyIncorrect"));
+
+            TValue typedValue;
+            try
+            {
+                typedValue = (TValue)value;
+            }
+            catch (InvalidCastException)
+            {
+                throw new ArgumentException(GetResource("ConcurrentDictionary_TypeOfValueIncorrect"));
+            }
+
+            ((IDictionary<TKey, TValue>)this).Add((TKey)key, typedValue);
+        }
+
+        /// <summary>
+        /// Gets whether the <see cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> contains an
+        /// element with the specified key.
+        /// </summary>
+        /// <param name="key">The key to locate in the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.</param>
+        /// <returns>true if the <see cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> contains
+        /// an element with the specified key; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentNullException"> <paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        bool IDictionary.Contains(object key)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            return (key is TKey) && ((ConcurrentDictionary<TKey, TValue>)this).ContainsKey((TKey)key);
+        }
+
+        /// <summary>Provides an <see cref="T:System.Collections.Generics.IDictionaryEnumerator"/> for the
+        /// <see cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.</summary>
+        /// <returns>An <see cref="T:System.Collections.Generics.IDictionaryEnumerator"/> for the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.</returns>
+        IDictionaryEnumerator IDictionary.GetEnumerator()
+        {
+            return new DictionaryEnumerator(this);
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> has a fixed size.
+        /// </summary>
+        /// <value>true if the <see cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> has a
+        /// fixed size; otherwise, false. For <see
+        /// cref="T:System.Collections.Generic.ConcurrentDictionary{TKey,TValue}"/>, this property always
+        /// returns false.</value>
+        bool IDictionary.IsFixedSize
+        {
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> is read-only.
+        /// </summary>
+        /// <value>true if the <see cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/> is
+        /// read-only; otherwise, false. For <see
+        /// cref="T:System.Collections.Generic.ConcurrentDictionary{TKey,TValue}"/>, this property always
+        /// returns false.</value>
+        bool IDictionary.IsReadOnly
+        {
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Gets an <see cref="T:System.Collections.ICollection"/> containing the keys of the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.ICollection"/> containing the keys of the <see
+        /// cref="T:System.Collections.Generic.IDictionary{TKey,TValue}"/>.</value>
+        ICollection IDictionary.Keys
+        {
+            get { return GetKeys(); }
+        }
+
+        /// <summary>
+        /// Removes the element with the specified key from the <see
+        /// cref="T:System.Collections.IDictionary"/>.
+        /// </summary>
+        /// <param name="key">The key of the element to remove.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        void IDictionary.Remove(object key)
+        {
+            if (key == null) throw new ArgumentNullException("key");
+
+            TValue throwAwayValue;
+            if (key is TKey)
+            {
+                this.TryRemove((TKey)key, out throwAwayValue);
+            }
+        }
+
+        /// <summary>
+        /// Gets an <see cref="T:System.Collections.ICollection"/> containing the values in the <see
+        /// cref="T:System.Collections.IDictionary"/>.
+        /// </summary>
+        /// <value>An <see cref="T:System.Collections.ICollection"/> containing the values in the <see
+        /// cref="T:System.Collections.IDictionary"/>.</value>
+        ICollection IDictionary.Values
+        {
+            get { return GetValues(); }
+        }
+
+        /// <summary>
+        /// Gets or sets the value associated with the specified key.
+        /// </summary>
+        /// <param name="key">The key of the value to get or set.</param>
+        /// <value>The value associated with the specified key, or a null reference (Nothing in Visual Basic)
+        /// if <paramref name="key"/> is not in the dictionary or <paramref name="key"/> is of a type that is
+        /// not assignable to the key type <typeparamref name="TKey"/> of the <see
+        /// cref="T:System.Collections.Generic.ConcurrentDictionary{TKey,TValue}"/>.</value>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="key"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentException">
+        /// A value is being assigned, and <paramref name="key"/> is of a type that is not assignable to the
+        /// key type <typeparamref name="TKey"/> of the <see
+        /// cref="T:System.Collections.Generic.ConcurrentDictionary{TKey,TValue}"/>. -or- A value is being
+        /// assigned, and <paramref name="key"/> is of a type that is not assignable to the value type
+        /// <typeparamref name="TValue"/> of the <see
+        /// cref="T:System.Collections.Generic.ConcurrentDictionary{TKey,TValue}"/>
+        /// </exception>
+        object IDictionary.this[object key]
+        {
+            get
+            {
+                if (key == null) throw new ArgumentNullException("key");
+
+                TValue value;
+                if (key is TKey && this.TryGetValue((TKey)key, out value))
+                {
+                    return value;
+                }
+
+                return null;
+            }
+            set
+            {
+                if (key == null) throw new ArgumentNullException("key");
+
+                if (!(key is TKey)) throw new ArgumentException(GetResource("ConcurrentDictionary_TypeOfKeyIncorrect"));
+                if (!(value is TValue)) throw new ArgumentException(GetResource("ConcurrentDictionary_TypeOfValueIncorrect"));
+
+                ((ConcurrentDictionary<TKey, TValue>)this)[(TKey)key] = (TValue)value;
+            }
+        }
+
+        #endregion
+
+        #region ICollection Members
+
+        /// <summary>
+        /// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an array, starting
+        /// at the specified array index.
+        /// </summary>
+        /// <param name="array">The one-dimensional array that is the destination of the elements copied from
+        /// the <see cref="T:System.Collections.ICollection"/>. The array must have zero-based
+        /// indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="T:System.ArgumentNullException"><paramref name="array"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// 0.</exception>
+        /// <exception cref="T:System.ArgumentException"><paramref name="index"/> is equal to or greater than
+        /// the length of the <paramref name="array"/>. -or- The number of elements in the source <see
+        /// cref="T:System.Collections.ICollection"/>
+        /// is greater than the available space from <paramref name="index"/> to the end of the destination
+        /// <paramref name="array"/>.</exception>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+        void ICollection.CopyTo(Array array, int index)
+        {
+            if (array == null) throw new ArgumentNullException("array");
+            if (index < 0) throw new ArgumentOutOfRangeException("index", GetResource("ConcurrentDictionary_IndexIsNegative"));
+
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+                Tables tables = m_tables;
+
+                int count = 0;
+
+                for (int i = 0; i < tables.m_locks.Length && count >= 0; i++)
+                {
+                    count += tables.m_countPerLock[i];
+                }
+
+                if (array.Length - count < index || count < 0) //"count" itself or "count + index" can overflow
+                {
+                    throw new ArgumentException(GetResource("ConcurrentDictionary_ArrayNotLargeEnough"));
+                }
+
+                // To be consistent with the behavior of ICollection.CopyTo() in Dictionary<TKey,TValue>,
+                // we recognize three types of target arrays:
+                //    - an array of KeyValuePair<TKey, TValue> structs
+                //    - an array of DictionaryEntry structs
+                //    - an array of objects
+
+                KeyValuePair<TKey, TValue>[] pairs = array as KeyValuePair<TKey, TValue>[];
+                if (pairs != null)
+                {
+                    CopyToPairs(pairs, index);
+                    return;
+                }
+
+                DictionaryEntry[] entries = array as DictionaryEntry[];
+                if (entries != null)
+                {
+                    CopyToEntries(entries, index);
+                    return;
+                }
+
+                object[] objects = array as object[];
+                if (objects != null)
+                {
+                    CopyToObjects(objects, index);
+                    return;
+                }
+
+                throw new ArgumentException(GetResource("ConcurrentDictionary_ArrayIncorrectType"), "array");
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"/> is
+        /// synchronized with the SyncRoot.
+        /// </summary>
+        /// <value>true if access to the <see cref="T:System.Collections.ICollection"/> is synchronized
+        /// (thread safe); otherwise, false. For <see
+        /// cref="T:System.Collections.Concurrent.ConcurrentDictionary{TKey,TValue}"/>, this property always
+        /// returns false.</value>
+        bool ICollection.IsSynchronized
+        {
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Gets an object that can be used to synchronize access to the <see
+        /// cref="T:System.Collections.ICollection"/>. This property is not supported.
+        /// </summary>
+        /// <exception cref="T:System.NotSupportedException">The SyncRoot property is not supported.</exception>
+        object ICollection.SyncRoot
+        {
+            get
+            {
+                throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported"));
+            }
+        }
+
+        #endregion
+
+        /// <summary>
+        /// Replaces the bucket table with a larger one. To prevent multiple threads from resizing the
+        /// table as a result of a race condition, the Tables instance that holds the table of buckets deemed too
+        /// small is passed in as an argument to GrowTable(). GrowTable() obtains a lock, and then checks
+        /// the Tables instance has been replaced in the meantime or not. 
+        /// The <paramref name="rehashCount"/> will be used to ensure that we don't do two subsequent resizes
+        /// because of a collision
+        /// </summary>
+        private void GrowTable(Tables tables, IEqualityComparer<TKey> newComparer, bool regenerateHashKeys, int rehashCount)
+        {
+            int locksAcquired = 0;
+            try
+            {
+                // The thread that first obtains m_locks[0] will be the one doing the resize operation
+                AcquireLocks(0, 1, ref locksAcquired);
+
+                if (regenerateHashKeys && rehashCount == m_keyRehashCount)
+                {
+                    // This method is called with regenerateHashKeys==true when we detected 
+                    // more than HashHelpers.HashCollisionThreshold collisions when adding a new element.
+                    // In that case we are in the process of switching to another (randomized) comparer
+                    // and we have to re-hash all the keys in the table.
+                    // We are only going to do this if we did not just rehash the entire table while waiting for the lock
+                    tables = m_tables;
+                }
+                else
+                {
+                    // If we don't require a regeneration of hash keys we want to make sure we don't do work when
+                    // we don't have to
+                    if (tables != m_tables)
+                    {
+                        // We assume that since the table reference is different, it was already resized (or the budget
+                        // was adjusted). If we ever decide to do table shrinking, or replace the table for other reasons,
+                        // we will have to revisit this logic.
+                        return;
+                    }
+
+                    // Compute the (approx.) total size. Use an Int64 accumulation variable to avoid an overflow.
+                    long approxCount = 0;
+                    for (int i = 0; i < tables.m_countPerLock.Length; i++)
+                    {
+                        approxCount += tables.m_countPerLock[i];
+                    }
+
+                    //
+                    // If the bucket array is too empty, double the budget instead of resizing the table
+                    //
+                    if (approxCount < tables.m_buckets.Length / 4)
+                    {
+                        m_budget = 2 * m_budget;
+                        if (m_budget < 0)
+                        {
+                            m_budget = int.MaxValue;
+                        }
+
+                        return;
+                    }
+                }
+                // Compute the new table size. We find the smallest integer larger than twice the previous table size, and not divisible by
+                // 2,3,5 or 7. We can consider a different table-sizing policy in the future.
+                int newLength = 0;
+                bool maximizeTableSize = false;
+                try
+                {
+                    checked
+                    {
+                        // Double the size of the buckets table and add one, so that we have an odd integer.
+                        newLength = tables.m_buckets.Length * 2 + 1;
+
+                        // Now, we only need to check odd integers, and find the first that is not divisible
+                        // by 3, 5 or 7.
+                        while (newLength % 3 == 0 || newLength % 5 == 0 || newLength % 7 == 0)
+                        {
+                            newLength += 2;
+                        }
+
+                        Assert(newLength % 2 != 0);
+
+                        if (newLength > Array.MaxArrayLength)
+                        {
+                            maximizeTableSize = true;
+                        }
+                    }
+                }
+                catch (OverflowException)
+                {
+                    maximizeTableSize = true;
+                }
+
+                if (maximizeTableSize)
+                {
+                    newLength = Array.MaxArrayLength;
+
+                    // We want to make sure that GrowTable will not be called again, since table is at the maximum size.
+                    // To achieve that, we set the budget to int.MaxValue.
+                    //
+                    // (There is one special case that would allow GrowTable() to be called in the future: 
+                    // calling Clear() on the ConcurrentDictionary will shrink the table and lower the budget.)
+                    m_budget = int.MaxValue;
+                }
+
+                // Now acquire all other locks for the table
+                AcquireLocks(1, tables.m_locks.Length, ref locksAcquired);
+
+                object[] newLocks = tables.m_locks;
+
+                // Add more locks
+                if (m_growLockArray && tables.m_locks.Length < MAX_LOCK_NUMBER)
+                {
+                    newLocks = new object[tables.m_locks.Length * 2];
+                    Array.Copy(tables.m_locks, newLocks, tables.m_locks.Length);
+
+                    for (int i = tables.m_locks.Length; i < newLocks.Length; i++)
+                    {
+                        newLocks[i] = new object();
+                    }
+                }
+
+                Node[] newBuckets = new Node[newLength];
+                int[] newCountPerLock = new int[newLocks.Length];
+
+                // Copy all data into a new table, creating new nodes for all elements
+                for (int i = 0; i < tables.m_buckets.Length; i++)
+                {
+                    Node current = tables.m_buckets[i];
+                    while (current != null)
+                    {
+                        Node next = current.m_next;
+                        int newBucketNo, newLockNo;
+                        int nodeHashCode = current.m_hashcode;
+
+                        if (regenerateHashKeys)
+                        {
+                            // Recompute the hash from the key
+                            nodeHashCode = newComparer.GetHashCode(current.m_key);
+                        }
+
+                        GetBucketAndLockNo(nodeHashCode, out newBucketNo, out newLockNo, newBuckets.Length, newLocks.Length);
+
+                        newBuckets[newBucketNo] = new Node(current.m_key, current.m_value, nodeHashCode, newBuckets[newBucketNo]);
+
+                        checked
+                        {
+                            newCountPerLock[newLockNo]++;
+                        }
+
+                        current = next;
+                    }
+                }
+
+                // If this resize regenerated the hashkeys, increment the count
+                if (regenerateHashKeys)
+                {
+                    // We use unchecked here because we don't want to throw an exception if 
+                    // an overflow happens
+                    unchecked
+                    {
+                        m_keyRehashCount++;
+                    }
+                }
+
+                // Adjust the budget
+                m_budget = Math.Max(1, newBuckets.Length / newLocks.Length);
+
+                // Replace tables with the new versions
+                m_tables = new Tables(newBuckets, newLocks, newCountPerLock, newComparer);
+            }
+            finally
+            {
+                // Release all locks that we took earlier
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Computes the bucket and lock number for a particular key. 
+        /// </summary>
+        private void GetBucketAndLockNo(
+                int hashcode, out int bucketNo, out int lockNo, int bucketCount, int lockCount)
+        {
+            bucketNo = (hashcode & 0x7fffffff) % bucketCount;
+            lockNo = bucketNo % lockCount;
+
+            Assert(bucketNo >= 0 && bucketNo < bucketCount);
+            Assert(lockNo >= 0 && lockNo < lockCount);
+        }
+
+        /// <summary>
+        /// The number of concurrent writes for which to optimize by default.
+        /// </summary>
+        private static int DefaultConcurrencyLevel
+        {
+
+            get { return DEFAULT_CONCURRENCY_MULTIPLIER * PlatformHelper.ProcessorCount; }
+        }
+
+        /// <summary>
+        /// Acquires all locks for this hash table, and increments locksAcquired by the number
+        /// of locks that were successfully acquired. The locks are acquired in an increasing
+        /// order.
+        /// </summary>
+        private void AcquireAllLocks(ref int locksAcquired)
+        {
+#if !FEATURE_CORECLR
+            if (CDSCollectionETWBCLProvider.Log.IsEnabled())
+            {
+                CDSCollectionETWBCLProvider.Log.ConcurrentDictionary_AcquiringAllLocks(m_tables.m_buckets.Length);
+            }
+#endif //!FEATURE_CORECLR
+
+            // First, acquire lock 0
+            AcquireLocks(0, 1, ref locksAcquired);
+
+            // Now that we have lock 0, the m_locks array will not change (i.e., grow),
+            // and so we can safely read m_locks.Length.
+            AcquireLocks(1, m_tables.m_locks.Length, ref locksAcquired);
+            Assert(locksAcquired == m_tables.m_locks.Length);
+        }
+
+        /// <summary>
+        /// Acquires a contiguous range of locks for this hash table, and increments locksAcquired
+        /// by the number of locks that were successfully acquired. The locks are acquired in an
+        /// increasing order.
+        /// </summary>
+        private void AcquireLocks(int fromInclusive, int toExclusive, ref int locksAcquired)
+        {
+            Assert(fromInclusive <= toExclusive);
+            object[] locks = m_tables.m_locks;
+
+            for (int i = fromInclusive; i < toExclusive; i++)
+            {
+                bool lockTaken = false;
+                try
+                {
+                    Monitor.Enter(locks[i], ref lockTaken);
+                }
+                finally
+                {
+                    if (lockTaken)
+                    {
+                        locksAcquired++;
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Releases a contiguous range of locks.
+        /// </summary>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "Reviewed for thread safety")]
+        private void ReleaseLocks(int fromInclusive, int toExclusive)
+        {
+            Assert(fromInclusive <= toExclusive);
+
+            for (int i = fromInclusive; i < toExclusive; i++)
+            {
+                Monitor.Exit(m_tables.m_locks[i]);
+            }
+        }
+
+        /// <summary>
+        /// Gets a collection containing the keys in the dictionary.
+        /// </summary>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+        private ReadOnlyCollection<TKey> GetKeys()
+        {
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+                List<TKey> keys = new List<TKey>();
+
+                for (int i = 0; i < m_tables.m_buckets.Length; i++)
+                {
+                    Node current = m_tables.m_buckets[i];
+                    while (current != null)
+                    {
+                        keys.Add(current.m_key);
+                        current = current.m_next;
+                    }
+                }
+
+                return new ReadOnlyCollection<TKey>(keys);
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// Gets a collection containing the values in the dictionary.
+        /// </summary>
+        [SuppressMessage("Microsoft.Concurrency", "CA8001", Justification = "ConcurrencyCop just doesn't know about these locks")]
+        private ReadOnlyCollection<TValue> GetValues()
+        {
+            int locksAcquired = 0;
+            try
+            {
+                AcquireAllLocks(ref locksAcquired);
+                List<TValue> values = new List<TValue>();
+
+                for (int i = 0; i < m_tables.m_buckets.Length; i++)
+                {
+                    Node current = m_tables.m_buckets[i];
+                    while (current != null)
+                    {
+                        values.Add(current.m_value);
+                        current = current.m_next;
+                    }
+                }
+
+                return new ReadOnlyCollection<TValue>(values);
+            }
+            finally
+            {
+                ReleaseLocks(0, locksAcquired);
+            }
+        }
+
+        /// <summary>
+        /// A helper method for asserts.
+        /// </summary>
+        [Conditional("DEBUG")]
+        private void Assert(bool condition)
+        {
+            Contract.Assert(condition);
+        }
+
+        /// <summary>
+        /// A helper function to obtain the string for a particular resource key.
+        /// </summary>
+        /// <param name="key"></param>
+        /// <returns></returns>
+        private string GetResource(string key)
+        {
+            Assert(key != null);
+
+            return Environment.GetResourceString(key);
+        }
+
+        /// <summary>
+        /// A node in a singly-linked list representing a particular hash table bucket.
+        /// </summary>
+        private class Node
+        {
+            internal TKey m_key;
+            internal TValue m_value;
+            internal volatile Node m_next;
+            internal int m_hashcode;
+
+            internal Node(TKey key, TValue value, int hashcode, Node next)
+            {
+                m_key = key;
+                m_value = value;
+                m_next = next;
+                m_hashcode = hashcode;
+            }
+        }
+
+        /// <summary>
+        /// A private class to represent enumeration over the dictionary that implements the 
+        /// IDictionaryEnumerator interface.
+        /// </summary>
+        private class DictionaryEnumerator : IDictionaryEnumerator
+        {
+            IEnumerator<KeyValuePair<TKey, TValue>> m_enumerator; // Enumerator over the dictionary.
+
+            internal DictionaryEnumerator(ConcurrentDictionary<TKey, TValue> dictionary)
+            {
+                m_enumerator = dictionary.GetEnumerator();
+            }
+
+            public DictionaryEntry Entry
+            {
+                get { return new DictionaryEntry(m_enumerator.Current.Key, m_enumerator.Current.Value); }
+            }
+
+            public object Key
+            {
+                get { return m_enumerator.Current.Key; }
+            }
+
+            public object Value
+            {
+                get { return m_enumerator.Current.Value; }
+            }
+
+            public object Current
+            {
+                get { return this.Entry; }
+            }
+
+            public bool MoveNext()
+            {
+                return m_enumerator.MoveNext();
+            }
+
+            public void Reset()
+            {
+                m_enumerator.Reset();
+            }
+        }
+
+#if !FEATURE_CORECLR
+        /// <summary>
+        /// Get the data array to be serialized
+        /// </summary>
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            Tables tables = m_tables;
+
+            // save the data into the serialization array to be saved
+            m_serializationArray = ToArray();
+            m_serializationConcurrencyLevel = tables.m_locks.Length;
+            m_serializationCapacity = tables.m_buckets.Length;
+            m_comparer = (IEqualityComparer<TKey>)HashHelpers.GetEqualityComparerForSerialization(tables.m_comparer);
+        }
+
+        /// <summary>
+        /// Construct the dictionary from a previously serialized one
+        /// </summary>
+        [OnDeserialized]
+        private void OnDeserialized(StreamingContext context)
+        {
+            KeyValuePair<TKey, TValue>[] array = m_serializationArray;
+
+            var buckets = new Node[m_serializationCapacity];
+            var countPerLock = new int[m_serializationConcurrencyLevel];
+
+            var locks = new object[m_serializationConcurrencyLevel];
+            for (int i = 0; i < locks.Length; i++)
+            {
+                locks[i] = new object();
+            }
+
+            m_tables = new Tables(buckets, locks, countPerLock, m_comparer);
+
+            InitializeFromCollection(array);
+            m_serializationArray = null;
+
+        }
+#endif
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs
new file mode 100644
index 0000000000..9164eadad1
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentQueue.cs
@@ -0,0 +1,960 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+#pragma warning disable 0420
+
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// A lock-free, concurrent queue primitive, and its associated debugger view type.
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.Contracts;
+using System.Runtime.ConstrainedExecution;
+using System.Runtime.InteropServices;
+using System.Runtime.Serialization;
+using System.Security;
+using System.Security.Permissions;
+using System.Threading;
+
+namespace System.Collections.Concurrent
+{
+
+    /// <summary>
+    /// Represents a thread-safe first-in, first-out collection of objects.
+    /// </summary>
+    /// <typeparam name="T">Specifies the type of elements in the queue.</typeparam>
+    /// <remarks>
+    /// All public  and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used
+    /// concurrently from multiple threads.
+    /// </remarks>
+    [ComVisible(false)]
+    [DebuggerDisplay("Count = {Count}")]
+    [DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))]
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+    [Serializable]
+    public class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>
+    {
+        //fields of ConcurrentQueue
+        [NonSerialized]
+        private volatile Segment m_head;
+
+        [NonSerialized]
+        private volatile Segment m_tail;
+
+        private T[] m_serializationArray; // Used for custom serialization.
+
+        private const int SEGMENT_SIZE = 32;
+
+        //number of snapshot takers, GetEnumerator(), ToList() and ToArray() operations take snapshot.
+        [NonSerialized]
+        internal volatile int m_numSnapshotTakers = 0;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class.
+        /// </summary>
+        public ConcurrentQueue()
+        {
+            m_head = m_tail = new Segment(0, this);
+        }
+
+        /// <summary>
+        /// Initializes the contents of the queue from an existing collection.
+        /// </summary>
+        /// <param name="collection">A collection from which to copy elements.</param>
+        private void InitializeFromCollection(IEnumerable<T> collection)
+        {
+            Segment localTail = new Segment(0, this);//use this local variable to avoid the extra volatile read/write. this is safe because it is only called from ctor
+            m_head = localTail; 
+
+            int index = 0;
+            foreach (T element in collection)
+            {
+                Contract.Assert(index >= 0 && index < SEGMENT_SIZE);
+                localTail.UnsafeAdd(element);
+                index++;
+
+                if (index >= SEGMENT_SIZE)
+                {
+                    localTail = localTail.UnsafeGrow();
+                    index = 0;
+                }
+            }
+
+            m_tail = localTail;
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/>
+        /// class that contains elements copied from the specified collection
+        /// </summary>
+        /// <param name="collection">The collection whose elements are copied to the new <see
+        /// cref="ConcurrentQueue{T}"/>.</param>
+        /// <exception cref="T:System.ArgumentNullException">The <paramref name="collection"/> argument is
+        /// null.</exception>
+        public ConcurrentQueue(IEnumerable<T> collection)
+        {
+            if (collection == null)
+            {
+                throw new ArgumentNullException("collection");
+            }
+
+            InitializeFromCollection(collection);
+        }
+
+        /// <summary>
+        /// Get the data array to be serialized
+        /// </summary>
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            // save the data into the serialization array to be saved
+            m_serializationArray = ToArray();
+        }
+
+        /// <summary>
+        /// Construct the queue from a previously seiralized one
+        /// </summary>
+        [OnDeserialized]
+        private void OnDeserialized(StreamingContext context)
+        {
+            Contract.Assert(m_serializationArray != null);
+            InitializeFromCollection(m_serializationArray);
+            m_serializationArray = null;
+        }
+
+        /// <summary>
+        /// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an <see
+        /// cref="T:System.Array"/>, starting at a particular
+        /// <see cref="T:System.Array"/> index.
+        /// </summary>
+        /// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the
+        /// destination of the elements copied from the
+        /// <see cref="T:System.Collections.Concurrent.ConcurrentBag"/>. The <see
+        /// cref="T:System.Array">Array</see> must have zero-based indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
+        /// Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// zero.</exception>
+        /// <exception cref="ArgumentException">
+        /// <paramref name="array"/> is multidimensional. -or-
+        /// <paramref name="array"/> does not have zero-based indexing. -or-
+        /// <paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/>
+        /// -or- The number of elements in the source <see cref="T:System.Collections.ICollection"/> is
+        /// greater than the available space from <paramref name="index"/> to the end of the destination
+        /// <paramref name="array"/>. -or- The type of the source <see
+        /// cref="T:System.Collections.ICollection"/> cannot be cast automatically to the type of the
+        /// destination <paramref name="array"/>.
+        /// </exception>
+        void ICollection.CopyTo(Array array, int index)
+        {
+            // Validate arguments.
+            if (array == null)
+            {
+                throw new ArgumentNullException("array");
+            }
+
+            // We must be careful not to corrupt the array, so we will first accumulate an
+            // internal list of elements that we will then copy to the array. This requires
+            // some extra allocation, but is necessary since we don't know up front whether
+            // the array is sufficiently large to hold the stack's contents.
+            ((ICollection)ToList()).CopyTo(array, index);
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"/> is
+        /// synchronized with the SyncRoot.
+        /// </summary>
+        /// <value>true if access to the <see cref="T:System.Collections.ICollection"/> is synchronized
+        /// with the SyncRoot; otherwise, false. For <see cref="ConcurrentQueue{T}"/>, this property always
+        /// returns false.</value>
+        bool ICollection.IsSynchronized
+        {
+            // Gets a value indicating whether access to this collection is synchronized. Always returns
+            // false. The reason is subtle. While access is in face thread safe, it's not the case that
+            // locking on the SyncRoot would have prevented concurrent pushes and pops, as this property
+            // would typically indicate; that's because we internally use CAS operations vs. true locks.
+            get { return false; }
+        }
+
+
+        /// <summary>
+        /// Gets an object that can be used to synchronize access to the <see
+        /// cref="T:System.Collections.ICollection"/>. This property is not supported.
+        /// </summary>
+        /// <exception cref="T:System.NotSupportedException">The SyncRoot property is not supported.</exception>
+        object ICollection.SyncRoot
+        {
+            get
+            {
+                throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported"));
+            }
+        }
+
+        /// <summary>
+        /// Returns an enumerator that iterates through a collection.
+        /// </summary>
+        /// <returns>An <see cref="T:System.Collections.IEnumerator"/> that can be used to iterate through the collection.</returns>
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable<T>)this).GetEnumerator();
+        }
+
+        /// <summary>
+        /// Attempts to add an object to the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">The object to add to the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null
+        /// reference (Nothing in Visual Basic) for reference types.
+        /// </param>
+        /// <returns>true if the object was added successfully; otherwise, false.</returns>
+        /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will always add the object to the
+        /// end of the <see cref="ConcurrentQueue{T}"/>
+        /// and return true.</remarks>
+        bool IProducerConsumerCollection<T>.TryAdd(T item)
+        {
+            Enqueue(item);
+            return true;
+        }
+
+        /// <summary>
+        /// Attempts to remove and return an object from the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">
+        /// When this method returns, if the operation was successful, <paramref name="item"/> contains the
+        /// object removed. If no object was available to be removed, the value is unspecified.
+        /// </param>
+        /// <returns>true if an element was removed and returned succesfully; otherwise, false.</returns>
+        /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will attempt to remove the object
+        /// from the beginning of the <see cref="ConcurrentQueue{T}"/>.
+        /// </remarks>
+        bool IProducerConsumerCollection<T>.TryTake(out T item)
+        {
+            return TryDequeue(out item);
+        }
+
+        /// <summary>
+        /// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"/> is empty.
+        /// </summary>
+        /// <value>true if the <see cref="ConcurrentQueue{T}"/> is empty; otherwise, false.</value>
+        /// <remarks>
+        /// For determining whether the collection contains any items, use of this property is recommended
+        /// rather than retrieving the number of items from the <see cref="Count"/> property and comparing it
+        /// to 0.  However, as this collection is intended to be accessed concurrently, it may be the case
+        /// that another thread will modify the collection after <see cref="IsEmpty"/> returns, thus invalidating
+        /// the result.
+        /// </remarks>
+        public bool IsEmpty
+        {
+            get
+            {
+                Segment head = m_head;
+                if (!head.IsEmpty)
+                    //fast route 1:
+                    //if current head is not empty, then queue is not empty
+                    return false;
+                else if (head.Next == null)
+                    //fast route 2:
+                    //if current head is empty and it's the last segment
+                    //then queue is empty
+                    return true;
+                else
+                //slow route:
+                //current head is empty and it is NOT the last segment,
+                //it means another thread is growing new segment 
+                {
+                    SpinWait spin = new SpinWait();
+                    while (head.IsEmpty)
+                    {
+                        if (head.Next == null)
+                            return true;
+
+                        spin.SpinOnce();
+                        head = m_head;
+                    }
+                    return false;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Copies the elements stored in the <see cref="ConcurrentQueue{T}"/> to a new array.
+        /// </summary>
+        /// <returns>A new array containing a snapshot of elements copied from the <see
+        /// cref="ConcurrentQueue{T}"/>.</returns>
+        public T[] ToArray()
+        {
+            return ToList().ToArray();
+        }
+
+        /// <summary>
+        /// Copies the <see cref="ConcurrentQueue{T}"/> elements to a new <see
+        /// cref="T:System.Collections.Generic.List{T}"/>.
+        /// </summary>
+        /// <returns>A new <see cref="T:System.Collections.Generic.List{T}"/> containing a snapshot of
+        /// elements copied from the <see cref="ConcurrentQueue{T}"/>.</returns>
+        private List<T> ToList()
+        {
+            // Increments the number of active snapshot takers. This increment must happen before the snapshot is 
+            // taken. At the same time, Decrement must happen after list copying is over. Only in this way, can it
+            // eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0. 
+            Interlocked.Increment(ref m_numSnapshotTakers);
+
+            List<T> list = new List<T>();
+            try
+            {
+                //store head and tail positions in buffer, 
+                Segment head, tail;
+                int headLow, tailHigh;
+                GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
+
+                if (head == tail)
+                {
+                    head.AddToList(list, headLow, tailHigh);
+                }
+                else
+                {
+                    head.AddToList(list, headLow, SEGMENT_SIZE - 1);
+                    Segment curr = head.Next;
+                    while (curr != tail)
+                    {
+                        curr.AddToList(list, 0, SEGMENT_SIZE - 1);
+                        curr = curr.Next;
+                    }
+                    //Add tail segment
+                    tail.AddToList(list, 0, tailHigh);
+                }
+            }
+            finally
+            {
+                // This Decrement must happen after copying is over. 
+                Interlocked.Decrement(ref m_numSnapshotTakers);
+            }
+            return list;
+        }
+
+        /// <summary>
+        /// Store the position of the current head and tail positions.
+        /// </summary>
+        /// <param name="head">return the head segment</param>
+        /// <param name="tail">return the tail segment</param>
+        /// <param name="headLow">return the head offset, value range [0, SEGMENT_SIZE]</param>
+        /// <param name="tailHigh">return the tail offset, value range [-1, SEGMENT_SIZE-1]</param>
+        private void GetHeadTailPositions(out Segment head, out Segment tail,
+            out int headLow, out int tailHigh)
+        {
+            head = m_head;
+            tail = m_tail;
+            headLow = head.Low;
+            tailHigh = tail.High;
+            SpinWait spin = new SpinWait();
+
+            //we loop until the observed values are stable and sensible.  
+            //This ensures that any update order by other methods can be tolerated.
+            while (
+                //if head and tail changed, retry
+                head != m_head || tail != m_tail
+                //if low and high pointers, retry
+                || headLow != head.Low || tailHigh != tail.High
+                //if head jumps ahead of tail because of concurrent grow and dequeue, retry
+                || head.m_index > tail.m_index)
+            {
+                spin.SpinOnce();
+                head = m_head;
+                tail = m_tail;
+                headLow = head.Low;
+                tailHigh = tail.High;
+            }
+        }
+
+
+        /// <summary>
+        /// Gets the number of elements contained in the <see cref="ConcurrentQueue{T}"/>.
+        /// </summary>
+        /// <value>The number of elements contained in the <see cref="ConcurrentQueue{T}"/>.</value>
+        /// <remarks>
+        /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/>
+        /// property is recommended rather than retrieving the number of items from the <see cref="Count"/>
+        /// property and comparing it to 0.
+        /// </remarks>
+        public int Count
+        {
+            get
+            {
+                //store head and tail positions in buffer, 
+                Segment head, tail;
+                int headLow, tailHigh;
+                GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
+
+                if (head == tail)
+                {
+                    return tailHigh - headLow + 1;
+                }
+
+                //head segment
+                int count = SEGMENT_SIZE - headLow;
+
+                //middle segment(s), if any, are full.
+                //We don't deal with overflow to be consistent with the behavior of generic types in CLR.
+                count += SEGMENT_SIZE * ((int)(tail.m_index - head.m_index - 1));
+
+                //tail segment
+                count += tailHigh + 1;
+
+                return count;
+            }
+        }
+
+
+        /// <summary>
+        /// Copies the <see cref="ConcurrentQueue{T}"/> elements to an existing one-dimensional <see
+        /// cref="T:System.Array">Array</see>, starting at the specified array index.
+        /// </summary>
+        /// <param name="array">The one-dimensional <see cref="T:System.Array">Array</see> that is the
+        /// destination of the elements copied from the
+        /// <see cref="ConcurrentQueue{T}"/>. The <see cref="T:System.Array">Array</see> must have zero-based
+        /// indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
+        /// Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// zero.</exception>
+        /// <exception cref="ArgumentException"><paramref name="index"/> is equal to or greater than the
+        /// length of the <paramref name="array"/>
+        /// -or- The number of elements in the source <see cref="ConcurrentQueue{T}"/> is greater than the
+        /// available space from <paramref name="index"/> to the end of the destination <paramref
+        /// name="array"/>.
+        /// </exception>
+        public void CopyTo(T[] array, int index)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException("array");
+            }
+
+            // We must be careful not to corrupt the array, so we will first accumulate an
+            // internal list of elements that we will then copy to the array. This requires
+            // some extra allocation, but is necessary since we don't know up front whether
+            // the array is sufficiently large to hold the stack's contents.
+            ToList().CopyTo(array, index);
+        }
+
+
+        /// <summary>
+        /// Returns an enumerator that iterates through the <see
+        /// cref="ConcurrentQueue{T}"/>.
+        /// </summary>
+        /// <returns>An enumerator for the contents of the <see
+        /// cref="ConcurrentQueue{T}"/>.</returns>
+        /// <remarks>
+        /// The enumeration represents a moment-in-time snapshot of the contents
+        /// of the queue.  It does not reflect any updates to the collection after 
+        /// <see cref="GetEnumerator"/> was called.  The enumerator is safe to use
+        /// concurrently with reads from and writes to the queue.
+        /// </remarks>
+        public IEnumerator<T> GetEnumerator()
+        {
+            // Increments the number of active snapshot takers. This increment must happen before the snapshot is 
+            // taken. At the same time, Decrement must happen after the enumeration is over. Only in this way, can it
+            // eliminate race condition when Segment.TryRemove() checks whether m_numSnapshotTakers == 0. 
+            Interlocked.Increment(ref m_numSnapshotTakers);
+
+            // Takes a snapshot of the queue. 
+            // A design flaw here: if a Thread.Abort() happens, we cannot decrement m_numSnapshotTakers. But we cannot 
+            // wrap the following with a try/finally block, otherwise the decrement will happen before the yield return 
+            // statements in the GetEnumerator (head, tail, headLow, tailHigh) method.           
+            Segment head, tail;
+            int headLow, tailHigh;
+            GetHeadTailPositions(out head, out tail, out headLow, out tailHigh);
+
+            //If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of
+            // the queue is not taken when GetEnumerator is initialized but when MoveNext() is first called.
+            // This is inconsistent with existing generic collections. In order to prevent it, we capture the 
+            // value of m_head in a buffer and call out to a helper method.
+            //The old way of doing this was to return the ToList().GetEnumerator(), but ToList() was an 
+            // unnecessary perfomance hit.
+            return GetEnumerator(head, tail, headLow, tailHigh);
+        }
+
+        /// <summary>
+        /// Helper method of GetEnumerator to seperate out yield return statement, and prevent lazy evaluation. 
+        /// </summary>
+        private IEnumerator<T> GetEnumerator(Segment head, Segment tail, int headLow, int tailHigh)
+        {
+            try
+            {
+                SpinWait spin = new SpinWait();
+
+                if (head == tail)
+                {
+                    for (int i = headLow; i <= tailHigh; i++)
+                    {
+                        // If the position is reserved by an Enqueue operation, but the value is not written into,
+                        // spin until the value is available.
+                        spin.Reset();
+                        while (!head.m_state[i].m_value)
+                        {
+                            spin.SpinOnce();
+                        }
+                        yield return head.m_array[i];
+                    }
+                }
+                else
+                {
+                    //iterate on head segment
+                    for (int i = headLow; i < SEGMENT_SIZE; i++)
+                    {
+                        // If the position is reserved by an Enqueue operation, but the value is not written into,
+                        // spin until the value is available.
+                        spin.Reset();
+                        while (!head.m_state[i].m_value)
+                        {
+                            spin.SpinOnce();
+                        }
+                        yield return head.m_array[i];
+                    }
+                    //iterate on middle segments
+                    Segment curr = head.Next;
+                    while (curr != tail)
+                    {
+                        for (int i = 0; i < SEGMENT_SIZE; i++)
+                        {
+                            // If the position is reserved by an Enqueue operation, but the value is not written into,
+                            // spin until the value is available.
+                            spin.Reset();
+                            while (!curr.m_state[i].m_value)
+                            {
+                                spin.SpinOnce();
+                            }
+                            yield return curr.m_array[i];
+                        }
+                        curr = curr.Next;
+                    }
+
+                    //iterate on tail segment
+                    for (int i = 0; i <= tailHigh; i++)
+                    {
+                        // If the position is reserved by an Enqueue operation, but the value is not written into,
+                        // spin until the value is available.
+                        spin.Reset();
+                        while (!tail.m_state[i].m_value)
+                        {
+                            spin.SpinOnce();
+                        }
+                        yield return tail.m_array[i];
+                    }
+                }
+            }
+            finally
+            {
+                // This Decrement must happen after the enumeration is over. 
+                Interlocked.Decrement(ref m_numSnapshotTakers);
+            }
+        }
+
+        /// <summary>
+        /// Adds an object to the end of the <see cref="ConcurrentQueue{T}"/>.
+        /// </summary>
+        /// <param name="item">The object to add to the end of the <see
+        /// cref="ConcurrentQueue{T}"/>. The value can be a null reference
+        /// (Nothing in Visual Basic) for reference types.
+        /// </param>
+        public void Enqueue(T item)
+        {
+            SpinWait spin = new SpinWait();
+            while (true)
+            {
+                Segment tail = m_tail;
+                if (tail.TryAppend(item))
+                    return;
+                spin.SpinOnce();
+            }
+        }
+
+
+        /// <summary>
+        /// Attempts to remove and return the object at the beginning of the <see
+        /// cref="ConcurrentQueue{T}"/>.
+        /// </summary>
+        /// <param name="result">
+        /// When this method returns, if the operation was successful, <paramref name="result"/> contains the
+        /// object removed. If no object was available to be removed, the value is unspecified.
+        /// </param>
+        /// <returns>true if an element was removed and returned from the beggining of the <see
+        /// cref="ConcurrentQueue{T}"/>
+        /// succesfully; otherwise, false.</returns>
+        public bool TryDequeue(out T result)
+        {
+            while (!IsEmpty)
+            {
+                Segment head = m_head;
+                if (head.TryRemove(out result))
+                    return true;
+                //since method IsEmpty spins, we don't need to spin in the while loop
+            }
+            result = default(T);
+            return false;
+        }
+
+        /// <summary>
+        /// Attempts to return an object from the beginning of the <see cref="ConcurrentQueue{T}"/>
+        /// without removing it.
+        /// </summary>
+        /// <param name="result">When this method returns, <paramref name="result"/> contains an object from
+        /// the beginning of the <see cref="T:System.Collections.Concurrent.ConccurrentQueue{T}"/> or an
+        /// unspecified value if the operation failed.</param>
+        /// <returns>true if and object was returned successfully; otherwise, false.</returns>
+        public bool TryPeek(out T result)
+        {
+            Interlocked.Increment(ref m_numSnapshotTakers);
+
+            while (!IsEmpty)
+            {
+                Segment head = m_head;
+                if (head.TryPeek(out result))
+                {
+                    Interlocked.Decrement(ref m_numSnapshotTakers);
+                    return true;
+                }
+                //since method IsEmpty spins, we don't need to spin in the while loop
+            }
+            result = default(T);
+            Interlocked.Decrement(ref m_numSnapshotTakers);
+            return false;
+        }
+
+
+        /// <summary>
+        /// private class for ConcurrentQueue. 
+        /// a queue is a linked list of small arrays, each node is called a segment.
+        /// A segment contains an array, a pointer to the next segment, and m_low, m_high indices recording
+        /// the first and last valid elements of the array.
+        /// </summary>
+        private class Segment
+        {
+            //we define two volatile arrays: m_array and m_state. Note that the accesses to the array items 
+            //do not get volatile treatment. But we don't need to worry about loading adjacent elements or 
+            //store/load on adjacent elements would suffer reordering. 
+            // - Two stores:  these are at risk, but CLRv2 memory model guarantees store-release hence we are safe.
+            // - Two loads: because one item from two volatile arrays are accessed, the loads of the array references
+            //          are sufficient to prevent reordering of the loads of the elements.
+            internal volatile T[] m_array;
+
+            // For each entry in m_array, the corresponding entry in m_state indicates whether this position contains 
+            // a valid value. m_state is initially all false. 
+            internal volatile VolatileBool[] m_state;
+
+            //pointer to the next segment. null if the current segment is the last segment
+            private volatile Segment m_next;
+
+            //We use this zero based index to track how many segments have been created for the queue, and
+            //to compute how many active segments are there currently. 
+            // * The number of currently active segments is : m_tail.m_index - m_head.m_index + 1;
+            // * m_index is incremented with every Segment.Grow operation. We use Int64 type, and we can safely 
+            //   assume that it never overflows. To overflow, we need to do 2^63 increments, even at a rate of 4 
+            //   billion (2^32) increments per second, it takes 2^31 seconds, which is about 64 years.
+            internal readonly long m_index;
+
+            //indices of where the first and last valid values
+            // - m_low points to the position of the next element to pop from this segment, range [0, infinity)
+            //      m_low >= SEGMENT_SIZE implies the segment is disposable
+            // - m_high points to the position of the latest pushed element, range [-1, infinity)
+            //      m_high == -1 implies the segment is new and empty
+            //      m_high >= SEGMENT_SIZE-1 means this segment is ready to grow. 
+            //        and the thread who sets m_high to SEGMENT_SIZE-1 is responsible to grow the segment
+            // - Math.Min(m_low, SEGMENT_SIZE) > Math.Min(m_high, SEGMENT_SIZE-1) implies segment is empty
+            // - initially m_low =0 and m_high=-1;
+            private volatile int m_low;
+            private volatile int m_high;
+
+            private volatile ConcurrentQueue<T> m_source;
+
+            /// <summary>
+            /// Create and initialize a segment with the specified index.
+            /// </summary>
+            internal Segment(long index, ConcurrentQueue<T> source)
+            {
+                m_array = new T[SEGMENT_SIZE];
+                m_state = new VolatileBool[SEGMENT_SIZE]; //all initialized to false
+                m_high = -1;
+                Contract.Assert(index >= 0);
+                m_index = index;
+                m_source = source;
+            }
+
+            /// <summary>
+            /// return the next segment
+            /// </summary>
+            internal Segment Next
+            {
+                get { return m_next; }
+            }
+
+
+            /// <summary>
+            /// return true if the current segment is empty (doesn't have any element available to dequeue, 
+            /// false otherwise
+            /// </summary>
+            internal bool IsEmpty
+            {
+                get { return (Low > High); }
+            }
+
+            /// <summary>
+            /// Add an element to the tail of the current segment
+            /// exclusively called by ConcurrentQueue.InitializedFromCollection
+            /// InitializeFromCollection is responsible to guaratee that there is no index overflow,
+            /// and there is no contention
+            /// </summary>
+            /// <param name="value"></param>
+            internal void UnsafeAdd(T value)
+            {
+                Contract.Assert(m_high < SEGMENT_SIZE - 1);
+                m_high++;
+                m_array[m_high] = value;
+                m_state[m_high].m_value = true;
+            }
+
+            /// <summary>
+            /// Create a new segment and append to the current one
+            /// Does not update the m_tail pointer
+            /// exclusively called by ConcurrentQueue.InitializedFromCollection
+            /// InitializeFromCollection is responsible to guaratee that there is no index overflow,
+            /// and there is no contention
+            /// </summary>
+            /// <returns>the reference to the new Segment</returns>
+            internal Segment UnsafeGrow()
+            {
+                Contract.Assert(m_high >= SEGMENT_SIZE - 1);
+                Segment newSegment = new Segment(m_index + 1, m_source); //m_index is Int64, we don't need to worry about overflow
+                m_next = newSegment;
+                return newSegment;
+            }
+
+            /// <summary>
+            /// Create a new segment and append to the current one
+            /// Update the m_tail pointer
+            /// This method is called when there is no contention
+            /// </summary>
+            internal void Grow()
+            {
+                //no CAS is needed, since there is no contention (other threads are blocked, busy waiting)
+                Segment newSegment = new Segment(m_index + 1, m_source);  //m_index is Int64, we don't need to worry about overflow
+                m_next = newSegment;
+                Contract.Assert(m_source.m_tail == this);
+                m_source.m_tail = m_next;
+            }
+
+
+            /// <summary>
+            /// Try to append an element at the end of this segment.
+            /// </summary>
+            /// <param name="value">the element to append</param>
+            /// <param name="tail">The tail.</param>
+            /// <returns>true if the element is appended, false if the current segment is full</returns>
+            /// <remarks>if appending the specified element succeeds, and after which the segment is full, 
+            /// then grow the segment</remarks>
+            internal bool TryAppend(T value)
+            {
+                //quickly check if m_high is already over the boundary, if so, bail out
+                if (m_high >= SEGMENT_SIZE - 1)
+                {
+                    return false;
+                }
+
+                //Now we will use a CAS to increment m_high, and store the result in newhigh.
+                //Depending on how many free spots left in this segment and how many threads are doing this Increment
+                //at this time, the returning "newhigh" can be 
+                // 1) < SEGMENT_SIZE - 1 : we took a spot in this segment, and not the last one, just insert the value
+                // 2) == SEGMENT_SIZE - 1 : we took the last spot, insert the value AND grow the segment
+                // 3) > SEGMENT_SIZE - 1 : we failed to reserve a spot in this segment, we return false to 
+                //    Queue.Enqueue method, telling it to try again in the next segment.
+
+                int newhigh = SEGMENT_SIZE; //initial value set to be over the boundary
+
+                //We need do Interlocked.Increment and value/state update in a finally block to ensure that they run
+                //without interuption. This is to prevent anything from happening between them, and another dequeue
+                //thread maybe spinning forever to wait for m_state[] to be true;
+                try
+                { }
+                finally
+                {
+                    newhigh = Interlocked.Increment(ref m_high);
+                    if (newhigh <= SEGMENT_SIZE - 1)
+                    {
+                        m_array[newhigh] = value;
+                        m_state[newhigh].m_value = true;
+                    }
+
+                    //if this thread takes up the last slot in the segment, then this thread is responsible
+                    //to grow a new segment. Calling Grow must be in the finally block too for reliability reason:
+                    //if thread abort during Grow, other threads will be left busy spinning forever.
+                    if (newhigh == SEGMENT_SIZE - 1)
+                    {
+                        Grow();
+                    }
+                }
+
+                //if newhigh <= SEGMENT_SIZE-1, it means the current thread successfully takes up a spot
+                return newhigh <= SEGMENT_SIZE - 1;
+            }
+
+
+            /// <summary>
+            /// try to remove an element from the head of current segment
+            /// </summary>
+            /// <param name="result">The result.</param>
+            /// <param name="head">The head.</param>
+            /// <returns>return false only if the current segment is empty</returns>
+            internal bool TryRemove(out T result)
+            {
+                SpinWait spin = new SpinWait();
+                int lowLocal = Low, highLocal = High;
+                while (lowLocal <= highLocal)
+                {
+                    //try to update m_low
+                    if (Interlocked.CompareExchange(ref m_low, lowLocal + 1, lowLocal) == lowLocal)
+                    {
+                        //if the specified value is not available (this spot is taken by a push operation,
+                        // but the value is not written into yet), then spin
+                        SpinWait spinLocal = new SpinWait();
+                        while (!m_state[lowLocal].m_value)
+                        {
+                            spinLocal.SpinOnce();
+                        }
+                        result = m_array[lowLocal];
+
+                        // If there is no other thread taking snapshot (GetEnumerator(), ToList(), etc), reset the deleted entry to null.
+                        // It is ok if after this conditional check m_numSnapshotTakers becomes > 0, because new snapshots won't include 
+                        // the deleted entry at m_array[lowLocal]. 
+                        if (m_source.m_numSnapshotTakers <= 0)
+                        {
+                            m_array[lowLocal] = default(T); //release the reference to the object. 
+                        }
+
+                        //if the current thread sets m_low to SEGMENT_SIZE, which means the current segment becomes
+                        //disposable, then this thread is responsible to dispose this segment, and reset m_head 
+                        if (lowLocal + 1 >= SEGMENT_SIZE)
+                        {
+                            //  Invariant: we only dispose the current m_head, not any other segment
+                            //  In usual situation, disposing a segment is simply seting m_head to m_head.m_next
+                            //  But there is one special case, where m_head and m_tail points to the same and ONLY
+                            //segment of the queue: Another thread A is doing Enqueue and finds that it needs to grow,
+                            //while the *current* thread is doing *this* Dequeue operation, and finds that it needs to 
+                            //dispose the current (and ONLY) segment. Then we need to wait till thread A finishes its 
+                            //Grow operation, this is the reason of having the following while loop
+                            spinLocal = new SpinWait();
+                            while (m_next == null)
+                            {
+                                spinLocal.SpinOnce();
+                            }
+                            Contract.Assert(m_source.m_head == this);
+                            m_source.m_head = m_next;
+                        }
+                        return true;
+                    }
+                    else
+                    {
+                        //CAS failed due to contention: spin briefly and retry
+                        spin.SpinOnce();
+                        lowLocal = Low; highLocal = High;
+                    }
+                }//end of while
+                result = default(T);
+                return false;
+            }
+
+            /// <summary>
+            /// try to peek the current segment
+            /// </summary>
+            /// <param name="result">holds the return value of the element at the head position, 
+            /// value set to default(T) if there is no such an element</param>
+            /// <returns>true if there are elements in the current segment, false otherwise</returns>
+            internal bool TryPeek(out T result)
+            {
+                result = default(T);
+                int lowLocal = Low;
+                if (lowLocal > High)
+                    return false;
+                SpinWait spin = new SpinWait();
+                while (!m_state[lowLocal].m_value)
+                {
+                    spin.SpinOnce();
+                }
+                result = m_array[lowLocal];
+                return true;
+            }
+
+            /// <summary>
+            /// Adds part or all of the current segment into a List.
+            /// </summary>
+            /// <param name="list">the list to which to add</param>
+            /// <param name="start">the start position</param>
+            /// <param name="end">the end position</param>
+            internal void AddToList(List<T> list, int start, int end)
+            {
+                for (int i = start; i <= end; i++)
+                {
+                    SpinWait spin = new SpinWait();
+                    while (!m_state[i].m_value)
+                    {
+                        spin.SpinOnce();
+                    }
+                    list.Add(m_array[i]);
+                }
+            }
+
+            /// <summary>
+            /// return the position of the head of the current segment
+            /// Value range [0, SEGMENT_SIZE], if it's SEGMENT_SIZE, it means this segment is exhausted and thus empty
+            /// </summary>
+            internal int Low
+            {
+                get
+                {
+                    return Math.Min(m_low, SEGMENT_SIZE);
+                }
+            }
+
+            /// <summary>
+            /// return the logical position of the tail of the current segment      
+            /// Value range [-1, SEGMENT_SIZE-1]. When it's -1, it means this is a new segment and has no elemnet yet
+            /// </summary>
+            internal int High
+            {
+                get
+                {
+                    //if m_high > SEGMENT_SIZE, it means it's out of range, we should return
+                    //SEGMENT_SIZE-1 as the logical position
+                    return Math.Min(m_high, SEGMENT_SIZE - 1);
+                }
+            }
+
+        }
+    }//end of class Segment
+
+    /// <summary>
+    /// A wrapper struct for volatile bool, please note the copy of the struct it self will not be volatile
+    /// for example this statement will not include in volatilness operation volatileBool1 = volatileBool2 the jit will copy the struct and will ignore the volatile
+    /// </summary>
+    struct VolatileBool
+    {
+        public VolatileBool(bool value)
+        {
+            m_value = value;
+        }
+        public volatile bool m_value;
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/ConcurrentStack.cs b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentStack.cs
new file mode 100644
index 0000000000..15d4176cff
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/ConcurrentStack.cs
@@ -0,0 +1,840 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+#pragma warning disable 0420
+
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// A lock-free, concurrent stack primitive, and its associated debugger view type.
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Diagnostics.Contracts;
+using System.Runtime.ConstrainedExecution;
+using System.Runtime.Serialization;
+using System.Security;
+using System.Security.Permissions;
+using System.Threading;
+
+namespace System.Collections.Concurrent
+{
+    // A stack that uses CAS operations internally to maintain thread-safety in a lock-free
+    // manner. Attempting to push or pop concurrently from the stack will not trigger waiting,
+    // although some optimistic concurrency and retry is used, possibly leading to lack of
+    // fairness and/or livelock. The stack uses spinning and backoff to add some randomization,
+    // in hopes of statistically decreasing the possibility of livelock.
+    // 
+    // Note that we currently allocate a new node on every push. This avoids having to worry
+    // about potential ABA issues, since the CLR GC ensures that a memory address cannot be
+    // reused before all references to it have died.
+
+    /// <summary>
+    /// Represents a thread-safe last-in, first-out collection of objects.
+    /// </summary>
+    /// <typeparam name="T">Specifies the type of elements in the stack.</typeparam>
+    /// <remarks>
+    /// All public and protected members of <see cref="ConcurrentStack{T}"/> are thread-safe and may be used
+    /// concurrently from multiple threads.
+    /// </remarks>
+    [DebuggerDisplay("Count = {Count}")]
+    [DebuggerTypeProxy(typeof(SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<>))]
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+#if !FEATURE_CORECLR
+    [Serializable]
+#endif //!FEATURE_CORECLR
+    public class ConcurrentStack<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T>
+    {
+        /// <summary>
+        /// A simple (internal) node type used to store elements of concurrent stacks and queues.
+        /// </summary>
+        private class Node
+        {
+            internal readonly T m_value; // Value of the node.
+            internal Node m_next; // Next pointer.
+
+            /// <summary>
+            /// Constructs a new node with the specified value and no next node.
+            /// </summary>
+            /// <param name="value">The value of the node.</param>
+            internal Node(T value)
+            {
+                m_value = value;
+                m_next = null;
+            }
+        }
+
+#if !FEATURE_CORECLR
+        [NonSerialized]
+#endif //!FEATURE_CORECLR
+        private volatile Node m_head; // The stack is a singly linked list, and only remembers the head.
+
+#if !FEATURE_CORECLR
+        private T[] m_serializationArray; // Used for custom serialization.
+#endif //!FEATURE_CORECLR
+
+        private const int BACKOFF_MAX_YIELDS = 8; // Arbitrary number to cap backoff.
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentStack{T}"/>
+        /// class.
+        /// </summary>
+        public ConcurrentStack()
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ConcurrentStack{T}"/>
+        /// class that contains elements copied from the specified collection
+        /// </summary>
+        /// <param name="collection">The collection whose elements are copied to the new <see
+        /// cref="ConcurrentStack{T}"/>.</param>
+        /// <exception cref="T:System.ArgumentNullException">The <paramref name="collection"/> argument is
+        /// null.</exception>
+        public ConcurrentStack(IEnumerable<T> collection)
+        {
+            if (collection == null)
+            {
+                throw new ArgumentNullException("collection");
+            }
+            InitializeFromCollection(collection);
+        }
+
+        /// <summary>
+        /// Initializes the contents of the stack from an existing collection.
+        /// </summary>
+        /// <param name="collection">A collection from which to copy elements.</param>
+        private void InitializeFromCollection(IEnumerable<T> collection)
+        {
+            // We just copy the contents of the collection to our stack.
+            Node lastNode = null;
+            foreach (T element in collection)
+            {
+                Node newNode = new Node(element);
+                newNode.m_next = lastNode;
+                lastNode = newNode;
+            }
+
+            m_head = lastNode;
+        }
+
+#if !FEATURE_CORECLR
+        /// <summary>
+        /// Get the data array to be serialized
+        /// </summary>
+        [OnSerializing]
+        private void OnSerializing(StreamingContext context)
+        {
+            // save the data into the serialization array to be saved
+            m_serializationArray = ToArray();
+        }
+
+        /// <summary>
+        /// Construct the stack from a previously seiralized one
+        /// </summary>
+        [OnDeserialized]
+        private void OnDeserialized(StreamingContext context)
+        {
+            Contract.Assert(m_serializationArray != null);
+            // Add the elements to our stack.  We need to add them from head-to-tail, to
+            // preserve the original ordering of the stack before serialization.
+            Node prevNode = null;
+            Node head = null;
+            for (int i = 0; i < m_serializationArray.Length; i++)
+            {
+                Node currNode = new Node(m_serializationArray[i]);
+
+                if (prevNode == null)
+                {
+                    head = currNode;
+                }
+                else
+                {
+                    prevNode.m_next = currNode;
+                }
+
+                prevNode = currNode;
+            }
+
+            m_head = head;
+            m_serializationArray = null;
+        }
+#endif //!FEATURE_CORECLR
+
+
+        /// <summary>
+        /// Gets a value that indicates whether the <see cref="ConcurrentStack{T}"/> is empty.
+        /// </summary>
+        /// <value>true if the <see cref="ConcurrentStack{T}"/> is empty; otherwise, false.</value>
+        /// <remarks>
+        /// For determining whether the collection contains any items, use of this property is recommended
+        /// rather than retrieving the number of items from the <see cref="Count"/> property and comparing it
+        /// to 0.  However, as this collection is intended to be accessed concurrently, it may be the case
+        /// that another thread will modify the collection after <see cref="IsEmpty"/> returns, thus invalidating
+        /// the result.
+        /// </remarks>
+        public bool IsEmpty
+        {
+            // Checks whether the stack is empty. Clearly the answer may be out of date even prior to
+            // the function returning (i.e. if another thread concurrently adds to the stack). It does
+            // guarantee, however, that, if another thread does not mutate the stack, a subsequent call
+            // to TryPop will return true -- i.e. it will also read the stack as non-empty.
+            get { return m_head == null; }
+        }
+
+        /// <summary>
+        /// Gets the number of elements contained in the <see cref="ConcurrentStack{T}"/>.
+        /// </summary>
+        /// <value>The number of elements contained in the <see cref="ConcurrentStack{T}"/>.</value>
+        /// <remarks>
+        /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/>
+        /// property is recommended rather than retrieving the number of items from the <see cref="Count"/>
+        /// property and comparing it to 0.
+        /// </remarks>
+        public int Count
+        {
+            // Counts the number of entries in the stack. This is an O(n) operation. The answer may be out
+            // of date before returning, but guarantees to return a count that was once valid. Conceptually,
+            // the implementation snaps a copy of the list and then counts the entries, though physically
+            // this is not what actually happens.
+            get
+            {
+                int count = 0;
+
+                // Just whip through the list and tally up the number of nodes. We rely on the fact that
+                // node next pointers are immutable after being enqueued for the first time, even as
+                // they are being dequeued. If we ever changed this (e.g. to pool nodes somehow),
+                // we'd need to revisit this implementation.
+
+                for (Node curr = m_head; curr != null; curr = curr.m_next)
+                {
+                    count++; //we don't handle overflow, to be consistent with existing generic collection types in CLR
+                }
+
+                return count;
+            }
+        }
+
+
+        /// <summary>
+        /// Gets a value indicating whether access to the <see cref="T:System.Collections.ICollection"/> is
+        /// synchronized with the SyncRoot.
+        /// </summary>
+        /// <value>true if access to the <see cref="T:System.Collections.ICollection"/> is synchronized
+        /// with the SyncRoot; otherwise, false. For <see cref="ConcurrentStack{T}"/>, this property always
+        /// returns false.</value>
+        bool ICollection.IsSynchronized
+        {
+            // Gets a value indicating whether access to this collection is synchronized. Always returns
+            // false. The reason is subtle. While access is in face thread safe, it's not the case that
+            // locking on the SyncRoot would have prevented concurrent pushes and pops, as this property
+            // would typically indicate; that's because we internally use CAS operations vs. true locks.
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Gets an object that can be used to synchronize access to the <see
+        /// cref="T:System.Collections.ICollection"/>. This property is not supported.
+        /// </summary>
+        /// <exception cref="T:System.NotSupportedException">The SyncRoot property is not supported</exception>
+        object ICollection.SyncRoot
+        {
+            get
+            {
+                throw new NotSupportedException(Environment.GetResourceString("ConcurrentCollection_SyncRoot_NotSupported"));
+            }
+        }
+
+        /// <summary>
+        /// Removes all objects from the <see cref="ConcurrentStack{T}"/>.
+        /// </summary>
+        public void Clear()
+        {
+            // Clear the list by setting the head to null. We don't need to use an atomic
+            // operation for this: anybody who is mutating the head by pushing or popping
+            // will need to use an atomic operation to guarantee they serialize and don't
+            // overwrite our setting of the head to null.
+            m_head = null;
+        }
+
+        /// <summary>
+        /// Copies the elements of the <see cref="T:System.Collections.ICollection"/> to an <see
+        /// cref="T:System.Array"/>, starting at a particular
+        /// <see cref="T:System.Array"/> index.
+        /// </summary>
+        /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of
+        /// the elements copied from the
+        /// <see cref="ConcurrentStack{T}"/>. The <see cref="T:System.Array"/> must
+        /// have zero-based indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
+        /// Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// zero.</exception>
+        /// <exception cref="ArgumentException">
+        /// <paramref name="array"/> is multidimensional. -or-
+        /// <paramref name="array"/> does not have zero-based indexing. -or-
+        /// <paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/>
+        /// -or- The number of elements in the source <see cref="T:System.Collections.ICollection"/> is
+        /// greater than the available space from <paramref name="index"/> to the end of the destination
+        /// <paramref name="array"/>. -or- The type of the source <see
+        /// cref="T:System.Collections.ICollection"/> cannot be cast automatically to the type of the
+        /// destination <paramref name="array"/>.
+        /// </exception>
+        void ICollection.CopyTo(Array array, int index)
+        {
+            // Validate arguments.
+            if (array == null)
+            {
+                throw new ArgumentNullException("array");
+            }
+
+            // We must be careful not to corrupt the array, so we will first accumulate an
+            // internal list of elements that we will then copy to the array. This requires
+            // some extra allocation, but is necessary since we don't know up front whether
+            // the array is sufficiently large to hold the stack's contents.
+            ((ICollection)ToList()).CopyTo(array, index);
+        }
+
+        /// <summary>
+        /// Copies the <see cref="ConcurrentStack{T}"/> elements to an existing one-dimensional <see
+        /// cref="T:System.Array"/>, starting at the specified array index.
+        /// </summary>
+        /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of
+        /// the elements copied from the
+        /// <see cref="ConcurrentStack{T}"/>. The <see cref="T:System.Array"/> must have zero-based
+        /// indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
+        /// Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// zero.</exception>
+        /// <exception cref="ArgumentException"><paramref name="index"/> is equal to or greater than the
+        /// length of the <paramref name="array"/>
+        /// -or- The number of elements in the source <see cref="ConcurrentStack{T}"/> is greater than the
+        /// available space from <paramref name="index"/> to the end of the destination <paramref
+        /// name="array"/>.
+        /// </exception>
+        public void CopyTo(T[] array, int index)
+        {
+            if (array == null)
+            {
+                throw new ArgumentNullException("array");
+            }
+
+            // We must be careful not to corrupt the array, so we will first accumulate an
+            // internal list of elements that we will then copy to the array. This requires
+            // some extra allocation, but is necessary since we don't know up front whether
+            // the array is sufficiently large to hold the stack's contents.
+            ToList().CopyTo(array, index);
+        }
+
+
+        /// <summary>
+        /// Inserts an object at the top of the <see cref="ConcurrentStack{T}"/>.
+        /// </summary>
+        /// <param name="item">The object to push onto the <see cref="ConcurrentStack{T}"/>. The value can be
+        /// a null reference (Nothing in Visual Basic) for reference types.
+        /// </param>
+        public void Push(T item)
+        {
+            // Pushes a node onto the front of the stack thread-safely. Internally, this simply
+            // swaps the current head pointer using a (thread safe) CAS operation to accomplish
+            // lock freedom. If the CAS fails, we add some back off to statistically decrease
+            // contention at the head, and then go back around and retry.
+
+            Node newNode = new Node(item);
+            newNode.m_next = m_head;
+            if (Interlocked.CompareExchange(ref m_head, newNode, newNode.m_next) == newNode.m_next)
+            {
+                return;
+            }
+
+            // If we failed, go to the slow path and loop around until we succeed.
+            PushCore(newNode, newNode);
+        }
+
+        /// <summary>
+        /// Inserts multiple objects at the top of the <see cref="ConcurrentStack{T}"/> atomically.
+        /// </summary>
+        /// <param name="items">The objects to push onto the <see cref="ConcurrentStack{T}"/>.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="items"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <remarks>
+        /// When adding multiple items to the stack, using PushRange is a more efficient
+        /// mechanism than using <see cref="Push"/> one item at a time.  Additionally, PushRange
+        /// guarantees that all of the elements will be added atomically, meaning that no other threads will
+        /// be able to inject elements between the elements being pushed.  Items at lower indices in
+        /// the <paramref name="items"/> array will be pushed before items at higher indices.
+        /// </remarks>
+        public void PushRange(T[] items)
+        {
+            if (items == null)
+            {
+                throw new ArgumentNullException("items");
+            }
+            PushRange(items, 0, items.Length);
+        }
+
+        /// <summary>
+        /// Inserts multiple objects at the top of the <see cref="ConcurrentStack{T}"/> atomically.
+        /// </summary>
+        /// <param name="items">The objects to push onto the <see cref="ConcurrentStack{T}"/>.</param>
+        /// <param name="startIndex">The zero-based offset in <paramref name="items"/> at which to begin
+        /// inserting elements onto the top of the <see cref="ConcurrentStack{T}"/>.</param>
+        /// <param name="count">The number of elements to be inserted onto the top of the <see
+        /// cref="ConcurrentStack{T}"/>.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="items"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="startIndex"/> or <paramref
+        /// name="count"/> is negative. Or <paramref name="startIndex"/> is greater than or equal to the length 
+        /// of <paramref name="items"/>.</exception>
+        /// <exception cref="ArgumentException"><paramref name="startIndex"/> + <paramref name="count"/> is
+        /// greater than the length of <paramref name="items"/>.</exception>
+        /// <remarks>
+        /// When adding multiple items to the stack, using PushRange is a more efficient
+        /// mechanism than using <see cref="Push"/> one item at a time. Additionally, PushRange
+        /// guarantees that all of the elements will be added atomically, meaning that no other threads will
+        /// be able to inject elements between the elements being pushed. Items at lower indices in the
+        /// <paramref name="items"/> array will be pushed before items at higher indices.
+        /// </remarks>
+        public void PushRange(T[] items, int startIndex, int count)
+        {
+            ValidatePushPopRangeInput(items, startIndex, count);
+
+            // No op if the count is zero
+            if (count == 0)
+                return;
+
+
+            Node head, tail;
+            head = tail = new Node(items[startIndex]);
+            for (int i = startIndex + 1; i < startIndex + count; i++)
+            {
+                Node node = new Node(items[i]);
+                node.m_next = head;
+                head = node;
+            }
+
+            tail.m_next = m_head;
+            if (Interlocked.CompareExchange(ref m_head, head, tail.m_next) == tail.m_next)
+            {
+                return;
+            }
+
+            // If we failed, go to the slow path and loop around until we succeed.
+            PushCore(head, tail);
+
+        }
+
+
+        /// <summary>
+        /// Push one or many nodes into the stack, if head and tails are equal then push one node to the stack other wise push the list between head
+        /// and tail to the stack
+        /// </summary>
+        /// <param name="head">The head pointer to the new list</param>
+        /// <param name="tail">The tail pointer to the new list</param>
+        private void PushCore(Node head, Node tail)
+        {
+            SpinWait spin = new SpinWait();
+
+            // Keep trying to CAS the exising head with the new node until we succeed.
+            do
+            {
+                spin.SpinOnce();
+                // Reread the head and link our new node.
+                tail.m_next = m_head;
+            }
+            while (Interlocked.CompareExchange(
+                ref m_head, head, tail.m_next) != tail.m_next);
+
+#if !FEATURE_CORECLR
+            if (CDSCollectionETWBCLProvider.Log.IsEnabled())
+            {
+                CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPushFailed(spin.Count);
+            }
+#endif // !FEATURE_CORECLR
+        }
+
+        /// <summary>
+        /// Local helper function to validate the Pop Push range methods input
+        /// </summary>
+        private void ValidatePushPopRangeInput(T[] items, int startIndex, int count)
+        {
+            if (items == null)
+            {
+                throw new ArgumentNullException("items");
+            }
+            if (count < 0)
+            {
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ConcurrentStack_PushPopRange_CountOutOfRange"));
+            }
+            int length = items.Length;
+            if (startIndex >= length || startIndex < 0)
+            {
+                throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ConcurrentStack_PushPopRange_StartOutOfRange"));
+            }
+            if (length - count < startIndex) //instead of (startIndex + count > items.Length) to prevent overflow
+            {
+                throw new ArgumentException(Environment.GetResourceString("ConcurrentStack_PushPopRange_InvalidCount"));
+            }
+        }
+
+        /// <summary>
+        /// Attempts to add an object to the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">The object to add to the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null
+        /// reference (Nothing in Visual Basic) for reference types.
+        /// </param>
+        /// <returns>true if the object was added successfully; otherwise, false.</returns>
+        /// <remarks>For <see cref="ConcurrentStack{T}"/>, this operation
+        /// will always insert the object onto the top of the <see cref="ConcurrentStack{T}"/>
+        /// and return true.</remarks>
+        bool IProducerConsumerCollection<T>.TryAdd(T item)
+        {
+            Push(item);
+            return true;
+        }
+
+        /// <summary>
+        /// Attempts to return an object from the top of the <see cref="ConcurrentStack{T}"/>
+        /// without removing it.
+        /// </summary>
+        /// <param name="result">When this method returns, <paramref name="result"/> contains an object from
+        /// the top of the <see cref="T:System.Collections.Concurrent.ConccurrentStack{T}"/> or an
+        /// unspecified value if the operation failed.</param>
+        /// <returns>true if and object was returned successfully; otherwise, false.</returns>
+        public bool TryPeek(out T result)
+        {
+            Node head = m_head;
+
+            // If the stack is empty, return false; else return the element and true.
+            if (head == null)
+            {
+                result = default(T);
+                return false;
+            }
+            else
+            {
+                result = head.m_value;
+                return true;
+            }
+        }
+
+        /// <summary>
+        /// Attempts to pop and return the object at the top of the <see cref="ConcurrentStack{T}"/>.
+        /// </summary>
+        /// <param name="result">
+        /// When this method returns, if the operation was successful, <paramref name="result"/> contains the
+        /// object removed. If no object was available to be removed, the value is unspecified.
+        /// </param>
+        /// <returns>true if an element was removed and returned from the top of the <see
+        /// cref="ConcurrentStack{T}"/>
+        /// succesfully; otherwise, false.</returns>
+        public bool TryPop(out T result)
+        {
+            Node head = m_head;
+            //stack is empty
+            if (head == null)
+            {
+                result = default(T);
+                return false;
+            }
+            if (Interlocked.CompareExchange(ref m_head, head.m_next, head) == head)
+            {
+                result = head.m_value;
+                return true;
+            }
+
+            // Fall through to the slow path.
+            return TryPopCore(out result);
+        }
+
+        /// <summary>
+        /// Attempts to pop and return multiple objects from the top of the <see cref="ConcurrentStack{T}"/>
+        /// atomically.
+        /// </summary>
+        /// <param name="items">
+        /// The <see cref="T:System.Array"/> to which objects popped from the top of the <see
+        /// cref="ConcurrentStack{T}"/> will be added.
+        /// </param>
+        /// <returns>The number of objects successfully popped from the top of the <see
+        /// cref="ConcurrentStack{T}"/> and inserted in
+        /// <paramref name="items"/>.</returns>
+        /// <exception cref="ArgumentNullException"><paramref name="items"/> is a null argument (Nothing
+        /// in Visual Basic).</exception>
+        /// <remarks>
+        /// When popping multiple items, if there is little contention on the stack, using
+        /// TryPopRange can be more efficient than using <see cref="TryPop"/>
+        /// once per item to be removed.  Nodes fill the <paramref name="items"/>
+        /// with the first node to be popped at the startIndex, the second node to be popped
+        /// at startIndex + 1, and so on.
+        /// </remarks>
+        public int TryPopRange(T[] items)
+        {
+            if (items == null)
+            {
+                throw new ArgumentNullException("items");
+            }
+
+            return TryPopRange(items, 0, items.Length);
+        }
+
+        /// <summary>
+        /// Attempts to pop and return multiple objects from the top of the <see cref="ConcurrentStack{T}"/>
+        /// atomically.
+        /// </summary>
+        /// <param name="items">
+        /// The <see cref="T:System.Array"/> to which objects popped from the top of the <see
+        /// cref="ConcurrentStack{T}"/> will be added.
+        /// </param>
+        /// <param name="startIndex">The zero-based offset in <paramref name="items"/> at which to begin
+        /// inserting elements from the top of the <see cref="ConcurrentStack{T}"/>.</param>
+        /// <param name="count">The number of elements to be popped from top of the <see
+        /// cref="ConcurrentStack{T}"/> and inserted into <paramref name="items"/>.</param>
+        /// <returns>The number of objects successfully popped from the top of 
+        /// the <see cref="ConcurrentStack{T}"/> and inserted in <paramref name="items"/>.</returns>        
+        /// <exception cref="ArgumentNullException"><paramref name="items"/> is a null reference
+        /// (Nothing in Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="startIndex"/> or <paramref
+        /// name="count"/> is negative. Or <paramref name="startIndex"/> is greater than or equal to the length 
+        /// of <paramref name="items"/>.</exception>
+        /// <exception cref="ArgumentException"><paramref name="startIndex"/> + <paramref name="count"/> is
+        /// greater than the length of <paramref name="items"/>.</exception>
+        /// <remarks>
+        /// When popping multiple items, if there is little contention on the stack, using
+        /// TryPopRange can be more efficient than using <see cref="TryPop"/>
+        /// once per item to be removed.  Nodes fill the <paramref name="items"/>
+        /// with the first node to be popped at the startIndex, the second node to be popped
+        /// at startIndex + 1, and so on.
+        /// </remarks>
+        public int TryPopRange(T[] items, int startIndex, int count)
+        {
+            ValidatePushPopRangeInput(items, startIndex, count);
+
+            // No op if the count is zero
+            if (count == 0)
+                return 0;
+
+            Node poppedHead;
+            int nodesCount = TryPopCore(count, out poppedHead);
+            if (nodesCount > 0)
+            {
+                CopyRemovedItems(poppedHead, items, startIndex, nodesCount);
+
+            }
+            return nodesCount;
+
+        }
+
+        /// <summary>
+        /// Local helper function to Pop an item from the stack, slow path
+        /// </summary>
+        /// <param name="result">The popped item</param>
+        /// <returns>True if succeeded, false otherwise</returns>
+        private bool TryPopCore(out T result)
+        {
+            Node poppedNode;
+
+            if (TryPopCore(1, out poppedNode) == 1)
+            {
+                result = poppedNode.m_value;
+                return true;
+            }
+
+            result = default(T);
+            return false;
+
+        }
+
+        /// <summary>
+        /// Slow path helper for TryPop. This method assumes an initial attempt to pop an element
+        /// has already occurred and failed, so it begins spinning right away.
+        /// </summary>
+        /// <param name="count">The number of items to pop.</param>
+        /// <param name="poppedHead">
+        /// When this method returns, if the pop succeeded, contains the removed object. If no object was
+        /// available to be removed, the value is unspecified. This parameter is passed uninitialized.
+        /// </param>
+        /// <returns>True if an element was removed and returned; otherwise, false.</returns>
+        private int TryPopCore(int count, out Node poppedHead)
+        {
+            SpinWait spin = new SpinWait();
+
+            // Try to CAS the head with its current next.  We stop when we succeed or
+            // when we notice that the stack is empty, whichever comes first.
+            Node head;
+            Node next;
+            int backoff = 1;
+            Random r = new Random(Environment.TickCount & Int32.MaxValue); // avoid the case where TickCount could return Int32.MinValue
+            while (true)
+            {
+                head = m_head;
+                // Is the stack empty?
+                if (head == null)
+                {
+#if !FEATURE_CORECLR
+                    if (count == 1 && CDSCollectionETWBCLProvider.Log.IsEnabled())
+                    {
+                        CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPopFailed(spin.Count);
+                    }
+#endif //!FEATURE_CORECLR
+                    poppedHead = null;
+                    return 0;
+                }
+                next = head;
+                int nodesCount = 1;
+                for (; nodesCount < count && next.m_next != null; nodesCount++)
+                {
+                    next = next.m_next;
+                }
+
+                // Try to swap the new head.  If we succeed, break out of the loop.
+                if (Interlocked.CompareExchange(ref m_head, next.m_next, head) == head)
+                {
+#if !FEATURE_CORECLR
+                    if (count == 1 && CDSCollectionETWBCLProvider.Log.IsEnabled())
+                    {
+                        CDSCollectionETWBCLProvider.Log.ConcurrentStack_FastPopFailed(spin.Count);
+                    }
+#endif //!FEATURE_CORECLR
+                    // Return the popped Node.
+                    poppedHead = head;
+                    return nodesCount;
+                }
+
+                // We failed to CAS the new head.  Spin briefly and retry.
+                for (int i = 0; i < backoff; i++)
+                {
+                    spin.SpinOnce();
+                }
+
+                backoff = spin.NextSpinWillYield ? r.Next(1, BACKOFF_MAX_YIELDS) : backoff * 2;
+            }
+        }
+
+
+        /// <summary>
+        /// Local helper function to copy the poped elements into a given collection
+        /// </summary>
+        /// <param name="head">The head of the list to be copied</param>
+        /// <param name="collection">The collection to place the popped items in</param>
+        /// <param name="startIndex">the beginning of index of where to place the popped items</param>
+        /// <param name="nodesCount">The number of nodes.</param>
+        private void CopyRemovedItems(Node head, T[] collection, int startIndex, int nodesCount)
+        {
+            Node current = head;
+            for (int i = startIndex; i < startIndex + nodesCount; i++)
+            {
+                collection[i] = current.m_value;
+                current = current.m_next;
+            }
+
+        }
+
+        /// <summary>
+        /// Attempts to remove and return an object from the <see
+        /// cref="T:System.Collections.Concurrent.IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">
+        /// When this method returns, if the operation was successful, <paramref name="item"/> contains the
+        /// object removed. If no object was available to be removed, the value is unspecified.
+        /// </param>
+        /// <returns>true if an element was removed and returned succesfully; otherwise, false.</returns>
+        /// <remarks>For <see cref="ConcurrentStack{T}"/>, this operation will attempt to pope the object at
+        /// the top of the <see cref="ConcurrentStack{T}"/>.
+        /// </remarks>
+        bool IProducerConsumerCollection<T>.TryTake(out T item)
+        {
+            return TryPop(out item);
+        }
+
+        /// <summary>
+        /// Copies the items stored in the <see cref="ConcurrentStack{T}"/> to a new array.
+        /// </summary>
+        /// <returns>A new array containing a snapshot of elements copied from the <see
+        /// cref="ConcurrentStack{T}"/>.</returns>
+        public T[] ToArray()
+        {
+            return ToList().ToArray();
+        }
+
+        /// <summary>
+        /// Returns an array containing a snapshot of the list's contents, using
+        /// the target list node as the head of a region in the list.
+        /// </summary>
+        /// <returns>An array of the list's contents.</returns>
+        private List<T> ToList()
+        {
+            List<T> list = new List<T>();
+
+            Node curr = m_head;
+            while (curr != null)
+            {
+                list.Add(curr.m_value);
+                curr = curr.m_next;
+            }
+
+            return list;
+        }
+
+        /// <summary>
+        /// Returns an enumerator that iterates through the <see cref="ConcurrentStack{T}"/>.
+        /// </summary>
+        /// <returns>An enumerator for the <see cref="ConcurrentStack{T}"/>.</returns>
+        /// <remarks>
+        /// The enumeration represents a moment-in-time snapshot of the contents
+        /// of the stack.  It does not reflect any updates to the collection after 
+        /// <see cref="GetEnumerator"/> was called.  The enumerator is safe to use
+        /// concurrently with reads from and writes to the stack.
+        /// </remarks>
+        public IEnumerator<T> GetEnumerator()
+        {
+            // Returns an enumerator for the stack. This effectively takes a snapshot
+            // of the stack's contents at the time of the call, i.e. subsequent modifications
+            // (pushes or pops) will not be reflected in the enumerator's contents.
+
+            //If we put yield-return here, the iterator will be lazily evaluated. As a result a snapshot of
+            //the stack is not taken when GetEnumerator is initialized but when MoveNext() is first called.
+            //This is inconsistent with existing generic collections. In order to prevent it, we capture the 
+            //value of m_head in a buffer and call out to a helper method
+            return GetEnumerator(m_head);
+        }
+
+        private IEnumerator<T> GetEnumerator(Node head)
+        {
+            Node current = head;
+            while (current != null)
+            {
+                yield return current.m_value;
+                current = current.m_next;
+            }
+        }
+
+        /// <summary>
+        /// Returns an enumerator that iterates through a collection.
+        /// </summary>
+        /// <returns>An <see cref="T:System.Collections.IEnumerator"/> that can be used to iterate through
+        /// the collection.</returns>
+        /// <remarks>
+        /// The enumeration represents a moment-in-time snapshot of the contents of the stack. It does not
+        /// reflect any updates to the collection after
+        /// <see cref="GetEnumerator"/> was called. The enumerator is safe to use concurrently with reads
+        /// from and writes to the stack.
+        /// </remarks>
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            return ((IEnumerable<T>)this).GetEnumerator();
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/IProducerConsumerCollection.cs b/src/mscorlib/src/System/Collections/Concurrent/IProducerConsumerCollection.cs
new file mode 100644
index 0000000000..a74f69069a
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/IProducerConsumerCollection.cs
@@ -0,0 +1,116 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// A common interface for all concurrent collections.
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics;
+
+namespace System.Collections.Concurrent
+{
+
+    /// <summary>
+    /// Defines methods to manipulate thread-safe collections intended for producer/consumer usage.
+    /// </summary>
+    /// <typeparam name="T">Specifies the type of elements in the collection.</typeparam>
+    /// <remarks>
+    /// All implementations of this interface must enable all members of this interface
+    /// to be used concurrently from multiple threads.
+    /// </remarks>
+    public interface IProducerConsumerCollection<T> : IEnumerable<T>, ICollection
+    {
+
+        /// <summary>
+        /// Copies the elements of the <see cref="IProducerConsumerCollection{T}"/> to
+        /// an
+        /// <see cref="T:System.Array"/>, starting at a specified index.
+        /// </summary>
+        /// <param name="array">The one-dimensional <see cref="T:System.Array"/> that is the destination of
+        /// the elements copied from the <see cref="IProducerConsumerCollection{T}"/>.
+        /// The array must have zero-based indexing.</param>
+        /// <param name="index">The zero-based index in <paramref name="array"/> at which copying
+        /// begins.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in
+        /// Visual Basic).</exception>
+        /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than
+        /// zero.</exception>
+        /// <exception cref="ArgumentException"><paramref name="index"/> is equal to or greater than the
+        /// length of the <paramref name="array"/>
+        /// -or- The number of elements in the source <see cref="ConcurrentQueue{T}"/> is greater than the
+        /// available space from <paramref name="index"/> to the end of the destination <paramref
+        /// name="array"/>.
+        /// </exception>
+        void CopyTo(T[] array, int index);
+
+        /// <summary>
+        /// Attempts to add an object to the <see
+        /// cref="IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">The object to add to the <see
+        /// cref="IProducerConsumerCollection{T}"/>.</param>
+        /// <returns>true if the object was added successfully; otherwise, false.</returns>
+        /// <exception cref="T:System.ArgumentException">The <paramref name="item"/> was invalid for this collection.</exception>
+        bool TryAdd(T item);
+
+        /// <summary>
+        /// Attempts to remove and return an object from the <see cref="IProducerConsumerCollection{T}"/>.
+        /// </summary>
+        /// <param name="item">
+        /// When this method returns, if the object was removed and returned successfully, <paramref
+        /// name="item"/> contains the removed object. If no object was available to be removed, the value is
+        /// unspecified.
+        /// </param>
+        /// <returns>true if an object was removed and returned successfully; otherwise, false.</returns>
+        bool TryTake(out T item);
+
+        /// <summary>
+        /// Copies the elements contained in the <see cref="IProducerConsumerCollection{T}"/> to a new array.
+        /// </summary>
+        /// <returns>A new array containing the elements copied from the <see cref="IProducerConsumerCollection{T}"/>.</returns>
+        T[] ToArray();
+
+    }
+
+
+    /// <summary>
+    /// A debugger view of the IProducerConsumerCollection that makes it simple to browse the
+    /// collection's contents at a point in time.
+    /// </summary>
+    /// <typeparam name="T">The type of elements stored within.</typeparam>
+    internal sealed class SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView<T>
+    {
+        private IProducerConsumerCollection<T> m_collection; // The collection being viewed.
+
+        /// <summary>
+        /// Constructs a new debugger view object for the provided collection object.
+        /// </summary>
+        /// <param name="collection">A collection to browse in the debugger.</param>
+        public SystemCollectionsConcurrent_ProducerConsumerCollectionDebugView(IProducerConsumerCollection<T> collection)
+        {
+            if (collection == null)
+            {
+                throw new ArgumentNullException("collection");
+            }
+
+            m_collection = collection;
+        }
+
+        /// <summary>
+        /// Returns a snapshot of the underlying collection's elements.
+        /// </summary>
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public T[] Items
+        {
+            get { return m_collection.ToArray(); }
+        }
+
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/OrderablePartitioner.cs b/src/mscorlib/src/System/Collections/Concurrent/OrderablePartitioner.cs
new file mode 100644
index 0000000000..02263b7f97
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/OrderablePartitioner.cs
@@ -0,0 +1,281 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// 
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System;
+using System.Collections.Generic;
+using System.Security.Permissions;
+using System.Threading;
+
+namespace System.Collections.Concurrent
+{
+
+    /// <summary>
+    /// Represents a particular manner of splitting an orderable data source into multiple partitions.
+    /// </summary>
+    /// <typeparam name="TSource">Type of the elements in the collection.</typeparam>
+    /// <remarks>
+    /// <para>
+    /// Each element in each partition has an integer index associated with it, which determines the relative
+    /// order of that element against elements in other partitions.
+    /// </para>
+    /// <para>
+    /// Inheritors of <see cref="OrderablePartitioner{TSource}"/> must adhere to the following rules:
+    /// <ol>
+    /// <li>All indices must be unique, such that there may not be duplicate indices. If all indices are not
+    /// unique, the output ordering may be scrambled.</li>
+    /// <li>All indices must be non-negative. If any indices are negative, consumers of the implementation
+    /// may throw exceptions.</li>
+    /// <li><see cref="GetPartitions"/> and <see cref="GetOrderablePartitions"/> should throw a
+    /// <see cref="T:System.ArgumentOutOfRangeException"/> if the requested partition count is less than or
+    /// equal to zero.</li>
+    /// <li><see cref="GetPartitions"/> and <see cref="GetOrderablePartitions"/> should always return a number
+    /// of enumerables equal to the requested partition count. If the partitioner runs out of data and cannot
+    /// create as many partitions as requested, an empty enumerator should be returned for each of the
+    /// remaining partitions. If this rule is not followed, consumers of the implementation may throw a <see
+    /// cref="T:System.InvalidOperationException"/>.</li>
+    /// <li><see cref="GetPartitions"/>, <see cref="GetOrderablePartitions"/>,
+    /// <see cref="GetDynamicPartitions"/>, and <see cref="GetOrderableDynamicPartitions"/>
+    /// should never return null. If null is returned, a consumer of the implementation may throw a
+    /// <see cref="T:System.InvalidOperationException"/>.</li>
+    /// <li><see cref="GetPartitions"/>, <see cref="GetOrderablePartitions"/>,
+    /// <see cref="GetDynamicPartitions"/>, and <see cref="GetOrderableDynamicPartitions"/>
+    /// should always return partitions that can fully and uniquely enumerate the input data source. All of
+    /// the data and only the data contained in the input source should be enumerated, with no duplication
+    /// that was not already in the input, unless specifically required by the particular partitioner's
+    /// design. If this is not followed, the output ordering may be scrambled.</li>
+    /// <li>If <see cref="KeysOrderedInEachPartition"/> returns true, each partition must return elements
+    /// with increasing key indices.</li>
+    /// <li>If <see cref="KeysOrderedAcrossPartitions"/> returns true, all the keys in partition numbered N
+    /// must be larger than all the keys in partition numbered N-1.</li>
+    /// <li>If <see cref="KeysNormalized"/> returns true, all indices must be monotonically increasing from
+    /// 0, though not necessarily within a single partition.</li>
+    /// </ol>
+    /// </para>
+    /// </remarks>
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+    public abstract class OrderablePartitioner<TSource> : Partitioner<TSource>
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="OrderablePartitioner{TSource}"/> class with the
+        /// specified constraints on the index keys.
+        /// </summary>
+        /// <param name="keysOrderedInEachPartition">
+        /// Indicates whether the elements in each partition are yielded in the order of
+        /// increasing keys.
+        /// </param>
+        /// <param name="keysOrderedAcrossPartitions">
+        /// Indicates whether elements in an earlier partition always come before
+        /// elements in a later partition. If true, each element in partition 0 has a smaller order key than
+        /// any element in partition 1, each element in partition 1 has a smaller order key than any element
+        /// in partition 2, and so on.
+        /// </param>
+        /// <param name="keysNormalized">
+        /// Indicates whether keys are normalized. If true, all order keys are distinct
+        /// integers in the range [0 .. numberOfElements-1]. If false, order keys must still be dictinct, but
+        /// only their relative order is considered, not their absolute values.
+        /// </param>
+        protected OrderablePartitioner(bool keysOrderedInEachPartition, bool keysOrderedAcrossPartitions, bool keysNormalized)
+        {
+            KeysOrderedInEachPartition = keysOrderedInEachPartition;
+            KeysOrderedAcrossPartitions = keysOrderedAcrossPartitions;
+            KeysNormalized = keysNormalized;
+        }
+
+        /// <summary>
+        /// Partitions the underlying collection into the specified number of orderable partitions.
+        /// </summary>
+        /// <remarks>
+        /// Each partition is represented as an enumerator over key-value pairs.
+        /// The value of the pair is the element itself, and the key is an integer which determines
+        /// the relative ordering of this element against other elements in the data source.
+        /// </remarks>
+        /// <param name="partitionCount">The number of partitions to create.</param>
+        /// <returns>A list containing <paramref name="partitionCount"/> enumerators.</returns>
+        public abstract IList<IEnumerator<KeyValuePair<long, TSource>>> GetOrderablePartitions(int partitionCount);
+
+        /// <summary>
+        /// Creates an object that can partition the underlying collection into a variable number of
+        /// partitions.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// The returned object implements the <see
+        /// cref="T:System.Collections.Generic.IEnumerable{TSource}"/> interface. Calling <see
+        /// cref="System.Collections.Generic.IEnumerable{TSource}.GetEnumerator">GetEnumerator</see> on the
+        /// object creates another partition over the sequence.
+        /// </para>
+        /// <para>
+        /// Each partition is represented as an enumerator over key-value pairs. The value in the pair is the element
+        /// itself, and the key is an integer which determines the relative ordering of this element against
+        /// other elements.
+        /// </para>
+        /// <para>
+        /// The <see cref="GetOrderableDynamicPartitions"/> method is only supported if the <see
+        /// cref="System.Collections.Concurrent.Partitioner{TSource}.SupportsDynamicPartitions">SupportsDynamicPartitions</see>
+        /// property returns true.
+        /// </para>
+        /// </remarks>
+        /// <returns>An object that can create partitions over the underlying data source.</returns>
+        /// <exception cref="NotSupportedException">Dynamic partitioning is not supported by this
+        /// partitioner.</exception>
+        public virtual IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions()
+        {
+            throw new NotSupportedException(Environment.GetResourceString("Partitioner_DynamicPartitionsNotSupported"));
+        }
+
+        /// <summary>
+        /// Gets whether elements in each partition are yielded in the order of increasing keys.
+        /// </summary>
+        public bool KeysOrderedInEachPartition { get; private set; }
+
+        /// <summary>
+        /// Gets whether elements in an earlier partition always come before elements in a later partition.
+        /// </summary>
+        /// <remarks>
+        /// If <see cref="KeysOrderedAcrossPartitions"/> returns true, each element in partition 0 has a
+        /// smaller order key than any element in partition 1, each element in partition 1 has a smaller
+        /// order key than any element in partition 2, and so on.
+        /// </remarks>
+        public bool KeysOrderedAcrossPartitions { get; private set; }
+
+        /// <summary>
+        /// Gets whether order keys are normalized.
+        /// </summary>
+        /// <remarks>
+        /// If <see cref="KeysNormalized"/> returns true, all order keys are distinct integers in the range
+        /// [0 .. numberOfElements-1]. If the property returns false, order keys must still be dictinct, but
+        /// only their relative order is considered, not their absolute values.
+        /// </remarks>
+        public bool KeysNormalized { get; private set; }
+
+        /// <summary>
+        /// Partitions the underlying collection into the given number of ordered partitions.
+        /// </summary>
+        /// <remarks>
+        /// The default implementation provides the same behavior as <see cref="GetOrderablePartitions"/> except
+        /// that the returned set of partitions does not provide the keys for the elements.
+        /// </remarks>
+        /// <param name="partitionCount">The number of partitions to create.</param>
+        /// <returns>A list containing <paramref name="partitionCount"/> enumerators.</returns>
+        public override IList<IEnumerator<TSource>> GetPartitions(int partitionCount)
+        {
+            IList<IEnumerator<KeyValuePair<long, TSource>>> orderablePartitions = GetOrderablePartitions(partitionCount);
+
+            if (orderablePartitions.Count != partitionCount)
+            {
+                throw new InvalidOperationException("OrderablePartitioner_GetPartitions_WrongNumberOfPartitions");
+            }
+
+            IEnumerator<TSource>[] partitions = new IEnumerator<TSource>[partitionCount];
+            for (int i = 0; i < partitionCount; i++)
+            {
+                partitions[i] = new EnumeratorDropIndices(orderablePartitions[i]);
+            }
+            return partitions;
+        }
+
+        /// <summary>
+        /// Creates an object that can partition the underlying collection into a variable number of
+        /// partitions.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// The returned object implements the <see
+        /// cref="T:System.Collections.Generic.IEnumerable{TSource}"/> interface. Calling <see
+        /// cref="System.Collections.Generic.IEnumerable{TSource}.GetEnumerator">GetEnumerator</see> on the
+        /// object creates another partition over the sequence.
+        /// </para>
+        /// <para>
+        /// The default implementation provides the same behavior as <see cref="GetOrderableDynamicPartitions"/> except
+        /// that the returned set of partitions does not provide the keys for the elements.
+        /// </para>
+        /// <para>
+        /// The <see cref="GetDynamicPartitions"/> method is only supported if the <see
+        /// cref="System.Collections.Concurrent.Partitioner{TSource}.SupportsDynamicPartitions"/>
+        /// property returns true.
+        /// </para>
+        /// </remarks>
+        /// <returns>An object that can create partitions over the underlying data source.</returns>
+        /// <exception cref="NotSupportedException">Dynamic partitioning is not supported by this
+        /// partitioner.</exception>
+        public override IEnumerable<TSource> GetDynamicPartitions()
+        {
+            IEnumerable<KeyValuePair<long, TSource>> orderablePartitions = GetOrderableDynamicPartitions();
+            return new EnumerableDropIndices(orderablePartitions);
+        }
+
+        /// <summary>
+        /// Converts an enumerable over key-value pairs to an enumerable over values.
+        /// </summary>
+        private class EnumerableDropIndices : IEnumerable<TSource>, IDisposable
+        {
+            private readonly IEnumerable<KeyValuePair<long, TSource>> m_source;
+            public EnumerableDropIndices(IEnumerable<KeyValuePair<long, TSource>> source)
+            {
+                m_source = source;
+            }
+            public IEnumerator<TSource> GetEnumerator()
+            {
+                return new EnumeratorDropIndices(m_source.GetEnumerator());
+            }
+            IEnumerator IEnumerable.GetEnumerator()
+            {
+                return ((EnumerableDropIndices)this).GetEnumerator();
+            }
+            public void Dispose()
+            {
+                IDisposable d = m_source as IDisposable;
+                if (d != null)
+                {
+                    d.Dispose();
+                }
+            }
+        }
+
+        private class EnumeratorDropIndices : IEnumerator<TSource>
+        {
+            private readonly IEnumerator<KeyValuePair<long, TSource>> m_source;
+            public EnumeratorDropIndices(IEnumerator<KeyValuePair<long, TSource>> source)
+            {
+                m_source = source;
+            }
+            public bool MoveNext()
+            {
+                return m_source.MoveNext();
+            }
+            public TSource Current
+            {
+                get
+                {
+                    return m_source.Current.Value;
+                }
+            }
+            Object IEnumerator.Current
+            {
+                get
+                {
+                    return ((EnumeratorDropIndices)this).Current;
+                }
+            }
+            public void Dispose()
+            {
+                m_source.Dispose();
+            }
+            public void Reset()
+            {
+                m_source.Reset();
+            }
+        }
+
+    }
+
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/Partitioner.cs b/src/mscorlib/src/System/Collections/Concurrent/Partitioner.cs
new file mode 100644
index 0000000000..3d54c1471b
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/Partitioner.cs
@@ -0,0 +1,102 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// Represents a particular way of splitting a collection into multiple partitions.
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System;
+using System.Collections.Generic;
+using System.Security.Permissions;
+using System.Threading;
+
+namespace System.Collections.Concurrent
+{
+    /// <summary>
+    /// Represents a particular manner of splitting a data source into multiple partitions.
+    /// </summary>
+    /// <typeparam name="TSource">Type of the elements in the collection.</typeparam>
+    /// <remarks>
+    /// <para>
+    /// Inheritors of <see cref="Partitioner{TSource}"/> must adhere to the following rules:
+    /// <ol>
+    /// <li><see cref="GetPartitions"/> should throw a
+    /// <see cref="T:System.ArgumentOutOfRangeException"/> if the requested partition count is less than or
+    /// equal to zero.</li>
+    /// <li><see cref="GetPartitions"/> should always return a number of enumerables equal to the requested
+    /// partition count. If the partitioner runs out of data and cannot create as many partitions as 
+    /// requested, an empty enumerator should be returned for each of the remaining partitions. If this rule
+    /// is not followed, consumers of the implementation may throw a <see
+    /// cref="T:System.InvalidOperationException"/>.</li>
+    /// <li><see cref="GetPartitions"/> and <see cref="GetDynamicPartitions"/>
+    /// should never return null. If null is returned, a consumer of the implementation may throw a
+    /// <see cref="T:System.InvalidOperationException"/>.</li>
+    /// <li><see cref="GetPartitions"/> and <see cref="GetDynamicPartitions"/> should always return
+    /// partitions that can fully and uniquely enumerate the input data source. All of the data and only the
+    /// data contained in the input source should be enumerated, with no duplication that was not already in
+    /// the input, unless specifically required by the particular partitioner's design. If this is not
+    /// followed, the output ordering may be scrambled.</li>
+    /// </ol>
+    /// </para>
+    /// </remarks>
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+    public abstract class Partitioner<TSource>
+    {
+        /// <summary>
+        /// Partitions the underlying collection into the given number of partitions.
+        /// </summary>
+        /// <param name="partitionCount">The number of partitions to create.</param>
+        /// <returns>A list containing <paramref name="partitionCount"/> enumerators.</returns>
+        public abstract IList<IEnumerator<TSource>> GetPartitions(int partitionCount);
+
+        /// <summary>
+        /// Gets whether additional partitions can be created dynamically.
+        /// </summary>
+        /// <returns>
+        /// true if the <see cref="Partitioner{TSource}"/> can create partitions dynamically as they are
+        /// requested; false if the <see cref="Partitioner{TSource}"/> can only allocate
+        /// partitions statically.
+        /// </returns>
+        /// <remarks>
+        /// <para>
+        /// If a derived class does not override and implement <see cref="GetDynamicPartitions"/>,
+        /// <see cref="SupportsDynamicPartitions"/> should return false. The value of <see
+        /// cref="SupportsDynamicPartitions"/> should not vary over the lifetime of this instance.
+        /// </para>
+        /// </remarks>
+        public virtual bool SupportsDynamicPartitions
+        {
+            get { return false; }
+        }
+
+        /// <summary>
+        /// Creates an object that can partition the underlying collection into a variable number of
+        /// partitions.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// The returned object implements the <see
+        /// cref="T:System.Collections.Generic.IEnumerable{TSource}"/> interface. Calling <see
+        /// cref="System.Collections.Generic.IEnumerable{TSource}.GetEnumerator">GetEnumerator</see> on the
+        /// object creates another partition over the sequence.
+        /// </para>
+        /// <para>
+        /// The <see cref="GetDynamicPartitions"/> method is only supported if the <see
+        /// cref="SupportsDynamicPartitions"/>
+        /// property returns true.
+        /// </para>
+        /// </remarks>
+        /// <returns>An object that can create partitions over the underlying data source.</returns>
+        /// <exception cref="NotSupportedException">Dynamic partitioning is not supported by this
+        /// partitioner.</exception>
+        public virtual IEnumerable<TSource> GetDynamicPartitions()
+        {
+            throw new NotSupportedException(Environment.GetResourceString("Partitioner_DynamicPartitionsNotSupported"));
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Concurrent/PartitionerStatic.cs b/src/mscorlib/src/System/Collections/Concurrent/PartitionerStatic.cs
new file mode 100644
index 0000000000..2169c6dee7
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Concurrent/PartitionerStatic.cs
@@ -0,0 +1,1733 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+#pragma warning disable 0420
+
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+//
+//
+// A class of default partitioners for Partitioner<TSource>
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+using System.Collections.Generic;
+using System.Security.Permissions;
+using System.Threading;
+using System.Diagnostics.Contracts;
+using System.Runtime.InteropServices;
+
+namespace System.Collections.Concurrent
+{
+    /// <summary>
+    /// Out-of-the-box partitioners are created with a set of default behaviors.  
+    /// For example, by default, some form of buffering and chunking will be employed to achieve 
+    /// optimal performance in the common scenario where an IEnumerable<> implementation is fast and 
+    /// non-blocking.  These behaviors can be overridden via this enumeration.
+    /// </summary>
+    [Flags]
+#if !FEATURE_CORECLR
+    [Serializable]
+#endif
+    public enum EnumerablePartitionerOptions
+    {
+        /// <summary>
+        /// Use the default behavior (i.e., use buffering to achieve optimal performance)
+        /// </summary>
+        None = 0x0,
+
+        /// <summary>
+        /// Creates a partitioner that will take items from the source enumerable one at a time
+        /// and will not use intermediate storage that can be accessed more efficiently by multiple threads.  
+        /// This option provides support for low latency (items will be processed as soon as they are available from 
+        /// the source) and partial support for dependencies between items (a thread cannot deadlock waiting for an item 
+        /// that it, itself, is responsible for processing).
+        /// </summary>
+        NoBuffering = 0x1
+    }
+
+    // The static class Partitioners implements 3 default partitioning strategies:
+    // 1. dynamic load balance partitioning for indexable data source (IList and arrays)
+    // 2. static partitioning for indexable data source (IList and arrays)
+    // 3. dynamic load balance partitioning for enumerables. Enumerables have indexes, which are the natural order
+    //    of elements, but enuemrators are not indexable 
+    // - data source of type IList/arrays have both dynamic and static partitioning, as 1 and 3.
+    //   We assume that the source data of IList/Array is not changing concurrently.
+    // - data source of type IEnumerable can only be partitioned dynamically (load-balance)
+    // - Dynamic partitioning methods 1 and 3 are same, both being dynamic and load-balance. But the 
+    //   implementation is different for data source of IList/Array vs. IEnumerable:
+    //   * When the source collection is IList/Arrays, we use Interlocked on the shared index; 
+    //   * When the source collection is IEnumerable, we use Monitor to wrap around the access to the source 
+    //     enumerator.
+
+    /// <summary>
+    /// Provides common partitioning strategies for arrays, lists, and enumerables.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The static methods on <see cref="Partitioner"/> are all thread-safe and may be used concurrently
+    /// from multiple threads. However, while a created partitioner is in use, the underlying data source
+    /// should not be modified, whether from the same thread that's using a partitioner or from a separate
+    /// thread.
+    /// </para>
+    /// </remarks>
+    [HostProtection(Synchronization = true, ExternalThreading = true)]
+    public static class Partitioner
+    {
+        /// <summary>
+        /// Creates an orderable partitioner from an <see cref="System.Collections.Generic.IList{T}"/>
+        /// instance.
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in source list.</typeparam>
+        /// <param name="list">The list to be partitioned.</param>
+        /// <param name="loadBalance">
+        /// A Boolean value that indicates whether the created partitioner should dynamically
+        /// load balance between partitions rather than statically partition.
+        /// </param>
+        /// <returns>
+        /// An orderable partitioner based on the input list.
+        /// </returns>
+        public static OrderablePartitioner<TSource> Create<TSource>(IList<TSource> list, bool loadBalance)
+        {
+            if (list == null)
+            {
+                throw new ArgumentNullException("list");
+            }
+            if (loadBalance)
+            {
+                return (new DynamicPartitionerForIList<TSource>(list));
+            }
+            else
+            {
+                return (new StaticIndexRangePartitionerForIList<TSource>(list));
+            }
+        }
+
+        /// <summary>
+        /// Creates an orderable partitioner from a <see cref="System.Array"/> instance.
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in source array.</typeparam>
+        /// <param name="array">The array to be partitioned.</param>
+        /// <param name="loadBalance">
+        /// A Boolean value that indicates whether the created partitioner should dynamically load balance
+        /// between partitions rather than statically partition.
+        /// </param>
+        /// <returns>
+        /// An orderable partitioner based on the input array.
+        /// </returns>
+        public static OrderablePartitioner<TSource> Create<TSource>(TSource[] array, bool loadBalance)
+        {
+            // This implementation uses 'ldelem' instructions for element retrieval, rather than using a
+            // method call.
+
+            if (array == null)
+            {
+                throw new ArgumentNullException("array");
+            }
+            if (loadBalance)
+            {
+                return (new DynamicPartitionerForArray<TSource>(array));
+            }
+            else
+            {
+                return (new StaticIndexRangePartitionerForArray<TSource>(array));
+            }
+        }
+
+        /// <summary>
+        /// Creates an orderable partitioner from a <see cref="System.Collections.Generic.IEnumerable{TSource}"/> instance.
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in source enumerable.</typeparam>
+        /// <param name="source">The enumerable to be partitioned.</param>
+        /// <returns>
+        /// An orderable partitioner based on the input array.
+        /// </returns>
+        /// <remarks>
+        /// The ordering used in the created partitioner is determined by the natural order of the elements 
+        /// as retrieved from the source enumerable.
+        /// </remarks>
+        public static OrderablePartitioner<TSource> Create<TSource>(IEnumerable<TSource> source)
+        {
+            return Create<TSource>(source, EnumerablePartitionerOptions.None);
+        }
+
+        /// <summary>
+        /// Creates an orderable partitioner from a <see cref="System.Collections.Generic.IEnumerable{TSource}"/> instance.
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in source enumerable.</typeparam>
+        /// <param name="source">The enumerable to be partitioned.</param>
+        /// <param name="partitionerOptions">Options to control the buffering behavior of the partitioner.</param>
+        /// <exception cref="T:System.ArgumentOutOfRangeException">
+        /// The <paramref name="partitionerOptions"/> argument specifies an invalid value for <see
+        /// cref="T:System.Collections.Concurrent.EnumerablePartitionerOptions"/>.
+        /// </exception>
+        /// <returns>
+        /// An orderable partitioner based on the input array.
+        /// </returns>
+        /// <remarks>
+        /// The ordering used in the created partitioner is determined by the natural order of the elements 
+        /// as retrieved from the source enumerable.
+        /// </remarks>
+        public static OrderablePartitioner<TSource> Create<TSource>(IEnumerable<TSource> source, EnumerablePartitionerOptions partitionerOptions)
+        {
+            if (source == null)
+            {
+                throw new ArgumentNullException("source");
+            }
+
+            if ((partitionerOptions & (~EnumerablePartitionerOptions.NoBuffering)) != 0)
+                throw new ArgumentOutOfRangeException("partitionerOptions");
+
+            return (new DynamicPartitionerForIEnumerable<TSource>(source, partitionerOptions));
+        }
+
+        /// <summary>Creates a partitioner that chunks the user-specified range.</summary>
+        /// <param name="fromInclusive">The lower, inclusive bound of the range.</param>
+        /// <param name="toExclusive">The upper, exclusive bound of the range.</param>
+        /// <returns>A partitioner.</returns>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="toExclusive"/> argument is 
+        /// less than or equal to the <paramref name="fromInclusive"/> argument.</exception>
+        public static OrderablePartitioner<Tuple<long, long>> Create(long fromInclusive, long toExclusive)
+        {
+            // How many chunks do we want to divide the range into?  If this is 1, then the
+            // answer is "one chunk per core".  Generally, though, you'll achieve better
+            // load balancing on a busy system if you make it higher than 1.
+            int coreOversubscriptionRate = 3;
+
+            if (toExclusive <= fromInclusive) throw new ArgumentOutOfRangeException("toExclusive");
+            long rangeSize = (toExclusive - fromInclusive) /
+                (PlatformHelper.ProcessorCount * coreOversubscriptionRate);
+            if (rangeSize == 0) rangeSize = 1;
+            return Partitioner.Create(CreateRanges(fromInclusive, toExclusive, rangeSize), EnumerablePartitionerOptions.NoBuffering); // chunk one range at a time
+        }
+
+        /// <summary>Creates a partitioner that chunks the user-specified range.</summary>
+        /// <param name="fromInclusive">The lower, inclusive bound of the range.</param>
+        /// <param name="toExclusive">The upper, exclusive bound of the range.</param>
+        /// <param name="rangeSize">The size of each subrange.</param>
+        /// <returns>A partitioner.</returns>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="toExclusive"/> argument is 
+        /// less than or equal to the <paramref name="fromInclusive"/> argument.</exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="rangeSize"/> argument is 
+        /// less than or equal to 0.</exception>
+        public static OrderablePartitioner<Tuple<long, long>> Create(long fromInclusive, long toExclusive, long rangeSize)
+        {
+            if (toExclusive <= fromInclusive) throw new ArgumentOutOfRangeException("toExclusive");
+            if (rangeSize <= 0) throw new ArgumentOutOfRangeException("rangeSize");
+            return Partitioner.Create(CreateRanges(fromInclusive, toExclusive, rangeSize), EnumerablePartitionerOptions.NoBuffering); // chunk one range at a time
+        }
+
+        // Private method to parcel out range tuples.
+        private static IEnumerable<Tuple<long, long>> CreateRanges(long fromInclusive, long toExclusive, long rangeSize)
+        {
+            // Enumerate all of the ranges
+            long from, to;
+            bool shouldQuit = false;
+
+            for (long i = fromInclusive; (i < toExclusive) && !shouldQuit; i += rangeSize)
+            {
+                from = i;
+                try { checked { to = i + rangeSize; } }
+                catch (OverflowException)
+                {
+                    to = toExclusive;
+                    shouldQuit = true;
+                }
+                if (to > toExclusive) to = toExclusive;
+                yield return new Tuple<long, long>(from, to);
+            }
+        }
+
+        /// <summary>Creates a partitioner that chunks the user-specified range.</summary>
+        /// <param name="fromInclusive">The lower, inclusive bound of the range.</param>
+        /// <param name="toExclusive">The upper, exclusive bound of the range.</param>
+        /// <returns>A partitioner.</returns>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="toExclusive"/> argument is 
+        /// less than or equal to the <paramref name="fromInclusive"/> argument.</exception>
+        public static OrderablePartitioner<Tuple<int, int>> Create(int fromInclusive, int toExclusive)
+        {
+            // How many chunks do we want to divide the range into?  If this is 1, then the
+            // answer is "one chunk per core".  Generally, though, you'll achieve better
+            // load balancing on a busy system if you make it higher than 1.
+            int coreOversubscriptionRate = 3;
+
+            if (toExclusive <= fromInclusive) throw new ArgumentOutOfRangeException("toExclusive");
+            int rangeSize = (toExclusive - fromInclusive) /
+                (PlatformHelper.ProcessorCount * coreOversubscriptionRate);
+            if (rangeSize == 0) rangeSize = 1;
+            return Partitioner.Create(CreateRanges(fromInclusive, toExclusive, rangeSize), EnumerablePartitionerOptions.NoBuffering); // chunk one range at a time
+        }
+
+        /// <summary>Creates a partitioner that chunks the user-specified range.</summary>
+        /// <param name="fromInclusive">The lower, inclusive bound of the range.</param>
+        /// <param name="toExclusive">The upper, exclusive bound of the range.</param>
+        /// <param name="rangeSize">The size of each subrange.</param>
+        /// <returns>A partitioner.</returns>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="toExclusive"/> argument is 
+        /// less than or equal to the <paramref name="fromInclusive"/> argument.</exception>
+        /// <exception cref="T:System.ArgumentOutOfRangeException"> The <paramref name="rangeSize"/> argument is 
+        /// less than or equal to 0.</exception>
+        public static OrderablePartitioner<Tuple<int, int>> Create(int fromInclusive, int toExclusive, int rangeSize)
+        {
+            if (toExclusive <= fromInclusive) throw new ArgumentOutOfRangeException("toExclusive");
+            if (rangeSize <= 0) throw new ArgumentOutOfRangeException("rangeSize");
+            return Partitioner.Create(CreateRanges(fromInclusive, toExclusive, rangeSize), EnumerablePartitionerOptions.NoBuffering); // chunk one range at a time
+        }
+
+        // Private method to parcel out range tuples.
+        private static IEnumerable<Tuple<int, int>> CreateRanges(int fromInclusive, int toExclusive, int rangeSize)
+        {
+            // Enumerate all of the ranges
+            int from, to;
+            bool shouldQuit = false;
+
+            for (int i = fromInclusive; (i < toExclusive) && !shouldQuit; i += rangeSize)
+            {
+                from = i;
+                try { checked { to = i + rangeSize; } }
+                catch (OverflowException)
+                {
+                    to = toExclusive;
+                    shouldQuit = true;
+                }
+                if (to > toExclusive) to = toExclusive;
+                yield return new Tuple<int, int>(from, to);
+            }
+        }
+
+        #region DynamicPartitionEnumerator_Abstract class
+        /// <summary>
+        /// DynamicPartitionEnumerator_Abstract defines the enumerator for each partition for the dynamic load-balance
+        /// partitioning algorithm. 
+        /// - Partition is an enumerator of KeyValuePairs, each corresponding to an item in the data source: 
+        ///   the key is the index in the source collection; the value is the item itself.
+        /// - a set of such partitions share a reader over data source. The type of the reader is specified by
+        ///   TSourceReader. 
+        /// - each partition requests a contiguous chunk of elements at a time from the source data. The chunk 
+        ///   size is initially 1, and doubles every time until it reaches the maximum chunk size. 
+        ///   The implementation for GrabNextChunk() method has two versions: one for data source of IndexRange 
+        ///   types (IList and the array), one for data source of IEnumerable.
+        /// - The method "Reset" is not supported for any partitioning algorithm.
+        /// - The implementation for MoveNext() method is same for all dynanmic partitioners, so we provide it
+        ///   in this abstract class.
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in the data source</typeparam>
+        /// <typeparam name="TSourceReader">Type of the reader on the data source</typeparam>
+        //TSourceReader is 
+        //  - IList<TSource>, when source data is IList<TSource>, the shared reader is source data itself
+        //  - TSource[], when source data is TSource[], the shared reader is source data itself
+        //  - IEnumerator<TSource>, when source data is IEnumerable<TSource>, and the shared reader is an 
+        //    enumerator of the source data
+        private abstract class DynamicPartitionEnumerator_Abstract<TSource, TSourceReader> : IEnumerator<KeyValuePair<long, TSource>>
+        {
+            //----------------- common fields and constructor for all dynamic partitioners -----------------
+            //--- shared by all dervied class with souce data type: IList, Array, and IEnumerator
+            protected readonly TSourceReader m_sharedReader;
+
+            protected static int s_defaultMaxChunkSize = GetDefaultChunkSize<TSource>();
+
+            //deferred allocating in MoveNext() with initial value 0, to avoid false sharing
+            //we also use the fact that: (m_currentChunkSize==null) means MoveNext is never called on this enumerator 
+            protected SharedInt m_currentChunkSize;
+
+            //deferring allocation in MoveNext() with initial value -1, to avoid false sharing
+            protected SharedInt m_localOffset;
+
+            private const int CHUNK_DOUBLING_RATE = 3; // Double the chunk size every this many grabs
+            private int m_doublingCountdown; // Number of grabs remaining until chunk size doubles
+            protected readonly int m_maxChunkSize; // s_defaultMaxChunkSize unless single-chunking is requested by the caller
+
+            // m_sharedIndex shared by this set of partitions, and particularly when m_sharedReader is IEnuerable
+            // it serves as tracking of the natual order of elements in m_sharedReader
+            // the value of this field is passed in from outside (already initialized) by the constructor, 
+            protected readonly SharedLong m_sharedIndex;
+
+            protected DynamicPartitionEnumerator_Abstract(TSourceReader sharedReader, SharedLong sharedIndex)
+                : this(sharedReader, sharedIndex, false)
+            {
+            }
+
+            protected DynamicPartitionEnumerator_Abstract(TSourceReader sharedReader, SharedLong sharedIndex, bool useSingleChunking)
+            {
+                m_sharedReader = sharedReader;
+                m_sharedIndex = sharedIndex;
+                m_maxChunkSize = useSingleChunking ? 1 : s_defaultMaxChunkSize;
+            }
+
+            // ---------------- abstract method declarations --------------
+
+            /// <summary>
+            /// Abstract method to request a contiguous chunk of elements from the source collection
+            /// </summary>
+            /// <param name="requestedChunkSize">specified number of elements requested</param>
+            /// <returns>
+            /// true if we successfully reserved at least one element (up to #=requestedChunkSize) 
+            /// false if all elements in the source collection have been reserved.
+            /// </returns>
+            //GrabNextChunk does the following: 
+            //  - grab # of requestedChunkSize elements from source data through shared reader, 
+            //  - at the time of function returns, m_currentChunkSize is updated with the number of 
+            //    elements actually got assigned (<=requestedChunkSize). 
+            //  - GrabNextChunk returns true if at least one element is assigned to this partition; 
+            //    false if the shared reader already hits the last element of the source data before 
+            //    we call GrabNextChunk
+            protected abstract bool GrabNextChunk(int requestedChunkSize);
+
+            /// <summary>
+            /// Abstract property, returns whether or not the shared reader has already read the last 
+            /// element of the source data 
+            /// </summary>
+            protected abstract bool HasNoElementsLeft { get; set; }
+
+            /// <summary>
+            /// Get the current element in the current partition. Property required by IEnumerator interface
+            /// This property is abstract because the implementation is different depending on the type
+            /// of the source data: IList, Array or IEnumerable
+            /// </summary>
+            public abstract KeyValuePair<long, TSource> Current { get; }
+
+            /// <summary>
+            /// Dispose is abstract, and depends on the type of the source data:
+            /// - For source data type IList and Array, the type of the shared reader is just the dataitself.
+            ///   We don't do anything in Dispose method for IList and Array. 
+            /// - For source data type IEnumerable, the type of the shared reader is an enumerator we created.
+            ///   Thus we need to dispose this shared reader enumerator, when there is no more active partitions
+            ///   left.
+            /// </summary>
+            public abstract void Dispose();
+
+            /// <summary>
+            /// Reset on partitions is not supported
+            /// </summary>
+            public void Reset()
+            {
+                throw new NotSupportedException();
+            }
+
+
+            /// <summary>
+            /// Get the current element in the current partition. Property required by IEnumerator interface
+            /// </summary>
+            Object IEnumerator.Current
+            {
+                get
+                {
+                    return ((DynamicPartitionEnumerator_Abstract<TSource, TSourceReader>)this).Current;
+                }
+            }
+
+            /// <summary>
+            /// Moves to the next element if any.
+            /// Try current chunk first, if the current chunk do not have any elements left, then we 
+            /// attempt to grab a chunk from the source collection.
+            /// </summary>
+            /// <returns>
+            /// true if successfully moving to the next position;
+            /// false otherwise, if and only if there is no more elements left in the current chunk 
+            /// AND the source collection is exhausted. 
+            /// </returns>
+            public bool MoveNext()
+            {
+                //perform deferred allocating of the local variables. 
+                if (m_localOffset == null)
+                {
+                    Contract.Assert(m_currentChunkSize == null);
+                    m_localOffset = new SharedInt(-1);
+                    m_currentChunkSize = new SharedInt(0);
+                    m_doublingCountdown = CHUNK_DOUBLING_RATE;
+                }
+
+                if (m_localOffset.Value < m_currentChunkSize.Value - 1)
+                //attempt to grab the next element from the local chunk
+                {
+                    m_localOffset.Value++;
+                    return true;
+                }
+                else
+                //otherwise it means we exhausted the local chunk
+                //grab a new chunk from the source enumerator
+                {
+                    // The second part of the || condition is necessary to handle the case when MoveNext() is called
+                    // after a previous MoveNext call returned false.
+                    Contract.Assert(m_localOffset.Value == m_currentChunkSize.Value - 1 || m_currentChunkSize.Value == 0);
+
+                    //set the requested chunk size to a proper value
+                    int requestedChunkSize;
+                    if (m_currentChunkSize.Value == 0) //first time grabbing from source enumerator
+                    {
+                        requestedChunkSize = 1;
+                    }
+                    else if (m_doublingCountdown > 0)
+                    {
+                        requestedChunkSize = m_currentChunkSize.Value;
+                    }
+                    else
+                    {
+                        requestedChunkSize = Math.Min(m_currentChunkSize.Value * 2, m_maxChunkSize);
+                        m_doublingCountdown = CHUNK_DOUBLING_RATE; // reset
+                    }
+
+                    // Decrement your doubling countdown
+                    m_doublingCountdown--;
+
+                    Contract.Assert(requestedChunkSize > 0 && requestedChunkSize <= m_maxChunkSize);
+                    //GrabNextChunk will update the value of m_currentChunkSize
+                    if (GrabNextChunk(requestedChunkSize))
+                    {
+                        Contract.Assert(m_currentChunkSize.Value <= requestedChunkSize && m_currentChunkSize.Value > 0);
+                        m_localOffset.Value = 0;
+                        return true;
+                    }
+                    else
+                    {
+                        return false;
+                    }
+                }
+            }
+        }
+        #endregion
+
+        #region Dynamic Partitioner for source data of IEnuemrable<> type
+        /// <summary>
+        /// Inherits from DynamicPartitioners
+        /// Provides customized implementation of GetOrderableDynamicPartitions_Factory method, to return an instance
+        /// of EnumerableOfPartitionsForIEnumerator defined internally
+        /// </summary>
+        /// <typeparam name="TSource">Type of elements in the source data</typeparam>
+        private class DynamicPartitionerForIEnumerable<TSource> : OrderablePartitioner<TSource>
+        {
+            IEnumerable<TSource> m_source;
+            readonly bool m_useSingleChunking;
+
+            //constructor
+            internal DynamicPartitionerForIEnumerable(IEnumerable<TSource> source, EnumerablePartitionerOptions partitionerOptions)
+                : base(true, false, true)
+            {
+                m_source = source;
+                m_useSingleChunking = ((partitionerOptions & EnumerablePartitionerOptions.NoBuffering) != 0);
+            }
+
+            /// <summary>
+            /// Overrides OrderablePartitioner.GetOrderablePartitions.
+            /// Partitions the underlying collection into the given number of orderable partitions.
+            /// </summary>
+            /// <param name="partitionCount">number of partitions requested</param>
+            /// <returns>A list containing <paramref name="partitionCount"/> enumerators.</returns>
+            override public IList<IEnumerator<KeyValuePair<long, TSource>>> GetOrderablePartitions(int partitionCount)
+            {
+                if (partitionCount <= 0)
+                {
+                    throw new ArgumentOutOfRangeException("partitionCount");
+                }
+                IEnumerator<KeyValuePair<long, TSource>>[] partitions
+                    = new IEnumerator<KeyValuePair<long, TSource>>[partitionCount];
+
+                IEnumerable<KeyValuePair<long, TSource>> partitionEnumerable = new InternalPartitionEnumerable(m_source.GetEnumerator(), m_useSingleChunking, true);
+                for (int i = 0; i < partitionCount; i++)
+                {
+                    partitions[i] = partitionEnumerable.GetEnumerator();
+                }
+                return partitions;
+            }
+
+            /// <summary>
+            /// Overrides OrderablePartitioner.GetOrderableDyanmicPartitions
+            /// </summary>
+            /// <returns>a enumerable collection of orderable partitions</returns>
+            override public IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions()
+            {
+                return new InternalPartitionEnumerable(m_source.GetEnumerator(), m_useSingleChunking, false);
+            }
+
+            /// <summary>
+            /// Whether additional partitions can be created dynamically.
+            /// </summary>
+            override public bool SupportsDynamicPartitions
+            {
+                get { return true; }
+            }
+
+            #region Internal classes:  InternalPartitionEnumerable, InternalPartitionEnumerator
+            /// <summary>
+            /// Provides customized implementation for source data of IEnumerable
+            /// Different from the counterpart for IList/Array, this enumerable maintains several additional fields
+            /// shared by the partitions it owns, including a boolean "m_hasNoElementsLef", a shared lock, and a 
+            /// shared count "m_activePartitionCount" used to track active partitions when they were created statically
+            /// </summary>
+            private class InternalPartitionEnumerable : IEnumerable<KeyValuePair<long, TSource>>, IDisposable
+            {
+                //reader through which we access the source data
+                private readonly IEnumerator<TSource> m_sharedReader;
+                private SharedLong m_sharedIndex;//initial value -1
+
+                private volatile KeyValuePair<long, TSource>[] m_FillBuffer;  // intermediate buffer to reduce locking
+                private volatile int m_FillBufferSize;               // actual number of elements in m_FillBuffer. Will start
+                                                                    // at m_FillBuffer.Length, and might be reduced during the last refill
+                private volatile int m_FillBufferCurrentPosition;    //shared value to be accessed by Interlock.Increment only
+                private volatile int m_activeCopiers;               //number of active copiers
+
+                //fields shared by all partitions that this Enumerable owns, their allocation is deferred
+                private SharedBool m_hasNoElementsLeft; // no elements left at all.
+                private SharedBool m_sourceDepleted;    // no elements left in the enumerator, but there may be elements in the Fill Buffer
+
+                //shared synchronization lock, created by this Enumerable
+                private object m_sharedLock;//deferring allocation by enumerator
+
+                private bool m_disposed;
+
+                // If dynamic partitioning, then m_activePartitionCount == null
+                // If static partitioning, then it keeps track of active partition count
+                private SharedInt m_activePartitionCount;
+
+                // records whether or not the user has requested single-chunking behavior
+                private readonly bool m_useSingleChunking;
+
+                internal InternalPartitionEnumerable(IEnumerator<TSource> sharedReader, bool useSingleChunking, bool isStaticPartitioning)
+                {
+                    m_sharedReader = sharedReader;
+                    m_sharedIndex = new SharedLong(-1);
+                    m_hasNoElementsLeft = new SharedBool(false);
+                    m_sourceDepleted = new SharedBool(false);
+                    m_sharedLock = new object();
+                    m_useSingleChunking = useSingleChunking;
+
+                    // Only allocate the fill-buffer if single-chunking is not in effect
+                    if (!m_useSingleChunking)
+                    {
+                        // Time to allocate the fill buffer which is used to reduce the contention on the shared lock.
+                        // First pick the buffer size multiplier. We use 4 for when there are more than 4 cores, and just 1 for below. This is based on empirical evidence.
+                        int fillBufferMultiplier = (PlatformHelper.ProcessorCount > 4) ? 4 : 1;
+
+                        // and allocate the fill buffer using these two numbers
+                        m_FillBuffer = new KeyValuePair<long, TSource>[fillBufferMultiplier * Partitioner.GetDefaultChunkSize<TSource>()];
+                    }
+
+                    if (isStaticPartitioning)
+                    {
+                        // If this object is created for static partitioning (ie. via GetPartitions(int partitionCount), 
+                        // GetOrderablePartitions(int partitionCount)), we track the active partitions, in order to dispose 
+                        // this object when all the partitions have been disposed.
+                        m_activePartitionCount = new SharedInt(0);
+                    }
+                    else
+                    {
+                        // Otherwise this object is created for dynamic partitioning (ie, via GetDynamicPartitions(),
+                        // GetOrderableDynamicPartitions()), we do not need tracking. This object must be disposed
+                        // explicitly
+                        m_activePartitionCount = null;
+                    }
+                }
+
+                public IEnumerator<KeyValuePair<long, TSource>> GetEnumerator()
+                {
+                    if (m_disposed)
+                    {
+                        throw new ObjectDisposedException(Environment.GetResourceString("PartitionerStatic_CanNotCallGetEnumeratorAfterSourceHasBeenDisposed"));
+                    }
+                    else
+                    {
+                        return new InternalPartitionEnumerator(m_sharedReader, m_sharedIndex,
+                            m_hasNoElementsLeft, m_sharedLock, m_activePartitionCount, this, m_useSingleChunking);
+                    }
+                }
+
+
+                IEnumerator IEnumerable.GetEnumerator()
+                {
+                    return ((InternalPartitionEnumerable)this).GetEnumerator();
+                }
+
+
+                ///////////////////
+                //
+                // Used by GrabChunk_Buffered()
+                private void TryCopyFromFillBuffer(KeyValuePair<long, TSource>[] destArray, 
+                                                  int requestedChunkSize, 
+                                                  ref int actualNumElementsGrabbed)
+                {                    
+                    actualNumElementsGrabbed = 0;
+
+
+                    // making a local defensive copy of the fill buffer reference, just in case it gets nulled out
+                    KeyValuePair<long, TSource>[] fillBufferLocalRef = m_FillBuffer;
+                    if (fillBufferLocalRef == null) return;
+
+                    // first do a quick check, and give up if the current position is at the end
+                    // so that we don't do an unncessary pair of Interlocked.Increment / Decrement calls
+                    if (m_FillBufferCurrentPosition >= m_FillBufferSize)
+                    {                        
+                        return; // no elements in the buffer to copy from
+                    }
+
+                    // We might have a chance to grab elements from the buffer. We will know for sure 
+                    // when we do the Interlocked.Add below. 
+                    // But first we must register as a potential copier in order to make sure 
+                    // the elements we're copying from don't get overwritten by another thread 
+                    // that starts refilling the buffer right after our Interlocked.Add.
+                    Interlocked.Increment(ref m_activeCopiers);
+
+                    int endPos = Interlocked.Add(ref m_FillBufferCurrentPosition, requestedChunkSize);
+                    int beginPos = endPos - requestedChunkSize;
+
+                    if (beginPos < m_FillBufferSize)
+                    {
+                        // adjust index and do the actual copy
+                        actualNumElementsGrabbed = (endPos < m_FillBufferSize) ? endPos : m_FillBufferSize - beginPos;
+                        Array.Copy(fillBufferLocalRef, beginPos, destArray, 0, actualNumElementsGrabbed);
+                    }
+
+                    // let the record show we are no longer accessing the buffer
+                    Interlocked.Decrement(ref m_activeCopiers);
+                }
+
+                /// <summary>
+                /// This is the common entry point for consuming items from the source enumerable
+                /// </summary>
+                /// <returns>
+                /// true if we successfully reserved at least one element 
+                /// false if all elements in the source collection have been reserved.
+                /// </returns>
+                internal bool GrabChunk(KeyValuePair<long, TSource>[] destArray, int requestedChunkSize, ref int actualNumElementsGrabbed)
+                {
+                    actualNumElementsGrabbed = 0;
+
+                    if (m_hasNoElementsLeft.Value)
+                    {
+                        return false;
+                    }
+
+                    if (m_useSingleChunking)
+                    {
+                        return GrabChunk_Single(destArray, requestedChunkSize, ref actualNumElementsGrabbed);
+                    }
+                    else
+                    {
+                        return GrabChunk_Buffered(destArray, requestedChunkSize, ref actualNumElementsGrabbed);
+                    }
+                }
+
+                /// <summary>
+                /// Version of GrabChunk that grabs a single element at a time from the source enumerable
+                /// </summary>
+                /// <returns>
+                /// true if we successfully reserved an element 
+                /// false if all elements in the source collection have been reserved.
+                /// </returns>
+                internal bool GrabChunk_Single(KeyValuePair<long,TSource>[] destArray, int requestedChunkSize, ref int actualNumElementsGrabbed)
+                {
+                    Contract.Assert(m_useSingleChunking, "Expected m_useSingleChecking to be true");
+                    Contract.Assert(requestedChunkSize == 1, "Got requested chunk size of " + requestedChunkSize + " when single-chunking was on");
+                    Contract.Assert(actualNumElementsGrabbed == 0, "Expected actualNumElementsGrabbed == 0, instead it is " + actualNumElementsGrabbed);
+                    Contract.Assert(destArray.Length == 1, "Expected destArray to be of length 1, instead its length is " + destArray.Length);
+
+                    lock (m_sharedLock)
+                    {
+                        if (m_hasNoElementsLeft.Value) return false;
+
+                        try
+                        {
+                            if (m_sharedReader.MoveNext())
+                            {
+                                m_sharedIndex.Value = checked(m_sharedIndex.Value + 1);
+                                destArray[0]
+                                    = new KeyValuePair<long, TSource>(m_sharedIndex.Value,
+                                                                        m_sharedReader.Current);
+                                actualNumElementsGrabbed = 1;
+                                return true;
+                            }
+                            else
+                            {
+                                //if MoveNext() return false, we set the flag to inform other partitions
+                                m_sourceDepleted.Value = true;
+                                m_hasNoElementsLeft.Value = true;
+                                return false;
+                            }
+                        }
+                        catch
+                        {
+                            // On an exception, make sure that no additional items are hereafter enumerated
+                            m_sourceDepleted.Value = true;
+                            m_hasNoElementsLeft.Value = true;
+                            throw;
+                        }
+                    }
+                }
+
+
+
+                /// <summary>
+                /// Version of GrabChunk that uses buffering scheme to grab items out of source enumerable
+                /// </summary>
+                /// <returns>
+                /// true if we successfully reserved at least one element (up to #=requestedChunkSize) 
+                /// false if all elements in the source collection have been reserved.
+                /// </returns>
+                internal bool GrabChunk_Buffered(KeyValuePair<long,TSource>[] destArray, int requestedChunkSize, ref int actualNumElementsGrabbed)
+                {
+                    Contract.Assert(requestedChunkSize > 0);
+                    Contract.Assert(!m_useSingleChunking, "Did not expect to be in single-chunking mode");
+
+                    TryCopyFromFillBuffer(destArray, requestedChunkSize, ref actualNumElementsGrabbed);
+                    
+                    if (actualNumElementsGrabbed == requestedChunkSize)
+                    {
+                        // that was easy.
+                        return true;
+                    }
+                    else if (m_sourceDepleted.Value)
+                    {
+                        // looks like we both reached the end of the fill buffer, and the source was depleted previously
+                        // this means no more work to do for any other worker
+                        m_hasNoElementsLeft.Value = true;
+                        m_FillBuffer = null;
+                        return (actualNumElementsGrabbed > 0);
+                    }
+
+
+                    //
+                    //  now's the time to take the shared lock and enumerate
+                    //
+                    lock (m_sharedLock)
+                    {
+                        if (m_sourceDepleted.Value)
+                        {
+                            return (actualNumElementsGrabbed > 0);
+                        }
+                        
+                        try
+                        {
+                            // we need to make sure all array copiers are finished
+                            if (m_activeCopiers > 0)
+                            {                                    
+                                SpinWait sw = new SpinWait();
+                                while( m_activeCopiers > 0) sw.SpinOnce();
+                            }
+
+                            Contract.Assert(m_sharedIndex != null); //already been allocated in MoveNext() before calling GrabNextChunk
+
+                            // Now's the time to actually enumerate the source
+
+                            // We first fill up the requested # of elements in the caller's array
+                            // continue from the where TryCopyFromFillBuffer() left off
+                            for (; actualNumElementsGrabbed < requestedChunkSize; actualNumElementsGrabbed++)
+                            {
+                                if (m_sharedReader.MoveNext())
+                                {
+                                    m_sharedIndex.Value = checked(m_sharedIndex.Value + 1);
+                                    destArray[actualNumElementsGrabbed]
+                                        = new KeyValuePair<long, TSource>(m_sharedIndex.Value,
+                                                                          m_sharedReader.Current);
+                                }
+                                else
+                                {
+                                    //if MoveNext() return false, we set the flag to inform other partitions
+                                    m_sourceDepleted.Value = true;
+                                    break;
+                                }
+                            }
+
+                            // taking a local snapshot of m_FillBuffer in case some other thread decides to null out m_FillBuffer 
+                            // in the entry of this method after observing m_sourceCompleted = true
+                            var localFillBufferRef = m_FillBuffer;
+
+                            // If the big buffer seems to be depleted, we will also fill that up while we are under the lock
+                            // Note that this is the only place that m_FillBufferCurrentPosition can be reset
+                            if (m_sourceDepleted.Value == false && localFillBufferRef != null && 
+                                m_FillBufferCurrentPosition >= localFillBufferRef.Length)
+                            {
+                                for (int i = 0; i < localFillBufferRef.Length; i++)
+                                {
+                                    if( m_sharedReader.MoveNext())
+                                    {
+                                        m_sharedIndex.Value = checked(m_sharedIndex.Value + 1);
+                                        localFillBufferRef[i]
+                                            = new KeyValuePair<long, TSource>(m_sharedIndex.Value,
+                                                                              m_sharedReader.Current);
+                                    }
+                                    else
+                                    {
+                                        // No more elements left in the enumerator.
+                                        // Record this, so that the next request can skip the lock
+                                        m_sourceDepleted.Value = true;
+
+                                        // also record the current count in m_FillBufferSize
+                                        m_FillBufferSize = i;
+
+                                        // and exit the for loop so that we don't keep incrementing m_FillBufferSize
+                                        break;
+                                    }
+
+                                }
+
+                                m_FillBufferCurrentPosition = 0;
+                            }
+
+
+                        }
+                        catch
+                        {
+                            // If an exception occurs, don't let the other enumerators try to enumerate.
+                            // NOTE: this could instead throw an InvalidOperationException, but that would be unexpected 
+                            // and not helpful to the end user.  We know the root cause is being communicated already.)
+                            m_sourceDepleted.Value = true;
+                            m_hasNoElementsLeft.Value = true;
+                            throw;
+                        }
+                    }
+
+                    return (actualNumElementsGrabbed > 0);
+                }
+
+                public void Dispose()
+                {
+                    if (!m_disposed)
+                    {
+                        m_disposed = true;
+                        m_sharedReader.Dispose();
+                    }
+                }
+            }
+
+            /// <summary>
+            /// Inherits from DynamicPartitionEnumerator_Abstract directly
+            /// Provides customized implementation for: GrabNextChunk, HasNoElementsLeft, Current, Dispose
+            /// </summary>
+            private class InternalPartitionEnumerator : DynamicPartitionEnumerator_Abstract<TSource, IEnumerator<TSource>>
+            {
+                //---- fields ----
+                //cached local copy of the current chunk
+                private KeyValuePair<long, TSource>[] m_localList; //defer allocating to avoid false sharing
+
+                // the values of the following two fields are passed in from
+                // outside(already initialized) by the constructor, 
+                private readonly SharedBool m_hasNoElementsLeft;
+                private readonly object m_sharedLock;
+                private readonly SharedInt m_activePartitionCount;
+                private InternalPartitionEnumerable m_enumerable;
+
+                //constructor
+                internal InternalPartitionEnumerator(
+                    IEnumerator<TSource> sharedReader,
+                    SharedLong sharedIndex,
+                    SharedBool hasNoElementsLeft,
+                    object sharedLock,
+                    SharedInt activePartitionCount,
+                    InternalPartitionEnumerable enumerable,
+                    bool useSingleChunking)
+                    : base(sharedReader, sharedIndex, useSingleChunking)
+                {
+                    m_hasNoElementsLeft = hasNoElementsLeft;
+                    m_sharedLock = sharedLock;
+                    m_enumerable = enumerable;
+                    m_activePartitionCount = activePartitionCount;
+
+                    if (m_activePartitionCount != null)
+                    {
+                        // If static partitioning, we need to increase the active partition count.
+                        Interlocked.Increment(ref m_activePartitionCount.Value);
+                    }
+                }
+
+                //overriding methods
+
+                /// <summary>
+                /// Reserves a contiguous range of elements from source data
+                /// </summary>
+                /// <param name="requestedChunkSize">specified number of elements requested</param>
+                /// <returns>
+                /// true if we successfully reserved at least one element (up to #=requestedChunkSize) 
+                /// false if all elements in the source collection have been reserved.
+                /// </returns>
+                override protected bool GrabNextChunk(int requestedChunkSize)
+                {
+                    Contract.Assert(requestedChunkSize > 0);
+
+                    if (HasNoElementsLeft)
+                    {
+                        return false;
+                    }
+
+                    // defer allocation to avoid false sharing
+                    if (m_localList == null)
+                    {
+                        m_localList = new KeyValuePair<long, TSource>[m_maxChunkSize];
+                    }
+
+                    // make the actual call to the enumerable that grabs a chunk
+                    return m_enumerable.GrabChunk(m_localList, requestedChunkSize, ref m_currentChunkSize.Value);
+                }
+
+                /// <summary>
+                /// Returns whether or not the shared reader has already read the last 
+                /// element of the source data 
+                /// </summary>
+                /// <remarks>
+                /// We cannot call m_sharedReader.MoveNext(), to see if it hits the last element
+                /// or not, because we can't undo MoveNext(). Thus we need to maintain a shared 
+                /// boolean value m_hasNoElementsLeft across all partitions
+                /// </remarks>
+                override protected bool HasNoElementsLeft
+                {
+                    get { return m_hasNoElementsLeft.Value; }
+                    set
+                    {
+                        //we only set it from false to true once
+                        //we should never set it back in any circumstances
+                        Contract.Assert(value);
+                        Contract.Assert(!m_hasNoElementsLeft.Value);
+                        m_hasNoElementsLeft.Value = true;
+                    }
+                }
+
+                override public KeyValuePair<long, TSource> Current
+                {
+                    get
+                    {
+                        //verify that MoveNext is at least called once before Current is called 
+                        if (m_currentChunkSize == null)
+                        {
+                            throw new InvalidOperationException(Environment.GetResourceString("PartitionerStatic_CurrentCalledBeforeMoveNext"));
+                        }
+                        Contract.Assert(m_localList != null);
+                        Contract.Assert(m_localOffset.Value >= 0 && m_localOffset.Value < m_currentChunkSize.Value);
+                        return (m_localList[m_localOffset.Value]);
+                    }
+                }
+
+                override public void Dispose()
+                {
+                    // If this is static partitioning, ie. m_activePartitionCount != null, since the current partition 
+                    // is disposed, we decrement the number of active partitions for the shared reader. 
+                    if (m_activePartitionCount != null && Interlocked.Decrement(ref m_activePartitionCount.Value) == 0)
+                    {
+                        // If the number of active partitions becomes 0, we need to dispose the shared 
+                        // reader we created in the m_enumerable object.
+                        m_enumerable.Dispose();
+                    }
+                    // If this is dynamic partitioning, ie. m_activePartitionCount != null, then m_enumerable needs to
+                    // be disposed explicitly by the user, and we do not need to anything here
+                }
+            }
+            #endregion
+
+        }
+        #endregion
+
+        #region Dynamic Partitioner for source data of IndexRange types (IList<> and Array<>)
+        /// <summary>
+        /// Dynamic load-balance partitioner. This class is abstract and to be derived from by 
+        /// the customized partitioner classes for IList, Array, and IEnumerable
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in the source data</typeparam>
+        /// <typeparam name="TCollection"> Type of the source data collection</typeparam>
+        private abstract class DynamicPartitionerForIndexRange_Abstract<TSource, TCollection> : OrderablePartitioner<TSource>
+        {
+            // TCollection can be: IList<TSource>, TSource[] and IEnumerable<TSource>
+            // Derived classes specify TCollection, and implement the abstract method GetOrderableDynamicPartitions_Factory accordingly
+            TCollection m_data;
+
+            /// <summary>
+            /// Constructs a new orderable partitioner 
+            /// </summary>
+            /// <param name="data">source data collection</param>
+            protected DynamicPartitionerForIndexRange_Abstract(TCollection data)
+                : base(true, false, true)
+            {
+                m_data = data;
+            }
+
+            /// <summary>
+            /// Partition the source data and create an enumerable over the resulting partitions. 
+            /// </summary>
+            /// <param name="data">the source data collection</param>
+            /// <returns>an enumerable of partitions of </returns>
+            protected abstract IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions_Factory(TCollection data);
+
+            /// <summary>
+            /// Overrides OrderablePartitioner.GetOrderablePartitions.
+            /// Partitions the underlying collection into the given number of orderable partitions.
+            /// </summary>
+            /// <param name="partitionCount">number of partitions requested</param>
+            /// <returns>A list containing <paramref name="partitionCount"/> enumerators.</returns>
+            override public IList<IEnumerator<KeyValuePair<long, TSource>>> GetOrderablePartitions(int partitionCount)
+            {
+                if (partitionCount <= 0)
+                {
+                    throw new ArgumentOutOfRangeException("partitionCount");
+                }
+                IEnumerator<KeyValuePair<long, TSource>>[] partitions
+                    = new IEnumerator<KeyValuePair<long, TSource>>[partitionCount];
+                IEnumerable<KeyValuePair<long, TSource>> partitionEnumerable = GetOrderableDynamicPartitions_Factory(m_data);
+                for (int i = 0; i < partitionCount; i++)
+                {
+                    partitions[i] = partitionEnumerable.GetEnumerator();
+                }
+                return partitions;
+            }
+
+            /// <summary>
+            /// Overrides OrderablePartitioner.GetOrderableDyanmicPartitions
+            /// </summary>
+            /// <returns>a enumerable collection of orderable partitions</returns>
+            override public IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions()
+            {
+                return GetOrderableDynamicPartitions_Factory(m_data);
+            }
+
+            /// <summary>
+            /// Whether additional partitions can be created dynamically.
+            /// </summary>
+            override public bool SupportsDynamicPartitions
+            {
+                get { return true; }
+            }
+
+        }
+
+        /// <summary>
+        /// Defines dynamic partition for source data of IList and Array. 
+        /// This class inherits DynamicPartitionEnumerator_Abstract
+        ///   - implements GrabNextChunk, HasNoElementsLeft, and Dispose methods for IList and Array
+        ///   - Current property still remains abstract, implementation is different for IList and Array
+        ///   - introduces another abstract method SourceCount, which returns the number of elements in
+        ///     the source data. Implementation differs for IList and Array
+        /// </summary>
+        /// <typeparam name="TSource">Type of the elements in the data source</typeparam>
+        /// <typeparam name="TSourceReader">Type of the reader on the source data</typeparam>
+        private abstract class DynamicPartitionEnumeratorForIndexRange_Abstract<TSource, TSourceReader> : DynamicPartitionEnumerator_Abstract<TSource, TSourceReader>
+        {
+            //fields
+            protected int m_startIndex; //initially zero
+
+            //constructor
+            protected DynamicPartitionEnumeratorForIndexRange_Abstract(TSourceReader sharedReader, SharedLong sharedIndex)
+                : base(sharedReader, sharedIndex)
+            {
+            }
+
+            //abstract methods
+            //the Current property is still abstract, and will be implemented by derived classes
+            //we add another abstract method SourceCount to get the number of elements from the source reader
+
+            /// <summary>
+            /// Get the number of elements from the source reader.
+            /// It calls IList.Count or Array.Length
+            /// </summary>
+            protected abstract int SourceCount { get; }
+
+            //overriding methods
+
+            /// <summary>
+            /// Reserves a contiguous range of elements from source data
+            /// </summary>
+            /// <param name="requestedChunkSize">specified number of elements requested</param>
+            /// <returns>
+            /// true if we successfully reserved at least one element (up to #=requestedChunkSize) 
+            /// false if all elements in the source collection have been reserved.
+            /// </returns>
+            override protected bool GrabNextChunk(int requestedChunkSize)
+            {
+                Contract.Assert(requestedChunkSize > 0);
+
+                while (!HasNoElementsLeft)
+                {
+                    Contract.Assert(m_sharedIndex != null);
+                    // use the new Volatile.Read method because it is cheaper than Interlocked.Read on AMD64 architecture
+                    long oldSharedIndex = Volatile.Read(ref m_sharedIndex.Value);
+
+                    if (HasNoElementsLeft)
+                    {
+                        //HasNoElementsLeft situation changed from false to true immediately
+                        //and oldSharedIndex becomes stale
+                        return false;
+                    }
+
+                    //there won't be overflow, because the index of IList/array is int, and we 
+                    //have casted it to long. 
+                    long newSharedIndex = Math.Min(SourceCount - 1, oldSharedIndex + requestedChunkSize);
+
+
+                    //the following CAS, if successful, reserves a chunk of elements [oldSharedIndex+1, newSharedIndex] 
+                    //inclusive in the source collection
+                    if (Interlocked.CompareExchange(ref m_sharedIndex.Value, newSharedIndex, oldSharedIndex)
+                        == oldSharedIndex)
+                    {
+                        //set up local indexes.
+                        //m_currentChunkSize is always set to requestedChunkSize when source data had 
+                        //enough elements of what we requested
+                        m_currentChunkSize.Value = (int)(newSharedIndex - oldSharedIndex);
+                        m_localOffset.Value = -1;
+                        m_startIndex = (int)(oldSharedIndex + 1);
+                        return true;
+                    }
+                }
+                //didn't get any element, return false;
+                return false;
+            }
+
+            /// <summary>
+            /// Returns whether or not the shared reader has already read the last 
+            /// element of the source data 
+            /// </summary>
+            override protected bool HasNoElementsLeft
+            {
+                get
+                {
+                    Contract.Assert(m_sharedIndex != null);
+                    // use the new Volatile.Read method because it is cheaper than Interlocked.Read on AMD64 architecture
+                    return Volatile.Read(ref m_sharedIndex.Value) >= SourceCount - 1;
+                }
+                set
+                {
+                    Contract.Assert(false);
+                }
+            }
+
+            /// <summary>
+            /// For source data type IList and Array, the type of the shared reader is just the data itself.
+            /// We don't do anything in Dispose method for IList and Array. 
+            /// </summary>
+            override public void Dispose()
+            { }
+        }
+
+
+        /// <summary>
+        /// Inherits from DynamicPartitioners
+        /// Provides customized implementation of GetOrderableDynamicPartitions_Factory method, to return an instance
+        /// of EnumerableOfPartitionsForIList defined internally
+        /// </summary>
+        /// <typeparam name="TSource">Type of elements in the source data</typeparam>
+        private class DynamicPartitionerForIList<TSource> : DynamicPartitionerForIndexRange_Abstract<TSource, IList<TSource>>
+        {
+            //constructor
+            internal DynamicPartitionerForIList(IList<TSource> source)
+                : base(source)
+            { }
+
+            //override methods
+            override protected IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions_Factory(IList<TSource> m_data)
+            {
+                //m_data itself serves as shared reader
+                return new InternalPartitionEnumerable(m_data);
+            }
+
+            /// <summary>
+            /// Inherits from PartitionList_Abstract 
+            /// Provides customized implementation for source data of IList
+            /// </summary>
+            private class InternalPartitionEnumerable : IEnumerable<KeyValuePair<long, TSource>>
+            {
+                //reader through which we access the source data
+                private readonly IList<TSource> m_sharedReader;
+                private SharedLong m_sharedIndex;
+
+                internal InternalPartitionEnumerable(IList<TSource> sharedReader)
+                {
+                    m_sharedReader = sharedReader;
+                    m_sharedIndex = new SharedLong(-1);
+                }
+
+                public IEnumerator<KeyValuePair<long, TSource>> GetEnumerator()
+                {
+                    return new InternalPartitionEnumerator(m_sharedReader, m_sharedIndex);
+                }
+
+                IEnumerator IEnumerable.GetEnumerator()
+                {
+                    return ((InternalPartitionEnumerable)this).GetEnumerator();
+                }
+            }
+
+            /// <summary>
+            /// Inherits from DynamicPartitionEnumeratorForIndexRange_Abstract
+            /// Provides customized implementation of SourceCount property and Current property for IList
+            /// </summary>
+            private class InternalPartitionEnumerator : DynamicPartitionEnumeratorForIndexRange_Abstract<TSource, IList<TSource>>
+            {
+                //constructor
+                internal InternalPartitionEnumerator(IList<TSource> sharedReader, SharedLong sharedIndex)
+                    : base(sharedReader, sharedIndex)
+                { }
+
+                //overriding methods
+                override protected int SourceCount
+                {
+                    get { return m_sharedReader.Count; }
+                }
+                /// <summary>
+                /// return a KeyValuePair of the current element and its key 
+                /// </summary>
+                override public KeyValuePair<long, TSource> Current
+                {
+                    get
+                    {
+                        //verify that MoveNext is at least called once before Current is called 
+                        if (m_currentChunkSize == null)
+                        {
+                            throw new InvalidOperationException(Environment.GetResourceString("PartitionerStatic_CurrentCalledBeforeMoveNext"));
+                        }
+
+                        Contract.Assert(m_localOffset.Value >= 0 && m_localOffset.Value < m_currentChunkSize.Value);
+                        return new KeyValuePair<long, TSource>(m_startIndex + m_localOffset.Value,
+                            m_sharedReader[m_startIndex + m_localOffset.Value]);
+                    }
+                }
+            }
+        }
+
+
+
+        /// <summary>
+        /// Inherits from DynamicPartitioners
+        /// Provides customized implementation of GetOrderableDynamicPartitions_Factory method, to return an instance
+        /// of EnumerableOfPartitionsForArray defined internally
+        /// </summary>
+        /// <typeparam name="TSource">Type of elements in the source data</typeparam>
+        private class DynamicPartitionerForArray<TSource> : DynamicPartitionerForIndexRange_Abstract<TSource, TSource[]>
+        {
+            //constructor
+            internal DynamicPartitionerForArray(TSource[] source)
+                : base(source)
+            { }
+
+            //override methods
+            override protected IEnumerable<KeyValuePair<long, TSource>> GetOrderableDynamicPartitions_Factory(TSource[] m_data)
+            {
+                return new InternalPartitionEnumerable(m_data);
+            }
+
+            /// <summary>
+            /// Inherits from PartitionList_Abstract 
+            /// Provides customized implementation for source data of Array
+            /// </summary>
+            private class InternalPartitionEnumerable : IEnumerable<KeyValuePair<long, TSource>>
+            {
+                //reader through which we access the source data
+                private readonly TSource[] m_sharedReader;
+                private SharedLong m_sharedIndex;
+
+                internal InternalPartitionEnumerable(TSource[] sharedReader)
+                {
+                    m_sharedReader = sharedReader;
+                    m_sharedIndex = new SharedLong(-1);
+                }
+
+                IEnumerator IEnumerable.GetEnumerator()
+                {
+                    return ((InternalPartitionEnumerable)this).GetEnumerator();
+                }
+
+
+                public IEnumerator<KeyValuePair<long, TSource>> GetEnumerator()
+                {
+                    return new InternalPartitionEnumerator(m_sharedReader, m_sharedIndex);
+                }
+            }
+
+            /// <summary>
+            /// Inherits from DynamicPartitionEnumeratorForIndexRange_Abstract
+            /// Provides customized implementation of SourceCount property and Current property for Array
+            /// </summary>
+            private class InternalPartitionEnumerator : DynamicPartitionEnumeratorForIndexRange_Abstract<TSource, TSource[]>
+            {
+                //constructor
+                internal InternalPartitionEnumerator(TSource[] sharedReader, SharedLong sharedIndex)
+                    : base(sharedReader, sharedIndex)
+                { }
+
+                //overriding methods
+                override protected int SourceCount
+                {
+                    get { return m_sharedReader.Length; }
+                }
+
+                override public KeyValuePair<long, TSource> Current
+                {
+                    get
+                    {
+                        //verify that MoveNext is at least called once before Current is called 
+                        if (m_currentChunkSize == null)
+                        {
+                            throw new InvalidOperationException(Environment.GetResourceString("PartitionerStatic_CurrentCalledBeforeMoveNext"));
+                        }
+
+                        Contract.Assert(m_localOffset.Value >= 0 && m_localOffset.Value < m_currentChunkSize.Value);
+                        return new KeyValuePair<long, TSource>(m_startIndex + m_localOffset.Value,
+                            m_sharedReader[m_startIndex + m_localOffset.Value]);
+                    }
+                }
+            }
+        }
+        #endregion
+
+
+        #region Static partitioning for IList and Array, abstract classes
+        /// <summary>
+        /// Static partitioning over IList. 
+        /// - dynamic and load-balance
+        /// - Keys are ordered within each partition
+        /// - Keys are ordered across partitions
+        /// - Keys are normalized
+        /// - Number of partitions is fixed once specified, and the elements of the source data are 
+        /// distributed to each partition as evenly as possible. 
+        /// </summary>
+        /// <typeparam name="TSource">type of the elements</typeparam>        
+        /// <typeparam name="TCollection">Type of the source data collection</typeparam>
+        private abstract class StaticIndexRangePartitioner<TSource, TCollection> : OrderablePartitioner<TSource>
+        {
+            protected StaticIndexRangePartitioner()
+                : base(true, true, true)
+            { }
+
+            /// <summary>
+            /// Abstract method to return the number of elements in the source data
+            /// </summary>
+            protected abstract int SourceCount { get; }
+
+            /// <summary>
+            /// Abstract method to create a partition that covers a range over source data, 
+            /// starting from "startIndex", ending at "endIndex"
+            /// </summary>
+            /// <param name="startIndex">start index of the current partition on the source data</param>
+            /// <param name="endIndex">end index of the current partition on the source data</param>
+            /// <returns>a partition enumerator over the specified range</returns>
+            // The partitioning algorithm is implemented in GetOrderablePartitions method
+            // This method delegates according to source data type IList/Array
+            protected abstract IEnumerator<KeyValuePair<long, TSource>> CreatePartition(int startIndex, int endIndex);
+
+            /// <summary>
+            /// Overrides OrderablePartitioner.GetOrderablePartitions
+            /// Return a list of partitions, each of which enumerate a fixed part of the source data
+            /// The elements of the source data are distributed to each partition as evenly as possible. 
+            /// Specifically, if the total number of elements is N, and number of partitions is x, and N = a*x +b, 
+            /// where a is the quotient, and b is the remainder. Then the first b partitions each has a + 1 elements,
+            /// and the last x-b partitions each has a elements.
+            /// For example, if N=10, x =3, then 
+            ///    partition 0 ranges [0,3],
+            ///    partition 1 ranges [4,6],
+            ///    partition 2 ranges [7,9].
+            /// This also takes care of the situation of (x&gt;N), the last x-N partitions are empty enumerators. 
+            /// An empty enumerator is indicated by 
+            ///      (m_startIndex == list.Count &amp;&amp; m_endIndex == list.Count -1)
+            /// </summary>
+            /// <param name="partitionCount">specified number of partitions</param>
+            /// <returns>a list of partitions</returns>
+            override public IList<IEnumerator<KeyValuePair<long, TSource>>> GetOrderablePartitions(int partitionCount)
+            {
+                if (partitionCount <= 0)
+                {
+                    throw new ArgumentOutOfRangeException("partitionCount");
+                }
+
+                int quotient, remainder;
+                quotient = Math.DivRem(SourceCount, partitionCount, out remainder);
+
+                IEnumerator<KeyValuePair<long, TSource>>[] partitions = new IEnumerator<KeyValuePair<long, TSource>>[partitionCount];
+                int lastEndIndex = -1;
+                for (int i = 0; i < partitionCount; i++)
+                {
+                    int startIndex = lastEndIndex + 1;
+
+                    if (i < remainder)
+                    {
+                        lastEndIndex = startIndex + quotient;
+                    }
+                    else
+                    {
+                        lastEndIndex = startIndex + quotient - 1;
+                    }
+                    partitions[i] = CreatePartition(startIndex, lastEndIndex);
+                }
+                return partitions;
+            }
+        }
+
+        /// <summary>
+        /// Static Partition for IList/Array.
+        /// This class implements all methods required by IEnumerator interface, except for the Current property.
+        /// Current Property is different for IList and Array. Arrays calls 'ldelem' instructions for faster element 
+        /// retrieval.
+        /// </summary>
+        //We assume the source collection is not being updated concurrently. Otherwise it will break the  
+        //static partitioning, since each partition operates on the source collection directly, it does 
+        //not have a local cache of the elements assigned to them.  
+        private abstract class StaticIndexRangePartition<TSource> : IEnumerator<KeyValuePair<long, TSource>>
+        {
+            //the start and end position in the source collection for the current partition
+            //the partition is empty if and only if 
+            // (m_startIndex == m_data.Count && m_endIndex == m_data.Count-1)
+            protected readonly int m_startIndex;
+            protected readonly int m_endIndex;
+
+            //the current index of the current partition while enumerating on the source collection
+            protected volatile int m_offset;
+
+            /// <summary>
+            /// Constructs an instance of StaticIndexRangePartition
+            /// </summary>
+            /// <param name="startIndex">the start index in the source collection for the current partition </param>
+            /// <param name="endIndex">the end index in the source collection for the current partition</param>
+            protected StaticIndexRangePartition(int startIndex, int endIndex)
+            {
+                m_startIndex = startIndex;
+                m_endIndex = endIndex;
+                m_offset = startIndex - 1;
+            }
+
+            /// <summary>
+            /// Current Property is different for IList and Array. Arrays calls 'ldelem' instructions for faster 
+            /// element retrieval.
+            /// </summary>
+            public abstract KeyValuePair<long, TSource> Current { get; }
+
+            /// <summary>
+            /// We don't dispose the source for IList and array
+            /// </summary>
+            public void Dispose()
+            { }
+
+            public void Reset()
+            {
+                throw new NotSupportedException();
+            }
+
+            /// <summary>
+            /// Moves to the next item
+            /// Before the first MoveNext is called: m_offset == m_startIndex-1;
+            /// </summary>
+            /// <returns>true if successful, false if there is no item left</returns>
+            public bool MoveNext()
+            {
+                if (m_offset < m_endIndex)
+                {
+                    m_offset++;
+                    return true;
+                }
+                else
+                {
+                    //After we have enumerated over all elements, we set m_offset to m_endIndex +1.
+                    //The reason we do this is, for an empty enumerator, we need to tell the Current 
+                    //property whether MoveNext has been called or not. 
+                    //For an empty enumerator, it starts with (m_offset == m_startIndex-1 == m_endIndex), 
+                    //and we don't set a new value to m_offset, then the above condition will always be 
+                    //true, and the Current property will mistakenly assume MoveNext is never called.
+                    m_offset = m_endIndex + 1;
+                    return false;
+                }
+            }
+
+            Object IEnumerator.Current
+            {
+                get
+                {
+                    return ((StaticIndexRangePartition<TSource>)this).Current;
+                }
+            }
+        }
+        #endregion
+
+        #region Static partitioning for IList
+        /// <summary>
+        /// Inherits from StaticIndexRangePartitioner
+        /// Provides customized implementation of SourceCount and CreatePartition
+        /// </summary>
+        /// <typeparam name="TSource"></typeparam>
+        private class StaticIndexRangePartitionerForIList<TSource> : StaticIndexRangePartitioner<TSource, IList<TSource>>
+        {
+            IList<TSource> m_list;
+            internal StaticIndexRangePartitionerForIList(IList<TSource> list)
+                : base()
+            {
+                Contract.Assert(list != null);
+                m_list = list;
+            }
+            override protected int SourceCount
+            {
+                get { return m_list.Count; }
+            }
+            override protected IEnumerator<KeyValuePair<long, TSource>> CreatePartition(int startIndex, int endIndex)
+            {
+                return new StaticIndexRangePartitionForIList<TSource>(m_list, startIndex, endIndex);
+            }
+        }
+
+        /// <summary>
+        /// Inherits from StaticIndexRangePartition
+        /// Provides customized implementation of Current property
+        /// </summary>
+        /// <typeparam name="TSource"></typeparam>
+        private class StaticIndexRangePartitionForIList<TSource> : StaticIndexRangePartition<TSource>
+        {
+            //the source collection shared by all partitions
+            private volatile IList<TSource> m_list;
+
+            internal StaticIndexRangePartitionForIList(IList<TSource> list, int startIndex, int endIndex)
+                : base(startIndex, endIndex)
+            {
+                Contract.Assert(startIndex >= 0 && endIndex <= list.Count - 1);
+                m_list = list;
+            }
+
+            override public KeyValuePair<long, TSource> Current
+            {
+                get
+                {
+                    //verify that MoveNext is at least called once before Current is called 
+                    if (m_offset < m_startIndex)
+                    {
+                        throw new InvalidOperationException(Environment.GetResourceString("PartitionerStatic_CurrentCalledBeforeMoveNext"));
+                    }
+
+                    Contract.Assert(m_offset >= m_startIndex && m_offset <= m_endIndex);
+                    return (new KeyValuePair<long, TSource>(m_offset, m_list[m_offset]));
+                }
+            }
+        }
+        #endregion
+
+        #region static partitioning for Arrays
+        /// <summary>
+        /// Inherits from StaticIndexRangePartitioner
+        /// Provides customized implementation of SourceCount and CreatePartition for Array
+        /// </summary>
+        private class StaticIndexRangePartitionerForArray<TSource> : StaticIndexRangePartitioner<TSource, TSource[]>
+        {
+            TSource[] m_array;
+            internal StaticIndexRangePartitionerForArray(TSource[] array)
+                : base()
+            {
+                Contract.Assert(array != null);
+                m_array = array;
+            }
+            override protected int SourceCount
+            {
+                get { return m_array.Length; }
+            }
+            override protected IEnumerator<KeyValuePair<long, TSource>> CreatePartition(int startIndex, int endIndex)
+            {
+                return new StaticIndexRangePartitionForArray<TSource>(m_array, startIndex, endIndex);
+            }
+        }
+
+        /// <summary>
+        /// Inherits from StaticIndexRangePartitioner
+        /// Provides customized implementation of SourceCount and CreatePartition
+        /// </summary>
+        private class StaticIndexRangePartitionForArray<TSource> : StaticIndexRangePartition<TSource>
+        {
+            //the source collection shared by all partitions
+            private volatile TSource[] m_array;
+
+            internal StaticIndexRangePartitionForArray(TSource[] array, int startIndex, int endIndex)
+                : base(startIndex, endIndex)
+            {
+                Contract.Assert(startIndex >= 0 && endIndex <= array.Length - 1);
+                m_array = array;
+            }
+
+            override public KeyValuePair<long, TSource> Current
+            {
+                get
+                {
+                    //verify that MoveNext is at least called once before Current is called 
+                    if (m_offset < m_startIndex)
+                    {
+                        throw new InvalidOperationException(Environment.GetResourceString("PartitionerStatic_CurrentCalledBeforeMoveNext"));
+                    }
+
+                    Contract.Assert(m_offset >= m_startIndex && m_offset <= m_endIndex);
+                    return (new KeyValuePair<long, TSource>(m_offset, m_array[m_offset]));
+                }
+            }
+        }
+        #endregion
+
+
+        #region Utility functions
+        /// <summary>
+        /// A very simple primitive that allows us to share a value across multiple threads.
+        /// </summary>
+        /// <typeparam name="TSource"></typeparam>
+        private class SharedInt
+        {
+            internal volatile int Value;
+
+            internal SharedInt(int value)
+            {
+                this.Value = value;
+            }
+
+        }
+
+        /// <summary>
+        /// A very simple primitive that allows us to share a value across multiple threads.
+        /// </summary>
+        private class SharedBool
+        {
+            internal volatile bool Value;
+
+            internal SharedBool(bool value)
+            {
+                this.Value = value;
+            }
+
+        }
+
+        /// <summary>
+        /// A very simple primitive that allows us to share a value across multiple threads.
+        /// </summary>
+        private class SharedLong
+        {
+            internal long Value;
+            internal SharedLong(long value)
+            {
+                this.Value = value;
+            }
+
+        }
+
+        //--------------------
+        // The following part calculates the default chunk size. It is copied from System.Linq.Parallel.Scheduling,
+        // because mscorlib.dll cannot access System.Linq.Parallel.Scheduling
+        //--------------------
+
+        // The number of bytes we want "chunks" to be, when partitioning, etc. We choose 4 cache
+        // lines worth, assuming 128b cache line.  Most (popular) architectures use 64b cache lines,
+        // but choosing 128b works for 64b too whereas a multiple of 64b isn't necessarily sufficient
+        // for 128b cache systems.  So 128b it is.
+        private const int DEFAULT_BYTES_PER_CHUNK = 128 * 4;
+        
+        private static int GetDefaultChunkSize<TSource>()
+        {
+            int chunkSize;
+
+            if (typeof(TSource).IsValueType)
+            {
+#if !FEATURE_CORECLR // Marshal.SizeOf is not supported in CoreCLR
+
+                if (typeof(TSource).StructLayoutAttribute.Value == LayoutKind.Explicit)
+                {
+                    chunkSize = Math.Max(1, DEFAULT_BYTES_PER_CHUNK / Marshal.SizeOf(typeof(TSource)));
+                }
+                else
+                {
+                    // We choose '128' because this ensures, no matter the actual size of the value type,
+                    // the total bytes used will be a multiple of 128. This ensures it's cache aligned.
+                    chunkSize = 128;
+                }
+#else
+                chunkSize = 128;
+#endif
+            }
+            else
+            {
+                Contract.Assert((DEFAULT_BYTES_PER_CHUNK % IntPtr.Size) == 0, "bytes per chunk should be a multiple of pointer size");
+                chunkSize = (DEFAULT_BYTES_PER_CHUNK / IntPtr.Size);
+            }
+            return chunkSize;
+        }
+        #endregion
+
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/DictionaryEntry.cs b/src/mscorlib/src/System/Collections/DictionaryEntry.cs
new file mode 100644
index 0000000000..fc1d57fe55
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/DictionaryEntry.cs
@@ -0,0 +1,55 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  DictionaryEntry
+** 
+** 
+**
+**
+** Purpose: Return Value for IDictionaryEnumerator::GetEntry
+**
+** 
+===========================================================*/
+namespace System.Collections {
+    
+    using System;
+    // A DictionaryEntry holds a key and a value from a dictionary.
+    // It is returned by IDictionaryEnumerator::GetEntry().
+[System.Runtime.InteropServices.ComVisible(true)]
+    [Serializable]
+    public struct DictionaryEntry
+    {
+        private Object _key;
+        private Object _value;
+    
+        // Constructs a new DictionaryEnumerator by setting the Key
+        // and Value fields appropriately.
+        public DictionaryEntry(Object key, Object value) {
+            _key = key;
+            _value = value;
+        }
+
+        public Object Key {
+            get {
+                return _key;
+            }
+            
+            set {
+                _key = value;
+            }
+        }
+
+        public Object Value {
+            get {
+                return _value;
+            }
+
+            set {
+                _value = value;
+            }
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/EmptyReadOnlyDictionaryInternal.cs b/src/mscorlib/src/System/Collections/EmptyReadOnlyDictionaryInternal.cs
new file mode 100644
index 0000000000..6e28493ef8
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/EmptyReadOnlyDictionaryInternal.cs
@@ -0,0 +1,194 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+**
+** 
+**
+**
+** Purpose: List for exceptions.
+**
+** 
+===========================================================*/
+
+using System.Diagnostics.Contracts;
+
+namespace System.Collections {
+    ///    This is a simple implementation of IDictionary that is empty and readonly.
+    [Serializable]
+    internal sealed class EmptyReadOnlyDictionaryInternal: IDictionary {
+
+        // Note that this class must be agile with respect to AppDomains.  See its usage in
+        // System.Exception to understand why this is the case.
+
+        public EmptyReadOnlyDictionaryInternal() {
+        }
+
+        // IEnumerable members
+
+        IEnumerator IEnumerable.GetEnumerator() {
+            return new NodeEnumerator();
+        }
+
+        // ICollection members
+
+        public void CopyTo(Array array, int index)  {
+            if (array==null)
+                throw new ArgumentNullException("array");
+
+            if (array.Rank != 1)
+                throw new ArgumentException(Environment.GetResourceString("Arg_RankMultiDimNotSupported"));
+
+            if (index < 0)
+                throw new ArgumentOutOfRangeException("index", Environment.GetResourceString("ArgumentOutOfRange_NeedNonNegNum"));
+
+            if ( array.Length - index < this.Count ) 
+                throw new ArgumentException( Environment.GetResourceString("ArgumentOutOfRange_Index"), "index");
+            Contract.EndContractBlock();
+
+            // the actual copy is a NOP
+        }
+
+        public int Count {
+            get {
+                return 0;
+            }
+        }   
+
+        public Object SyncRoot {
+            get {
+                return this;
+            }
+        }
+
+        public bool IsSynchronized {
+            get {
+                return false;
+            }
+        }
+
+        // IDictionary members
+
+        public Object this[Object key] {
+            get {
+                if (key == null) {
+                    throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key"));
+                }
+                Contract.EndContractBlock();
+                return null;
+            }
+            set {
+                if (key == null) {
+                    throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key"));
+                }
+
+                if (!key.GetType().IsSerializable)                 
+                    throw new ArgumentException(Environment.GetResourceString("Argument_NotSerializable"), "key");                    
+
+                if( (value != null) && (!value.GetType().IsSerializable ) )
+                    throw new ArgumentException(Environment.GetResourceString("Argument_NotSerializable"), "value");                    
+                Contract.EndContractBlock();
+
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_ReadOnly"));
+            }
+        }
+
+        public ICollection Keys {
+            get {
+                return EmptyArray<Object>.Value;
+            }
+        }
+
+        public ICollection Values {
+            get {
+                return EmptyArray<Object>.Value;
+            }
+        }
+
+        public bool Contains(Object key) {
+            return false;
+        }
+
+        public void Add(Object key, Object value) {
+            if (key == null) {
+                throw new ArgumentNullException("key", Environment.GetResourceString("ArgumentNull_Key"));
+            }
+
+            if (!key.GetType().IsSerializable)                 
+                throw new ArgumentException(Environment.GetResourceString("Argument_NotSerializable"), "key" );                    
+
+            if( (value != null) && (!value.GetType().IsSerializable) )
+                throw new ArgumentException(Environment.GetResourceString("Argument_NotSerializable"), "value");                    
+            Contract.EndContractBlock();
+
+            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_ReadOnly"));
+        }
+
+        public void Clear() {
+            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_ReadOnly"));
+        }
+
+        public bool IsReadOnly {
+            get {
+                return true;
+            }
+        }
+
+        public bool IsFixedSize {
+            get {
+                return true;
+            }
+        }
+
+        public IDictionaryEnumerator GetEnumerator() {
+            return new NodeEnumerator();
+        }
+
+        public void Remove(Object key) {
+            throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_ReadOnly"));
+        }
+
+        private sealed class NodeEnumerator : IDictionaryEnumerator {
+
+            public NodeEnumerator() {
+            }
+
+            // IEnumerator members
+
+            public bool MoveNext() {
+                return false;
+            }
+
+            public Object Current {
+                get {
+                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumOpCantHappen"));
+                }
+            }
+
+            public void Reset() {
+            }
+
+            // IDictionaryEnumerator members
+
+            public Object Key {
+                get {
+                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumOpCantHappen"));
+                }
+            }
+
+            public Object Value {
+                get {
+                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumOpCantHappen"));
+                }
+            }
+
+            public DictionaryEntry Entry {
+                get {
+                    throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_EnumOpCantHappen"));
+                }
+            }
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/ArraySortHelper.cs b/src/mscorlib/src/System/Collections/Generic/ArraySortHelper.cs
new file mode 100644
index 0000000000..b2fed9d78f
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/ArraySortHelper.cs
@@ -0,0 +1,1558 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+**
+** 
+**
+**
+** Purpose: class to sort arrays
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic
+{
+    using System;
+    using System.Globalization;
+    using System.Runtime.CompilerServices;
+    using System.Diagnostics.Contracts;
+    using System.Runtime.Versioning;
+    
+    #region ArraySortHelper for single arrays
+
+    internal interface IArraySortHelper<TKey>
+    {
+        void Sort(TKey[] keys, int index, int length, IComparer<TKey> comparer);
+        int BinarySearch(TKey[] keys, int index, int length, TKey value, IComparer<TKey> comparer);
+    }
+
+    internal static class IntrospectiveSortUtilities
+    {
+        // This is the threshold where Introspective sort switches to Insertion sort.
+        // Imperically, 16 seems to speed up most cases without slowing down others, at least for integers.
+        // Large value types may benefit from a smaller number.
+        internal const int IntrosortSizeThreshold = 16;
+
+        internal const int QuickSortDepthThreshold = 32;
+
+        internal static int FloorLog2(int n)
+        {
+            int result = 0;
+            while (n >= 1)
+            {
+                result++;
+                n = n / 2;
+            }
+            return result;
+        }
+
+        internal static void ThrowOrIgnoreBadComparer(Object comparer) {
+            // This is hit when an invarant of QuickSort is violated due to a bad IComparer implementation (for
+            // example, imagine an IComparer that returns 0 when items are equal but -1 all other times).
+            //
+            // We could have thrown this exception on v4, but due to changes in v4.5 around how we partition arrays
+            // there are different sets of input where we would throw this exception.  In order to reduce overall risk from
+            // an app compat persective, we're changing to never throw on v4.  Instead, we'll return with a partially
+            // sorted array.
+            if(BinaryCompatibility.TargetsAtLeast_Desktop_V4_5) {
+                throw new ArgumentException(Environment.GetResourceString("Arg_BogusIComparer", comparer));
+            }
+        }
+
+    }
+
+    [TypeDependencyAttribute("System.Collections.Generic.GenericArraySortHelper`1")]     
+    internal class ArraySortHelper<T>  
+        : IArraySortHelper<T>
+    {
+        static volatile IArraySortHelper<T> defaultArraySortHelper;
+        
+        public static IArraySortHelper<T> Default
+        {
+            get
+            {
+                IArraySortHelper<T> sorter = defaultArraySortHelper;
+                if (sorter == null)
+                    sorter = CreateArraySortHelper();
+
+                return sorter;
+            }
+        }                
+        
+        [System.Security.SecuritySafeCritical]  // auto-generated
+        private static IArraySortHelper<T> CreateArraySortHelper()
+        {
+            if (typeof(IComparable<T>).IsAssignableFrom(typeof(T)))
+            {
+                defaultArraySortHelper = (IArraySortHelper<T>)RuntimeTypeHandle.Allocate(typeof(GenericArraySortHelper<string>).TypeHandle.Instantiate(new Type[] { typeof(T) }));
+            }
+            else
+            {
+                defaultArraySortHelper = new ArraySortHelper<T>();                        
+            }
+            return defaultArraySortHelper;
+        }
+
+        #region IArraySortHelper<T> Members
+
+        public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
+        {
+            Contract.Assert(keys != null, "Check the arguments in the caller!");
+            Contract.Assert( index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
+
+            // Add a try block here to detect IComparers (or their
+            // underlying IComparables, etc) that are bogus.
+            try
+            {
+                if (comparer == null)
+                {
+                    comparer = Comparer<T>.Default;
+                }
+
+#if FEATURE_CORECLR
+                // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                IntrospectiveSort(keys, index, length, comparer);
+#else
+                if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                {
+                    IntrospectiveSort(keys, index, length, comparer);
+                }
+                else
+                {
+                    DepthLimitedQuickSort(keys, index, length + index - 1, comparer, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                }
+#endif
+            }
+            catch (IndexOutOfRangeException)
+            {
+                IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer(comparer);
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T> comparer)
+        {
+            try
+            {
+                if (comparer == null)
+                {
+                    comparer = Comparer<T>.Default;
+                }
+
+                return InternalBinarySearch(array, index, length, value, comparer);
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        #endregion
+
+        internal static int InternalBinarySearch(T[] array, int index, int length, T value, IComparer<T> comparer)
+        {
+            Contract.Requires(array != null, "Check the arguments in the caller!");
+            Contract.Requires(index >= 0 && length >= 0 && (array.Length - index >= length), "Check the arguments in the caller!");
+
+            int lo = index;
+            int hi = index + length - 1;
+            while (lo <= hi)
+            {
+                int i = lo + ((hi - lo) >> 1);
+                int order = comparer.Compare(array[i], value);
+
+                if (order == 0) return i;
+                if (order < 0)
+                {
+                    lo = i + 1;
+                }
+                else
+                {
+                    hi = i - 1;
+                }
+            }
+
+            return ~lo;
+        }
+
+        private static void SwapIfGreater(T[] keys, IComparer<T> comparer, int a, int b)
+        {
+            if (a != b)
+            {
+                if (comparer.Compare(keys[a], keys[b]) > 0)
+                {
+                    T key = keys[a];
+                    keys[a] = keys[b];
+                    keys[b] = key;
+                }
+            }
+        }
+
+        private static void Swap(T[] a, int i, int j)
+        {
+            if(i != j)
+            {
+                T t = a[i];
+                a[i] = a[j];
+                a[j] = t;
+            }
+        }
+
+        internal static void DepthLimitedQuickSort(T[] keys, int left, int right, IComparer<T> comparer, int depthLimit)
+        {
+            do
+            {
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, left, right, comparer);
+                    return;
+                }
+
+                int i = left;
+                int j = right;
+
+                // pre-sort the low, middle (pivot), and high values in place.
+                // this improves performance in the face of already sorted data, or 
+                // data that is made up of multiple sorted runs appended together.
+                int middle = i + ((j - i) >> 1);
+                SwapIfGreater(keys, comparer, i, middle);  // swap the low with the mid point
+                SwapIfGreater(keys, comparer, i, j);   // swap the low with the high
+                SwapIfGreater(keys, comparer, middle, j); // swap the middle with the high
+
+                T x = keys[middle];
+                do
+                {
+                    while (comparer.Compare(keys[i], x) < 0) i++;
+                    while (comparer.Compare(x, keys[j]) < 0) j--;
+                    Contract.Assert(i >= left && j <= right, "(i>=left && j<=right)  Sort failed - Is your IComparer bogus?");
+                    if (i > j) break;
+                    if (i < j)
+                    {
+                        T key = keys[i];
+                        keys[i] = keys[j];
+                        keys[j] = key;
+                    }
+                    i++;
+                    j--;
+                } while (i <= j);
+
+                // The next iteration of the while loop is to "recursively" sort the larger half of the array and the
+                // following calls recursively sort the smaller half.  So we subtract one from depthLimit here so
+                // both sorts see the new value.
+                depthLimit--;
+
+                if (j - left <= right - i)
+                {
+                    if (left < j) DepthLimitedQuickSort(keys, left, j, comparer, depthLimit);
+                    left = i;
+                }
+                else
+                {
+                    if (i < right) DepthLimitedQuickSort(keys, i, right, comparer, depthLimit);
+                    right = j;
+                }
+            } while (left < right);
+        }
+
+        internal static void IntrospectiveSort(T[] keys, int left, int length, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(left >= 0);
+            Contract.Requires(length >= 0);
+            Contract.Requires(length <= keys.Length);
+            Contract.Requires(length + left <= keys.Length);
+
+            if (length < 2)
+                return;
+
+            IntroSort(keys, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length), comparer);
+        }
+
+        private static void IntroSort(T[] keys, int lo, int hi, int depthLimit, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi < keys.Length);
+
+            while (hi > lo)
+            {
+                int partitionSize = hi - lo + 1;
+                if (partitionSize <= IntrospectiveSortUtilities.IntrosortSizeThreshold)
+                {
+                    if (partitionSize == 1)
+                    {
+                        return;
+                    }
+                    if (partitionSize == 2)
+                    {
+                        SwapIfGreater(keys, comparer, lo, hi);
+                        return;
+                    }
+                    if (partitionSize == 3)
+                    {
+                        SwapIfGreater(keys, comparer, lo, hi-1);
+                        SwapIfGreater(keys, comparer, lo, hi);
+                        SwapIfGreater(keys, comparer, hi-1, hi);
+                        return;
+                    }
+
+                    InsertionSort(keys, lo, hi, comparer);
+                    return;
+                }
+
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, lo, hi, comparer);
+                    return;
+                }
+                depthLimit--;
+
+                int p = PickPivotAndPartition(keys, lo, hi, comparer);
+                // Note we've already partitioned around the pivot and do not have to move the pivot again.
+                IntroSort(keys, p + 1, hi, depthLimit, comparer);
+                hi = p - 1;
+            }
+        }
+
+        private static int PickPivotAndPartition(T[] keys, int lo, int hi, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+            Contract.Ensures(Contract.Result<int>() >= lo && Contract.Result<int>() <= hi);
+
+            // Compute median-of-three.  But also partition them, since we've done the comparison.
+            int middle = lo + ((hi - lo) / 2);
+
+            // Sort lo, mid and hi appropriately, then pick mid as the pivot.
+            SwapIfGreater(keys, comparer, lo, middle);  // swap the low with the mid point
+            SwapIfGreater(keys, comparer, lo, hi);   // swap the low with the high
+            SwapIfGreater(keys, comparer, middle, hi); // swap the middle with the high
+
+            T pivot = keys[middle];
+            Swap(keys, middle, hi - 1);
+            int left = lo, right = hi - 1;  // We already partitioned lo and hi and put the pivot in hi - 1.  And we pre-increment & decrement below.
+
+            while (left < right)
+            {
+                while (comparer.Compare(keys[++left], pivot) < 0) ;
+                while (comparer.Compare(pivot, keys[--right]) < 0) ;
+
+                if (left >= right)
+                    break;
+
+                Swap(keys, left, right);
+            }
+
+            // Put pivot in the right location.
+            Swap(keys, left, (hi - 1));
+            return left;
+        }
+
+        private static void Heapsort(T[] keys, int lo, int hi, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+
+            int n = hi - lo + 1;
+            for (int i = n / 2; i >= 1; i = i - 1)
+            {
+                DownHeap(keys, i, n, lo, comparer);
+            }
+            for (int i = n; i > 1; i = i - 1)
+            {
+                Swap(keys, lo, lo + i - 1);
+                DownHeap(keys, 1, i - 1, lo, comparer);
+            }
+        }
+
+        private static void DownHeap(T[] keys, int i, int n, int lo, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(lo < keys.Length);
+
+            T d = keys[lo + i - 1];
+            int child;
+            while (i <= n / 2)
+            {
+                child = 2 * i;
+                if (child < n && comparer.Compare(keys[lo + child - 1], keys[lo + child]) < 0)
+                {
+                    child++;
+                }
+                if (!(comparer.Compare(d, keys[lo + child - 1]) < 0))
+                    break;
+                keys[lo + i - 1] = keys[lo + child - 1];
+                i = child;
+            }
+            keys[lo + i - 1] = d;
+        }
+
+        private static void InsertionSort(T[] keys, int lo, int hi, IComparer<T> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi >= lo);
+            Contract.Requires(hi <= keys.Length);
+
+            int i, j;
+            T t;
+            for (i = lo; i < hi; i++)
+            {
+                j = i;
+                t = keys[i + 1];
+                while (j >= lo && comparer.Compare(t, keys[j]) < 0)
+                {
+                    keys[j + 1] = keys[j];
+                    j--;
+                }
+                keys[j + 1] = t;
+            }
+        }
+    }
+
+    [Serializable()]
+    internal class GenericArraySortHelper<T>
+        : IArraySortHelper<T>
+        where T : IComparable<T>
+    {
+        // Do not add a constructor to this class because ArraySortHelper<T>.CreateSortHelper will not execute it
+        
+        #region IArraySortHelper<T> Members
+
+        public void Sort(T[] keys, int index, int length, IComparer<T> comparer)
+        {
+            Contract.Assert(keys != null, "Check the arguments in the caller!");
+            Contract.Assert(index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
+
+            try
+            {
+                if (comparer == null || comparer == Comparer<T>.Default) {
+
+#if FEATURE_CORECLR
+                    // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                    // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                    // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                    IntrospectiveSort(keys, index, length);
+#else
+                    // call the faster version of our sort algorithm if the user doesn't provide a comparer
+                    if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                    {
+                        IntrospectiveSort(keys, index, length);
+                    }
+                    else
+                    {
+                        DepthLimitedQuickSort(keys, index, length + index - 1, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                    }
+#endif
+                }
+                else
+                {
+#if FEATURE_CORECLR
+                    // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                    // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                    // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                    ArraySortHelper<T>.IntrospectiveSort(keys, index, length, comparer);
+#else
+                    if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                    {
+                        ArraySortHelper<T>.IntrospectiveSort(keys, index, length, comparer);
+                    }
+                    else
+                    {
+                        ArraySortHelper<T>.DepthLimitedQuickSort(keys, index, length + index - 1, comparer, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                    }
+#endif
+                }
+            }
+            catch (IndexOutOfRangeException)
+            {
+                IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer(comparer);
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        public int BinarySearch(T[] array, int index, int length, T value, IComparer<T> comparer)
+        {
+            Contract.Assert(array != null, "Check the arguments in the caller!");
+            Contract.Assert(index >= 0 && length >= 0 && (array.Length - index >= length), "Check the arguments in the caller!");
+
+            try
+            {
+                if (comparer == null || comparer == Comparer<T>.Default)
+                {
+                    return BinarySearch(array, index, length, value);
+                }
+                else
+                {
+                    return ArraySortHelper<T>.InternalBinarySearch(array, index, length, value, comparer);
+                }
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        #endregion
+
+        // This function is called when the user doesn't specify any comparer.
+        // Since T is constrained here, we can call IComparable<T>.CompareTo here.
+        // We can avoid boxing for value type and casting for reference types.
+        private static int BinarySearch(T[] array, int index, int length, T value)
+        {
+            int lo = index;
+            int hi = index + length - 1;
+            while (lo <= hi)
+            {
+                int i = lo + ((hi - lo) >> 1);
+                int order;
+                if (array[i] == null)
+                {
+                    order = (value == null) ? 0 : -1;
+                }
+                else
+                {
+                    order = array[i].CompareTo(value);
+                }
+
+                if (order == 0)
+                {
+                    return i;
+                }
+
+                if (order < 0)
+                {
+                    lo = i + 1;
+                }
+                else
+                {
+                    hi = i - 1;
+                }
+            }
+
+            return ~lo;
+        }
+
+        private static void SwapIfGreaterWithItems(T[] keys, int a, int b)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(0 <= a && a < keys.Length);
+            Contract.Requires(0 <= b && b < keys.Length);
+
+            if (a != b)
+            {
+                if (keys[a] != null && keys[a].CompareTo(keys[b]) > 0)
+                {
+                    T key = keys[a];
+                    keys[a] = keys[b];
+                    keys[b] = key;
+                }
+            }
+        }
+
+        private static void Swap(T[] a, int i, int j)
+        {
+            if(i!=j)
+            {
+                T t = a[i];
+                a[i] = a[j];
+                a[j] = t;
+            }
+        }
+
+        private static void DepthLimitedQuickSort(T[] keys, int left, int right, int depthLimit)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(0 <= left && left < keys.Length);
+            Contract.Requires(0 <= right && right < keys.Length);
+
+            // The code in this function looks very similar to QuickSort in ArraySortHelper<T> class.
+            // The difference is that T is constrainted to IComparable<T> here.
+            // So the IL code will be different. This function is faster than the one in ArraySortHelper<T>.
+
+            do
+            {
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, left, right);
+                    return;
+                }
+
+                int i = left;
+                int j = right;
+
+                // pre-sort the low, middle (pivot), and high values in place.
+                // this improves performance in the face of already sorted data, or 
+                // data that is made up of multiple sorted runs appended together.
+                int middle = i + ((j - i) >> 1);
+                SwapIfGreaterWithItems(keys, i, middle); // swap the low with the mid point
+                SwapIfGreaterWithItems(keys, i, j);      // swap the low with the high
+                SwapIfGreaterWithItems(keys, middle, j); // swap the middle with the high
+
+                T x = keys[middle];
+                do
+                {
+                    if (x == null)
+                    {
+                        // if x null, the loop to find two elements to be switched can be reduced.
+                        while (keys[j] != null) j--;
+                    }
+                    else
+                    {
+                        while (x.CompareTo(keys[i]) > 0) i++;
+                        while (x.CompareTo(keys[j]) < 0) j--;
+                    }
+                    Contract.Assert(i >= left && j <= right, "(i>=left && j<=right)  Sort failed - Is your IComparer bogus?");
+                    if (i > j) break;
+                    if (i < j)
+                    {
+                        T key = keys[i];
+                        keys[i] = keys[j];
+                        keys[j] = key;
+                    }
+                    i++;
+                    j--;
+                } while (i <= j);
+
+                // The next iteration of the while loop is to "recursively" sort the larger half of the array and the
+                // following calls recursively sort the smaller half.  So we subtract one from depthLimit here so
+                // both sorts see the new value.
+                depthLimit--;
+
+                if (j - left <= right - i)
+                {
+                    if (left < j) DepthLimitedQuickSort(keys, left, j, depthLimit);
+                    left = i;
+                }
+                else
+                {
+                    if (i < right) DepthLimitedQuickSort(keys, i, right, depthLimit);
+                    right = j;
+                }
+            } while (left < right);
+        }
+
+        internal static void IntrospectiveSort(T[] keys, int left, int length)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(left >= 0);
+            Contract.Requires(length >= 0);
+            Contract.Requires(length <= keys.Length);
+            Contract.Requires(length + left <= keys.Length);
+
+            if (length < 2)
+                return;
+
+            IntroSort(keys, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length));
+        }
+
+        private static void IntroSort(T[] keys, int lo, int hi, int depthLimit)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi < keys.Length);
+
+            while (hi > lo)
+            {
+                int partitionSize = hi - lo + 1;
+                if (partitionSize <= IntrospectiveSortUtilities.IntrosortSizeThreshold)
+                {
+                    if (partitionSize == 1)
+                    {
+                        return;
+                    }
+                    if (partitionSize == 2)
+                    {
+                        SwapIfGreaterWithItems(keys, lo, hi);
+                        return;
+                    }
+                    if (partitionSize == 3)
+                    {
+                        SwapIfGreaterWithItems(keys, lo, hi-1);
+                        SwapIfGreaterWithItems(keys, lo, hi);
+                        SwapIfGreaterWithItems(keys, hi-1, hi);
+                        return;
+                    }
+
+                    InsertionSort(keys, lo, hi);
+                    return;
+                }
+
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, lo, hi);
+                    return;
+                }
+                depthLimit--;
+
+                int p = PickPivotAndPartition(keys, lo, hi);
+                // Note we've already partitioned around the pivot and do not have to move the pivot again.
+                IntroSort(keys, p + 1, hi, depthLimit);
+                hi = p - 1;
+            }
+        }
+
+        private static int PickPivotAndPartition(T[] keys, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+            Contract.Ensures(Contract.Result<int>() >= lo && Contract.Result<int>() <= hi);
+
+            // Compute median-of-three.  But also partition them, since we've done the comparison.
+            int middle = lo + ((hi - lo) / 2);
+
+            // Sort lo, mid and hi appropriately, then pick mid as the pivot.
+            SwapIfGreaterWithItems(keys, lo, middle);  // swap the low with the mid point
+            SwapIfGreaterWithItems(keys, lo, hi);   // swap the low with the high
+            SwapIfGreaterWithItems(keys, middle, hi); // swap the middle with the high
+
+            T pivot = keys[middle];
+            Swap(keys, middle, hi - 1);
+            int left = lo, right = hi - 1;  // We already partitioned lo and hi and put the pivot in hi - 1.  And we pre-increment & decrement below.
+
+            while (left < right)
+            {
+                if (pivot == null)
+                {
+                    while (left < (hi - 1) && keys[++left] == null) ;
+                    while (right > lo && keys[--right] != null) ;
+                }
+                else
+                {
+                    while (pivot.CompareTo(keys[++left]) > 0) ;
+                    while (pivot.CompareTo(keys[--right]) < 0) ;
+                }
+
+                if (left >= right)
+                    break;
+
+                Swap(keys, left, right);
+            }
+
+            // Put pivot in the right location.
+            Swap(keys, left, (hi - 1));
+            return left;
+        }
+
+        private static void Heapsort(T[] keys, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+
+            int n = hi - lo + 1;
+            for (int i = n / 2; i >= 1; i = i - 1)
+            {
+                DownHeap(keys, i, n, lo);
+            }
+            for (int i = n; i > 1; i = i - 1)
+            {
+                Swap(keys, lo, lo + i - 1);
+                DownHeap(keys, 1, i - 1, lo);
+            }
+        }
+
+        private static void DownHeap(T[] keys, int i, int n, int lo)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(lo < keys.Length);
+
+            T d = keys[lo + i - 1];
+            int child;
+            while (i <= n / 2)
+            {
+                child = 2 * i;
+                if (child < n && (keys[lo + child - 1] == null || keys[lo + child - 1].CompareTo(keys[lo + child]) < 0))
+                {
+                    child++;
+                }
+                if (keys[lo + child - 1] == null || keys[lo + child - 1].CompareTo(d) < 0)
+                    break;
+                keys[lo + i - 1] = keys[lo + child - 1];
+                i = child;
+            }
+            keys[lo + i - 1] = d;
+        }
+
+        private static void InsertionSort(T[] keys, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi >= lo);
+            Contract.Requires(hi <= keys.Length);
+
+            int i, j;
+            T t;
+            for (i = lo; i < hi; i++)
+            {
+                j = i;
+                t = keys[i + 1];
+                while (j >= lo && (t == null || t.CompareTo(keys[j]) < 0))
+                {
+                    keys[j + 1] = keys[j];
+                    j--;
+                }
+                keys[j + 1] = t;
+            }
+        }
+    }
+
+    #endregion
+
+    #region ArraySortHelper for paired key and value arrays
+
+    internal interface IArraySortHelper<TKey, TValue>
+    {
+        void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey> comparer);
+    }
+
+    [TypeDependencyAttribute("System.Collections.Generic.GenericArraySortHelper`2")]
+    internal class ArraySortHelper<TKey, TValue>
+        : IArraySortHelper<TKey, TValue>
+    {
+        static volatile IArraySortHelper<TKey, TValue> defaultArraySortHelper;
+
+        public static IArraySortHelper<TKey, TValue> Default
+        {
+            get
+            {
+                IArraySortHelper<TKey, TValue> sorter = defaultArraySortHelper;
+                if (sorter == null)
+                    sorter = CreateArraySortHelper();
+
+                return sorter;
+            }
+        }
+
+        [System.Security.SecuritySafeCritical]  // auto-generated
+        private static IArraySortHelper<TKey, TValue> CreateArraySortHelper()
+        {
+            if (typeof(IComparable<TKey>).IsAssignableFrom(typeof(TKey)))
+            {
+                defaultArraySortHelper = (IArraySortHelper<TKey, TValue>)RuntimeTypeHandle.Allocate(typeof(GenericArraySortHelper<string, string>).TypeHandle.Instantiate(new Type[] { typeof(TKey), typeof(TValue) }));
+            }
+            else
+            {
+                defaultArraySortHelper = new ArraySortHelper<TKey, TValue>();
+            }
+            return defaultArraySortHelper;
+        }
+
+        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey> comparer)
+        {
+            Contract.Assert(keys != null, "Check the arguments in the caller!");  // Precondition on interface method
+            Contract.Assert(index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
+
+            // Add a try block here to detect IComparers (or their
+            // underlying IComparables, etc) that are bogus.
+            try
+            {
+                if (comparer == null || comparer == Comparer<TKey>.Default)
+                {
+                    comparer = Comparer<TKey>.Default;
+                }
+
+#if FEATURE_CORECLR
+                // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                IntrospectiveSort(keys, values, index, length, comparer);
+#else
+                if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                {
+                    IntrospectiveSort(keys, values, index, length, comparer);
+                }
+                else
+                {
+                    DepthLimitedQuickSort(keys, values, index, length + index - 1, comparer, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                }
+#endif
+            }
+            catch (IndexOutOfRangeException)
+            {
+                IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer(comparer);
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        private static void SwapIfGreaterWithItems(TKey[] keys, TValue[] values, IComparer<TKey> comparer, int a, int b)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values == null || values.Length >= keys.Length);
+            Contract.Requires(comparer != null);
+            Contract.Requires(0 <= a && a < keys.Length);
+            Contract.Requires(0 <= b && b < keys.Length);
+
+            if (a != b)
+            {
+                if (comparer.Compare(keys[a], keys[b]) > 0)
+                {
+                    TKey key = keys[a];
+                    keys[a] = keys[b];
+                    keys[b] = key;
+                    if (values != null)
+                    {
+                        TValue value = values[a];
+                        values[a] = values[b];
+                        values[b] = value;
+                    }
+                }
+            }
+        }
+
+        private static void Swap(TKey[] keys, TValue[] values, int i, int j)
+        {
+            if(i!=j)
+            {
+                TKey k = keys[i];
+                keys[i] = keys[j];
+                keys[j] = k;
+                if(values != null)
+                {
+                    TValue v = values[i];
+                    values[i] = values[j];
+                    values[j] = v;
+                }
+            }
+        }
+
+        internal static void DepthLimitedQuickSort(TKey[] keys, TValue[] values, int left, int right, IComparer<TKey> comparer, int depthLimit)
+        {
+            do
+            {
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, values, left, right, comparer);
+                    return;
+                }
+
+                int i = left;
+                int j = right;
+
+                // pre-sort the low, middle (pivot), and high values in place.
+                // this improves performance in the face of already sorted data, or 
+                // data that is made up of multiple sorted runs appended together.
+                int middle = i + ((j - i) >> 1);
+                SwapIfGreaterWithItems(keys, values, comparer, i, middle);  // swap the low with the mid point
+                SwapIfGreaterWithItems(keys, values, comparer, i, j);   // swap the low with the high
+                SwapIfGreaterWithItems(keys, values, comparer, middle, j); // swap the middle with the high
+
+                TKey x = keys[middle];
+                do
+                {
+                    while (comparer.Compare(keys[i], x) < 0) i++;
+                    while (comparer.Compare(x, keys[j]) < 0) j--;
+                    Contract.Assert(i >= left && j <= right, "(i>=left && j<=right)  Sort failed - Is your IComparer bogus?");
+                    if (i > j) break;
+                    if (i < j)
+                    {
+                        TKey key = keys[i];
+                        keys[i] = keys[j];
+                        keys[j] = key;
+                        if (values != null)
+                        {
+                            TValue value = values[i];
+                            values[i] = values[j];
+                            values[j] = value;
+                        }
+                    }
+                    i++;
+                    j--;
+                } while (i <= j);
+
+                // The next iteration of the while loop is to "recursively" sort the larger half of the array and the
+                // following calls recursively sort the smaller half.  So we subtract one from depthLimit here so
+                // both sorts see the new value.
+                depthLimit--;
+
+                if (j - left <= right - i)
+                {
+                    if (left < j) DepthLimitedQuickSort(keys, values, left, j, comparer, depthLimit);
+                    left = i;
+                }
+                else
+                {
+                    if (i < right) DepthLimitedQuickSort(keys, values, i, right, comparer, depthLimit);
+                    right = j;
+                }
+            } while (left < right);
+        }
+
+        internal static void IntrospectiveSort(TKey[] keys, TValue[] values, int left, int length, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(left >= 0);
+            Contract.Requires(length >= 0);
+            Contract.Requires(length <= keys.Length);
+            Contract.Requires(length + left <= keys.Length);
+            Contract.Requires(length + left <= values.Length);
+
+            if (length < 2)
+                return;
+
+            IntroSort(keys, values, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length), comparer);
+        }
+
+        private static void IntroSort(TKey[] keys, TValue[] values, int lo, int hi, int depthLimit, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi < keys.Length);
+
+            while (hi > lo)
+            {
+                int partitionSize = hi - lo + 1;
+                if (partitionSize <= IntrospectiveSortUtilities.IntrosortSizeThreshold)
+                {
+                    if (partitionSize == 1)
+                    {
+                        return;
+                    }
+                    if (partitionSize == 2)
+                    {
+                        SwapIfGreaterWithItems(keys, values, comparer, lo, hi);
+                        return;
+                    }
+                    if (partitionSize == 3)
+                    {
+                        SwapIfGreaterWithItems(keys, values, comparer, lo, hi-1);
+                        SwapIfGreaterWithItems(keys, values, comparer, lo, hi);
+                        SwapIfGreaterWithItems(keys, values, comparer, hi-1, hi);
+                        return;
+                    }
+
+                    InsertionSort(keys, values, lo, hi, comparer);
+                    return;
+                }
+
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, values, lo, hi, comparer);
+                    return;
+                }
+                depthLimit--;
+
+                int p = PickPivotAndPartition(keys, values, lo, hi, comparer);
+                // Note we've already partitioned around the pivot and do not have to move the pivot again.
+                IntroSort(keys, values, p + 1, hi, depthLimit, comparer);
+                hi = p - 1;
+            }
+        }
+
+        private static int PickPivotAndPartition(TKey[] keys, TValue[] values, int lo, int hi, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+            Contract.Ensures(Contract.Result<int>() >= lo && Contract.Result<int>() <= hi);
+
+            // Compute median-of-three.  But also partition them, since we've done the comparison.
+            int middle = lo + ((hi - lo) / 2);
+
+            // Sort lo, mid and hi appropriately, then pick mid as the pivot.
+            SwapIfGreaterWithItems(keys, values, comparer, lo, middle);  // swap the low with the mid point
+            SwapIfGreaterWithItems(keys, values, comparer, lo, hi);   // swap the low with the high
+            SwapIfGreaterWithItems(keys, values, comparer, middle, hi); // swap the middle with the high
+
+            TKey pivot = keys[middle];
+            Swap(keys, values, middle, hi - 1);
+            int left = lo, right = hi - 1;  // We already partitioned lo and hi and put the pivot in hi - 1.  And we pre-increment & decrement below.
+
+            while (left < right)
+            {
+                while (comparer.Compare(keys[++left], pivot) < 0) ;
+                while (comparer.Compare(pivot, keys[--right]) < 0) ;
+
+                if (left >= right)
+                    break;
+
+                Swap(keys, values, left, right);
+            }
+
+            // Put pivot in the right location.
+            Swap(keys, values, left, (hi - 1));
+            return left;
+        }
+
+        private static void Heapsort(TKey[] keys, TValue[] values, int lo, int hi, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+
+            int n = hi - lo + 1;
+            for (int i = n / 2; i >= 1; i = i - 1)
+            {
+                DownHeap(keys, values, i, n, lo, comparer);
+            }
+            for (int i = n; i > 1; i = i - 1)
+            {
+                Swap(keys, values, lo, lo + i - 1);
+                DownHeap(keys, values, 1, i - 1, lo, comparer);
+            }
+        }
+
+        private static void DownHeap(TKey[] keys, TValue[] values, int i, int n, int lo, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(lo < keys.Length);
+
+            TKey d = keys[lo + i - 1];
+            TValue dValue = (values != null) ? values[lo + i - 1] : default(TValue);
+            int child;
+            while (i <= n / 2)
+            {
+                child = 2 * i;
+                if (child < n && comparer.Compare(keys[lo + child - 1], keys[lo + child]) < 0)
+                {
+                    child++;
+                }
+                if (!(comparer.Compare(d, keys[lo + child - 1]) < 0))
+                    break;
+                keys[lo + i - 1] = keys[lo + child - 1];
+                if(values != null)
+                    values[lo + i - 1] = values[lo + child - 1];
+                i = child;
+            }
+            keys[lo + i - 1] = d;
+            if(values != null)
+                values[lo + i - 1] = dValue;
+        }
+
+        private static void InsertionSort(TKey[] keys, TValue[] values, int lo, int hi, IComparer<TKey> comparer)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(comparer != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi >= lo);
+            Contract.Requires(hi <= keys.Length);
+
+            int i, j;
+            TKey t;
+            TValue tValue;
+            for (i = lo; i < hi; i++)
+            {
+                j = i;
+                t = keys[i + 1];
+                tValue = (values != null) ? values[i + 1] : default(TValue);
+                while (j >= lo && comparer.Compare(t, keys[j]) < 0)
+                {
+                    keys[j + 1] = keys[j];
+                    if(values != null)
+                        values[j + 1] = values[j];
+                    j--;
+                }
+                keys[j + 1] = t;
+                if(values != null)
+                    values[j + 1] = tValue;
+            }
+        }
+    }
+
+    internal class GenericArraySortHelper<TKey, TValue>
+        : IArraySortHelper<TKey, TValue>
+        where TKey : IComparable<TKey>
+    {
+        public void Sort(TKey[] keys, TValue[] values, int index, int length, IComparer<TKey> comparer)
+        {
+            Contract.Assert(keys != null, "Check the arguments in the caller!");
+            Contract.Assert( index >= 0 && length >= 0 && (keys.Length - index >= length), "Check the arguments in the caller!");
+            
+            // Add a try block here to detect IComparers (or their
+            // underlying IComparables, etc) that are bogus.
+            try
+            {
+                if (comparer == null || comparer == Comparer<TKey>.Default)
+                {
+#if FEATURE_CORECLR
+                    // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                    // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                    // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                    IntrospectiveSort(keys, values, index, length);
+#else
+                    // call the faster version of our sort algorithm if the user doesn't provide a comparer
+                    if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                    {
+                        IntrospectiveSort(keys, values, index, length);
+                    }
+                    else
+                    {
+                        DepthLimitedQuickSort(keys, values, index, length + index - 1, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                    }
+#endif
+                }
+                else
+                {
+#if FEATURE_CORECLR
+                    // Since QuickSort and IntrospectiveSort produce different sorting sequence for equal keys the upgrade 
+                    // to IntrospectiveSort was quirked. However since the phone builds always shipped with the new sort aka 
+                    // IntrospectiveSort and we would want to continue using this sort moving forward CoreCLR always uses the new sort.
+
+                    ArraySortHelper<TKey, TValue>.IntrospectiveSort(keys, values, index, length, comparer);
+#else
+                    if (BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                    {
+                        ArraySortHelper<TKey, TValue>.IntrospectiveSort(keys, values, index, length, comparer);
+                    }
+                    else
+                    {
+                        ArraySortHelper<TKey, TValue>.DepthLimitedQuickSort(keys, values, index, length + index - 1, comparer, IntrospectiveSortUtilities.QuickSortDepthThreshold);
+                    }
+#endif
+                }
+
+            }                    
+            catch (IndexOutOfRangeException)
+            {
+                IntrospectiveSortUtilities.ThrowOrIgnoreBadComparer(comparer);
+            }
+            catch (Exception e)
+            {
+                throw new InvalidOperationException(Environment.GetResourceString("InvalidOperation_IComparerFailed"), e);
+            }
+        }
+
+        private static void SwapIfGreaterWithItems(TKey[] keys, TValue[] values, int a, int b)
+        {
+            if (a != b)
+            {
+                if (keys[a] != null && keys[a].CompareTo(keys[b]) > 0)
+                {
+                    TKey key = keys[a];
+                    keys[a] = keys[b];
+                    keys[b] = key;
+                    if (values != null)
+                    {
+                        TValue value = values[a];
+                        values[a] = values[b];
+                        values[b] = value;
+                    }
+                }
+            }
+        }
+
+        private static void Swap(TKey[] keys, TValue[] values, int i, int j)
+        {
+            if(i != j)
+            {
+                TKey k = keys[i];
+                keys[i] = keys[j];
+                keys[j] = k;
+                if(values != null)
+                {
+                    TValue v = values[i];
+                    values[i] = values[j];
+                    values[j] = v;
+                }
+            }
+        }
+
+        private static void DepthLimitedQuickSort(TKey[] keys, TValue[] values, int left, int right, int depthLimit)
+        {
+            // The code in this function looks very similar to QuickSort in ArraySortHelper<T> class.
+            // The difference is that T is constrainted to IComparable<T> here.
+            // So the IL code will be different. This function is faster than the one in ArraySortHelper<T>.
+
+            do
+            {
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, values, left, right);
+                    return;
+                }
+
+                int i = left;
+                int j = right;
+
+                // pre-sort the low, middle (pivot), and high values in place.
+                // this improves performance in the face of already sorted data, or 
+                // data that is made up of multiple sorted runs appended together.
+                int middle = i + ((j - i) >> 1);
+                SwapIfGreaterWithItems(keys, values, i, middle); // swap the low with the mid point
+                SwapIfGreaterWithItems(keys, values, i, j);      // swap the low with the high
+                SwapIfGreaterWithItems(keys, values, middle, j); // swap the middle with the high
+
+                TKey x = keys[middle];
+                do
+                {
+                    if (x == null)
+                    {
+                        // if x null, the loop to find two elements to be switched can be reduced.
+                        while (keys[j] != null) j--;
+                    }
+                    else
+                    {
+                        while (x.CompareTo(keys[i]) > 0) i++;
+                        while (x.CompareTo(keys[j]) < 0) j--;
+                    }
+                    Contract.Assert(i >= left && j <= right, "(i>=left && j<=right)  Sort failed - Is your IComparer bogus?");
+                    if (i > j) break;
+                    if (i < j)
+                    {
+                        TKey key = keys[i];
+                        keys[i] = keys[j];
+                        keys[j] = key;
+                        if (values != null)
+                        {
+                            TValue value = values[i];
+                            values[i] = values[j];
+                            values[j] = value;
+                        }
+                    }
+                    i++;
+                    j--;
+                } while (i <= j);
+
+                // The next iteration of the while loop is to "recursively" sort the larger half of the array and the
+                // following calls recursively sort the smaller half.  So we subtract one from depthLimit here so
+                // both sorts see the new value.
+                depthLimit--;
+
+                if (j - left <= right - i)
+                {
+                    if (left < j) DepthLimitedQuickSort(keys, values, left, j, depthLimit);
+                    left = i;
+                }
+                else
+                {
+                    if (i < right) DepthLimitedQuickSort(keys, values, i, right, depthLimit);
+                    right = j;
+                }
+            } while (left < right);
+        }
+
+        internal static void IntrospectiveSort(TKey[] keys, TValue[] values, int left, int length)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(left >= 0);
+            Contract.Requires(length >= 0);
+            Contract.Requires(length <= keys.Length);
+            Contract.Requires(length + left <= keys.Length);
+            Contract.Requires(length + left <= values.Length);
+
+            if (length < 2)
+                return;
+
+            IntroSort(keys, values, left, length + left - 1, 2 * IntrospectiveSortUtilities.FloorLog2(keys.Length));
+        }
+
+        private static void IntroSort(TKey[] keys, TValue[] values, int lo, int hi, int depthLimit)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi < keys.Length);
+
+            while (hi > lo)
+            {
+                int partitionSize = hi - lo + 1;
+                if (partitionSize <= IntrospectiveSortUtilities.IntrosortSizeThreshold)
+                {
+                    if (partitionSize == 1)
+                    {
+                        return;
+                    }
+                    if (partitionSize == 2)
+                    {
+                        SwapIfGreaterWithItems(keys, values, lo, hi);
+                        return;
+                    }
+                    if (partitionSize == 3)
+                    {
+                        SwapIfGreaterWithItems(keys, values, lo, hi-1);
+                        SwapIfGreaterWithItems(keys, values, lo, hi);
+                        SwapIfGreaterWithItems(keys, values, hi-1, hi);
+                        return;
+                    }
+
+                    InsertionSort(keys, values, lo, hi);
+                    return;
+                }
+
+                if (depthLimit == 0)
+                {
+                    Heapsort(keys, values, lo, hi);
+                    return;
+                }
+                depthLimit--;
+
+                int p = PickPivotAndPartition(keys, values, lo, hi);
+                // Note we've already partitioned around the pivot and do not have to move the pivot again.
+                IntroSort(keys, values, p + 1, hi, depthLimit);
+                hi = p - 1;
+            }
+        }
+
+        private static int PickPivotAndPartition(TKey[] keys, TValue[] values, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+            Contract.Ensures(Contract.Result<int>() >= lo && Contract.Result<int>() <= hi);
+
+            // Compute median-of-three.  But also partition them, since we've done the comparison.
+            int middle = lo + ((hi - lo) / 2);
+
+            // Sort lo, mid and hi appropriately, then pick mid as the pivot.
+            SwapIfGreaterWithItems(keys, values, lo, middle);  // swap the low with the mid point
+            SwapIfGreaterWithItems(keys, values, lo, hi);   // swap the low with the high
+            SwapIfGreaterWithItems(keys, values, middle, hi); // swap the middle with the high
+
+            TKey pivot = keys[middle];
+            Swap(keys, values, middle, hi - 1);
+            int left = lo, right = hi - 1;  // We already partitioned lo and hi and put the pivot in hi - 1.  And we pre-increment & decrement below.
+
+            while (left < right)
+            {
+                if(pivot == null)
+                {
+                    while (left < (hi - 1) && keys[++left] == null) ;
+                    while (right > lo && keys[--right] != null);
+                }
+                else
+                {
+                    while (pivot.CompareTo(keys[++left]) > 0) ;
+                    while (pivot.CompareTo(keys[--right]) < 0) ;
+                }
+
+                if (left >= right)
+                    break;
+
+                Swap(keys, values, left, right);
+            }
+
+            // Put pivot in the right location.
+            Swap(keys, values, left, (hi - 1));
+            return left;
+        }
+
+        private static void Heapsort(TKey[] keys, TValue[] values, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi > lo);
+            Contract.Requires(hi < keys.Length);
+
+            int n = hi - lo + 1;
+            for (int i = n / 2; i >= 1; i = i - 1)
+            {
+                DownHeap(keys, values, i, n, lo);
+            }
+            for (int i = n; i > 1; i = i - 1)
+            {
+                Swap(keys, values, lo, lo + i - 1);
+                DownHeap(keys, values, 1, i - 1, lo);
+            }
+        }
+
+        private static void DownHeap(TKey[] keys, TValue[] values, int i, int n, int lo)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(lo < keys.Length);
+
+            TKey d = keys[lo + i - 1];
+            TValue dValue = (values != null) ? values[lo + i - 1] : default(TValue);
+            int child;
+            while (i <= n / 2)
+            {
+                child = 2 * i;
+                if (child < n && (keys[lo + child - 1] == null || keys[lo + child - 1].CompareTo(keys[lo + child]) < 0))
+                {
+                    child++;
+                }
+                if (keys[lo + child - 1] == null || keys[lo + child - 1].CompareTo(d) < 0)
+                    break;
+                keys[lo + i - 1] = keys[lo + child - 1];
+                if(values != null)
+                    values[lo + i - 1] = values[lo + child - 1];
+                i = child;
+            }
+            keys[lo + i - 1] = d;
+            if(values != null)
+                values[lo + i - 1] = dValue;
+        }
+
+        private static void InsertionSort(TKey[] keys, TValue[] values, int lo, int hi)
+        {
+            Contract.Requires(keys != null);
+            Contract.Requires(values != null);
+            Contract.Requires(lo >= 0);
+            Contract.Requires(hi >= lo);
+            Contract.Requires(hi <= keys.Length);
+
+            int i, j;
+            TKey t;
+            TValue tValue;
+            for (i = lo; i < hi; i++)
+            {
+                j = i;
+                t = keys[i + 1];
+                tValue = (values != null)? values[i + 1] : default(TValue);
+                while (j >= lo && (t == null || t.CompareTo(keys[j]) < 0))
+                {
+                    keys[j + 1] = keys[j];
+                    if(values != null)
+                        values[j + 1] = values[j];
+                    j--;
+                }
+                keys[j + 1] = t;
+                if(values != null)
+                    values[j + 1] = tValue;
+            }
+        }
+    }
+
+    #endregion
+}
+
+
diff --git a/src/mscorlib/src/System/Collections/Generic/Comparer.cs b/src/mscorlib/src/System/Collections/Generic/Comparer.cs
new file mode 100644
index 0000000000..9f1a8bff6f
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/Comparer.cs
@@ -0,0 +1,325 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// 
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Diagnostics.Contracts;
+//using System.Globalization;
+using System.Runtime.CompilerServices;
+using System.Security;
+using System.Runtime.Serialization;
+
+namespace System.Collections.Generic
+{    
+    [Serializable]
+    [TypeDependencyAttribute("System.Collections.Generic.ObjectComparer`1")] 
+    public abstract class Comparer<T> : IComparer, IComparer<T>
+    {
+        static readonly Comparer<T> defaultComparer = CreateComparer();
+
+        public static Comparer<T> Default {
+            get {
+                Contract.Ensures(Contract.Result<Comparer<T>>() != null);
+                return defaultComparer;
+            }
+        }
+
+        public static Comparer<T> Create(Comparison<T> comparison)
+        {
+            Contract.Ensures(Contract.Result<Comparer<T>>() != null);
+
+            if (comparison == null)
+                throw new ArgumentNullException("comparison");
+
+            return new ComparisonComparer<T>(comparison);
+        }
+
+        //
+        // Note that logic in this method is replicated in vm\compile.cpp to ensure that NGen
+        // saves the right instantiations
+        //
+        [System.Security.SecuritySafeCritical]  // auto-generated
+        private static Comparer<T> CreateComparer()
+        {
+            object result = null;
+            RuntimeType t = (RuntimeType)typeof(T);
+
+            // If T implements IComparable<T> return a GenericComparer<T>
+            if (typeof(IComparable<T>).IsAssignableFrom(t))
+            {
+                result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericComparer<int>), t);
+            }
+            else if (default(T) == null)
+            {
+                // If T is a Nullable<U> where U implements IComparable<U> return a NullableComparer<U>
+                if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>)) {
+                    RuntimeType u = (RuntimeType)t.GetGenericArguments()[0];
+                    if (typeof(IComparable<>).MakeGenericType(u).IsAssignableFrom(u)) {
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(NullableComparer<int>), u);
+                    }
+                }
+            }
+            else if (t.IsEnum)
+            {
+                // Explicitly call Enum.GetUnderlyingType here. Although GetTypeCode
+                // ends up doing this anyway, we end up avoiding an unnecessary P/Invoke
+                // and virtual method call.
+                TypeCode underlyingTypeCode = Type.GetTypeCode(Enum.GetUnderlyingType(t));
+                
+                // Depending on the enum type, we need to special case the comparers so that we avoid boxing
+                // Specialize differently for signed/unsigned types so we avoid problems with large numbers
+                switch (underlyingTypeCode)
+                {
+                    case TypeCode.SByte:
+                    case TypeCode.Int16:
+                    case TypeCode.Int32:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(Int32EnumComparer<int>), t);
+                        break;
+                    case TypeCode.Byte:
+                    case TypeCode.UInt16:
+                    case TypeCode.UInt32:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(UInt32EnumComparer<uint>), t);
+                        break;
+                    // 64-bit enums: use UnsafeEnumCastLong
+                    case TypeCode.Int64:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(Int64EnumComparer<long>), t);
+                        break;
+                    case TypeCode.UInt64:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(UInt64EnumComparer<ulong>), t);
+                        break;
+                }
+            }
+            
+            return result != null ?
+                (Comparer<T>)result :
+                new ObjectComparer<T>(); // Fallback to ObjectComparer, which uses boxing
+        }
+
+        public abstract int Compare(T x, T y);
+
+        int IComparer.Compare(object x, object y) {
+            if (x == null) return y == null ? 0 : -1;
+            if (y == null) return 1;
+            if (x is T && y is T) return Compare((T)x, (T)y);
+            ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArgumentForComparison);
+            return 0;
+        }
+    }
+    
+    // Note: although there is a lot of shared code in the following
+    // comparers, we do not incorporate it into a base class for perf
+    // reasons. Adding another base class (even one with no fields)
+    // means another generic instantiation, which can be costly esp.
+    // for value types.
+    
+    [Serializable]
+    internal sealed class GenericComparer<T> : Comparer<T> where T : IComparable<T>
+    {    
+        public override int Compare(T x, T y) {
+            if (x != null) {
+                if (y != null) return x.CompareTo(y);
+                return 1;
+            }
+            if (y != null) return -1;
+            return 0;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+    }
+
+    [Serializable]
+    internal sealed class NullableComparer<T> : Comparer<T?> where T : struct, IComparable<T>
+    {
+        public override int Compare(Nullable<T> x, Nullable<T> y) {
+            if (x.HasValue) {
+                if (y.HasValue) return x.value.CompareTo(y.value);
+                return 1;
+            }
+            if (y.HasValue) return -1;
+            return 0;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+    }
+
+    [Serializable]
+    internal sealed class ObjectComparer<T> : Comparer<T>
+    {
+        public override int Compare(T x, T y) {
+            return System.Collections.Comparer.Default.Compare(x, y);
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+    }
+
+    [Serializable]
+    internal sealed class ComparisonComparer<T> : Comparer<T>
+    {
+        private readonly Comparison<T> _comparison;
+
+        public ComparisonComparer(Comparison<T> comparison) {
+            _comparison = comparison;
+        }
+
+        public override int Compare(T x, T y) {
+            return _comparison(x, y);
+        }
+    }
+
+    // Enum comparers (specialized to avoid boxing)
+    // NOTE: Each of these needs to implement ISerializable
+    // and have a SerializationInfo/StreamingContext ctor,
+    // since we want to serialize as ObjectComparer for
+    // back-compat reasons (see below).
+
+    [Serializable]
+    internal sealed class Int32EnumComparer<T> : Comparer<T>, ISerializable where T : struct
+    {
+        public Int32EnumComparer()
+        {
+            Contract.Assert(typeof(T).IsEnum, "This type is only intended to be used to compare enums!");
+        }
+        
+        // Used by the serialization engine.
+        private Int32EnumComparer(SerializationInfo info, StreamingContext context) { }
+        
+        public override int Compare(T x, T y)
+        {
+            int ix = JitHelpers.UnsafeEnumCast(x);
+            int iy = JitHelpers.UnsafeEnumCast(y);
+            return ix.CompareTo(iy);
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            // Previously Comparer<T> was not specialized for enums,
+            // and instead fell back to ObjectComparer which uses boxing.
+            // Set the type as ObjectComparer here so code that serializes
+            // Comparer for enums will not break.
+            info.SetType(typeof(ObjectComparer<T>));
+        }
+    }
+
+    [Serializable]
+    internal sealed class UInt32EnumComparer<T> : Comparer<T>, ISerializable where T : struct
+    {
+        public UInt32EnumComparer()
+        {
+            Contract.Assert(typeof(T).IsEnum, "This type is only intended to be used to compare enums!");
+        }
+        
+        // Used by the serialization engine.
+        private UInt32EnumComparer(SerializationInfo info, StreamingContext context) { }
+        
+        public override int Compare(T x, T y)
+        {
+            uint ix = (uint)JitHelpers.UnsafeEnumCast(x);
+            uint iy = (uint)JitHelpers.UnsafeEnumCast(y);
+            return ix.CompareTo(iy);
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            info.SetType(typeof(ObjectComparer<T>));
+        }
+    }
+
+    [Serializable]
+    internal sealed class Int64EnumComparer<T> : Comparer<T>, ISerializable where T : struct
+    {
+        public Int64EnumComparer()
+        {
+            Contract.Assert(typeof(T).IsEnum, "This type is only intended to be used to compare enums!");
+        }
+        
+        // Used by the serialization engine.
+        private Int64EnumComparer(SerializationInfo info, StreamingContext context) { }
+        
+        public override int Compare(T x, T y)
+        {
+            long lx = JitHelpers.UnsafeEnumCastLong(x);
+            long ly = JitHelpers.UnsafeEnumCastLong(y);
+            return lx.CompareTo(ly);
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            info.SetType(typeof(ObjectComparer<T>));
+        }
+    }
+
+    [Serializable]
+    internal sealed class UInt64EnumComparer<T> : Comparer<T>, ISerializable where T : struct
+    {
+        public UInt64EnumComparer()
+        {
+            Contract.Assert(typeof(T).IsEnum, "This type is only intended to be used to compare enums!");
+        }
+        
+        // Used by the serialization engine.
+        private UInt64EnumComparer(SerializationInfo info, StreamingContext context) { }
+        
+        public override int Compare(T x, T y)
+        {
+            ulong lx = (ulong)JitHelpers.UnsafeEnumCastLong(x);
+            ulong ly = (ulong)JitHelpers.UnsafeEnumCastLong(y);
+            return lx.CompareTo(ly);
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) =>
+            obj != null && GetType() == obj.GetType();
+
+        public override int GetHashCode() =>
+            GetType().GetHashCode();
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            info.SetType(typeof(ObjectComparer<T>));
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/DebugView.cs b/src/mscorlib/src/System/Collections/Generic/DebugView.cs
new file mode 100644
index 0000000000..57b91eff51
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/DebugView.cs
@@ -0,0 +1,129 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*=============================================================================
+**
+**
+**
+** Purpose: DebugView class for generic collections
+** 
+** 
+**
+**
+=============================================================================*/
+
+namespace System.Collections.Generic {
+    using System;
+    using System.Collections.ObjectModel;
+    using System.Security.Permissions;
+    using System.Diagnostics;    
+    using System.Diagnostics.Contracts;
+
+    //
+    // VS IDE can't differentiate between types with the same name from different
+    // assembly. So we need to use different names for collection debug view for 
+    // collections in mscorlib.dll and system.dll.
+    //
+    internal sealed class Mscorlib_CollectionDebugView<T> {
+        private ICollection<T> collection; 
+        
+        public Mscorlib_CollectionDebugView(ICollection<T> collection) {
+            if (collection == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
+
+                this.collection = collection;
+        }
+       
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public T[] Items   { 
+            get {
+                T[] items = new T[collection.Count];
+                collection.CopyTo(items, 0);
+                return items;
+            }
+        }
+    }        
+
+    internal sealed class Mscorlib_DictionaryKeyCollectionDebugView<TKey, TValue> {
+        private ICollection<TKey> collection; 
+        
+        public Mscorlib_DictionaryKeyCollectionDebugView(ICollection<TKey> collection) {
+            if (collection == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
+
+                this.collection = collection;
+        }
+       
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public TKey[] Items   { 
+            get {
+                TKey[] items = new TKey[collection.Count];
+                collection.CopyTo(items, 0);
+                return items;
+            }
+        }
+    }        
+
+    internal sealed class Mscorlib_DictionaryValueCollectionDebugView<TKey, TValue> {
+        private ICollection<TValue> collection; 
+        
+        public Mscorlib_DictionaryValueCollectionDebugView(ICollection<TValue> collection) {
+            if (collection == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
+
+                this.collection = collection;
+        }
+       
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public TValue[] Items   { 
+            get {
+                TValue[] items = new TValue[collection.Count];
+                collection.CopyTo(items, 0);
+                return items;
+            }
+        }
+    }        
+
+    internal sealed class Mscorlib_DictionaryDebugView<K, V> {
+        private IDictionary<K, V> dict; 
+        
+        public Mscorlib_DictionaryDebugView(IDictionary<K, V> dictionary) {
+            if (dictionary == null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
+
+                this.dict = dictionary;
+        }
+       
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public KeyValuePair<K, V>[] Items   { 
+            get {
+                KeyValuePair<K, V>[] items = new KeyValuePair<K, V>[dict.Count];
+                dict.CopyTo(items, 0);
+                return items;
+            }
+        }
+    }        
+
+    internal sealed class Mscorlib_KeyedCollectionDebugView<K, T> {
+        private KeyedCollection<K, T> kc; 
+        
+        public Mscorlib_KeyedCollectionDebugView(KeyedCollection<K, T> keyedCollection) {
+            if (keyedCollection == null) {
+                throw new ArgumentNullException("keyedCollection");
+            }
+            Contract.EndContractBlock();
+
+            kc = keyedCollection;
+        }
+       
+        [DebuggerBrowsable(DebuggerBrowsableState.RootHidden)]
+        public T[] Items   { 
+            get {
+                T[] items = new T[kc.Count];
+                kc.CopyTo(items, 0);
+                return items;
+            }
+        }        
+    }            
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/Dictionary.cs b/src/mscorlib/src/System/Collections/Generic/Dictionary.cs
new file mode 100644
index 0000000000..9cbfff5a57
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/Dictionary.cs
@@ -0,0 +1,1187 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** 
+** 
+**
+** Purpose: Generic hash table implementation
+**
+** #DictionaryVersusHashtableThreadSafety
+** Hashtable has multiple reader/single writer (MR/SW) thread safety built into 
+** certain methods and properties, whereas Dictionary doesn't. If you're 
+** converting framework code that formerly used Hashtable to Dictionary, it's
+** important to consider whether callers may have taken a dependence on MR/SW
+** thread safety. If a reader writer lock is available, then that may be used
+** with a Dictionary to get the same thread safety guarantee. 
+** 
+** Reader writer locks don't exist in silverlight, so we do the following as a
+** result of removing non-generic collections from silverlight: 
+** 1. If the Hashtable was fully synchronized, then we replace it with a 
+**    Dictionary with full locks around reads/writes (same thread safety
+**    guarantee).
+** 2. Otherwise, the Hashtable has the default MR/SW thread safety behavior, 
+**    so we do one of the following on a case-by-case basis:
+**    a. If the race condition can be addressed by rearranging the code and using a temp
+**       variable (for example, it's only populated immediately after created)
+**       then we address the race condition this way and use Dictionary.
+**    b. If there's concern about degrading performance with the increased 
+**       locking, we ifdef with FEATURE_NONGENERIC_COLLECTIONS so we can at 
+**       least use Hashtable in the desktop build, but Dictionary with full 
+**       locks in silverlight builds. Note that this is heavier locking than 
+**       MR/SW, but this is the only option without rewriting (or adding back)
+**       the reader writer lock. 
+**    c. If there's no performance concern (e.g. debug-only code) we 
+**       consistently replace Hashtable with Dictionary plus full locks to 
+**       reduce complexity.
+**    d. Most of serialization is dead code in silverlight. Instead of updating
+**       those Hashtable occurences in serialization, we carved out references 
+**       to serialization such that this code doesn't need to build in 
+**       silverlight. 
+===========================================================*/
+namespace System.Collections.Generic {
+
+    using System;
+    using System.Collections;
+    using System.Diagnostics;
+    using System.Diagnostics.Contracts;
+    using System.Runtime.Serialization;
+    using System.Security.Permissions;
+
+    [DebuggerTypeProxy(typeof(Mscorlib_DictionaryDebugView<,>))]
+    [DebuggerDisplay("Count = {Count}")]
+    [Serializable]
+    [System.Runtime.InteropServices.ComVisible(false)]
+    public class Dictionary<TKey,TValue>: IDictionary<TKey,TValue>, IDictionary, IReadOnlyDictionary<TKey, TValue>, ISerializable, IDeserializationCallback  {
+    
+        private struct Entry {
+            public int hashCode;    // Lower 31 bits of hash code, -1 if unused
+            public int next;        // Index of next entry, -1 if last
+            public TKey key;           // Key of entry
+            public TValue value;         // Value of entry
+        }
+
+        private int[] buckets;
+        private Entry[] entries;
+        private int count;
+        private int version;
+        private int freeList;
+        private int freeCount;
+        private IEqualityComparer<TKey> comparer;
+        private KeyCollection keys;
+        private ValueCollection values;
+        private Object _syncRoot;
+        
+        // constants for serialization
+        private const String VersionName = "Version";
+        private const String HashSizeName = "HashSize";  // Must save buckets.Length
+        private const String KeyValuePairsName = "KeyValuePairs";
+        private const String ComparerName = "Comparer";
+
+        public Dictionary(): this(0, null) {}
+
+        public Dictionary(int capacity): this(capacity, null) {}
+
+        public Dictionary(IEqualityComparer<TKey> comparer): this(0, comparer) {}
+
+        public Dictionary(int capacity, IEqualityComparer<TKey> comparer) {
+            if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity);
+            if (capacity > 0) Initialize(capacity);
+            this.comparer = comparer ?? EqualityComparer<TKey>.Default;
+
+#if FEATURE_RANDOMIZED_STRING_HASHING && FEATURE_CORECLR
+            if (HashHelpers.s_UseRandomizedStringHashing && comparer == EqualityComparer<string>.Default)
+            {
+                this.comparer = (IEqualityComparer<TKey>) NonRandomizedStringEqualityComparer.Default;
+            }
+#endif // FEATURE_RANDOMIZED_STRING_HASHING && FEATURE_CORECLR
+        }
+
+        public Dictionary(IDictionary<TKey,TValue> dictionary): this(dictionary, null) {}
+
+        public Dictionary(IDictionary<TKey,TValue> dictionary, IEqualityComparer<TKey> comparer):
+            this(dictionary != null? dictionary.Count: 0, comparer) {
+
+            if( dictionary == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
+            }
+
+            // It is likely that the passed-in dictionary is Dictionary<TKey,TValue>. When this is the case,
+            // avoid the enumerator allocation and overhead by looping through the entries array directly.
+            // We only do this when dictionary is Dictionary<TKey,TValue> and not a subclass, to maintain
+            // back-compat with subclasses that may have overridden the enumerator behavior.
+            if (dictionary.GetType() == typeof(Dictionary<TKey,TValue>)) {
+                Dictionary<TKey,TValue> d = (Dictionary<TKey,TValue>)dictionary;
+                int count = d.count;
+                Entry[] entries = d.entries;
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0) {
+                        Add(entries[i].key, entries[i].value);
+                    }
+                }
+                return;
+            }
+
+            foreach (KeyValuePair<TKey,TValue> pair in dictionary) {
+                Add(pair.Key, pair.Value);
+            }
+        }
+
+        protected Dictionary(SerializationInfo info, StreamingContext context) {
+            //We can't do anything with the keys and values until the entire graph has been deserialized
+            //and we have a resonable estimate that GetHashCode is not going to fail.  For the time being,
+            //we'll just cache this.  The graph is not valid until OnDeserialization has been called.
+            HashHelpers.SerializationInfoTable.Add(this, info);
+        }
+            
+        public IEqualityComparer<TKey> Comparer {
+            get {
+                return comparer;                
+            }               
+        }
+        
+        public int Count {
+            get { return count - freeCount; }
+        }
+
+        public KeyCollection Keys {
+            get {
+                Contract.Ensures(Contract.Result<KeyCollection>() != null);
+                if (keys == null) keys = new KeyCollection(this);
+                return keys;
+            }
+        }
+
+        ICollection<TKey> IDictionary<TKey, TValue>.Keys {
+            get {                
+                if (keys == null) keys = new KeyCollection(this);                
+                return keys;
+            }
+        }
+
+        IEnumerable<TKey> IReadOnlyDictionary<TKey, TValue>.Keys {
+            get {                
+                if (keys == null) keys = new KeyCollection(this);                
+                return keys;
+            }
+        }
+
+        public ValueCollection Values {
+            get {
+                Contract.Ensures(Contract.Result<ValueCollection>() != null);
+                if (values == null) values = new ValueCollection(this);
+                return values;
+            }
+        }
+
+        ICollection<TValue> IDictionary<TKey, TValue>.Values {
+            get {                
+                if (values == null) values = new ValueCollection(this);
+                return values;
+            }
+        }
+
+        IEnumerable<TValue> IReadOnlyDictionary<TKey, TValue>.Values {
+            get {                
+                if (values == null) values = new ValueCollection(this);
+                return values;
+            }
+        }
+
+        public TValue this[TKey key] {
+            get {
+                int i = FindEntry(key);
+                if (i >= 0) return entries[i].value;
+                ThrowHelper.ThrowKeyNotFoundException();
+                return default(TValue);
+            }
+            set {
+                Insert(key, value, false);
+            }
+        }
+
+        public void Add(TKey key, TValue value) {
+            Insert(key, value, true);
+        }
+
+        void ICollection<KeyValuePair<TKey, TValue>>.Add(KeyValuePair<TKey, TValue> keyValuePair) {
+            Add(keyValuePair.Key, keyValuePair.Value);
+        }
+
+        bool ICollection<KeyValuePair<TKey, TValue>>.Contains(KeyValuePair<TKey, TValue> keyValuePair) {
+            int i = FindEntry(keyValuePair.Key);
+            if( i >= 0 && EqualityComparer<TValue>.Default.Equals(entries[i].value, keyValuePair.Value)) {
+                return true;
+            }
+            return false;
+        }
+
+        bool ICollection<KeyValuePair<TKey, TValue>>.Remove(KeyValuePair<TKey, TValue> keyValuePair) {
+            int i = FindEntry(keyValuePair.Key);
+            if( i >= 0 && EqualityComparer<TValue>.Default.Equals(entries[i].value, keyValuePair.Value)) {
+                Remove(keyValuePair.Key);
+                return true;
+            }
+            return false;
+        }
+
+        public void Clear() {
+            if (count > 0) {
+                for (int i = 0; i < buckets.Length; i++) buckets[i] = -1;
+                Array.Clear(entries, 0, count);
+                freeList = -1;
+                count = 0;
+                freeCount = 0;
+                version++;
+            }
+        }
+
+        public bool ContainsKey(TKey key) {
+            return FindEntry(key) >= 0;
+        }
+
+        public bool ContainsValue(TValue value) {
+            if (value == null) {
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0 && entries[i].value == null) return true;
+                }
+            }
+            else {
+                EqualityComparer<TValue> c = EqualityComparer<TValue>.Default;
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0 && c.Equals(entries[i].value, value)) return true;
+                }
+            }
+            return false;
+        }
+
+        private void CopyTo(KeyValuePair<TKey,TValue>[] array, int index) {
+            if (array == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            }
+            
+            if (index < 0 || index > array.Length ) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            }
+
+            if (array.Length - index < Count) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+            }
+
+            int count = this.count;
+            Entry[] entries = this.entries;
+            for (int i = 0; i < count; i++) {
+                if (entries[i].hashCode >= 0) {
+                    array[index++] = new KeyValuePair<TKey,TValue>(entries[i].key, entries[i].value);
+                }
+            }
+        }
+
+        public Enumerator GetEnumerator() {
+            return new Enumerator(this, Enumerator.KeyValuePair);
+        }
+
+        IEnumerator<KeyValuePair<TKey, TValue>> IEnumerable<KeyValuePair<TKey, TValue>>.GetEnumerator() {
+            return new Enumerator(this, Enumerator.KeyValuePair);
+        }        
+
+        [System.Security.SecurityCritical]  // auto-generated_required
+        public virtual void GetObjectData(SerializationInfo info, StreamingContext context) {
+            if (info==null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.info);
+            }
+            info.AddValue(VersionName, version);
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+            info.AddValue(ComparerName, HashHelpers.GetEqualityComparerForSerialization(comparer), typeof(IEqualityComparer<TKey>));
+#else
+            info.AddValue(ComparerName, comparer, typeof(IEqualityComparer<TKey>));
+#endif
+
+            info.AddValue(HashSizeName, buckets == null ? 0 : buckets.Length); //This is the length of the bucket array.
+            if( buckets != null) {
+                KeyValuePair<TKey, TValue>[] array = new KeyValuePair<TKey, TValue>[Count];
+                CopyTo(array, 0);
+                info.AddValue(KeyValuePairsName, array, typeof(KeyValuePair<TKey, TValue>[]));
+            }
+        }
+
+        private int FindEntry(TKey key) {
+            if( key == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
+            }
+
+            if (buckets != null) {
+                int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+                for (int i = buckets[hashCode % buckets.Length]; i >= 0; i = entries[i].next) {
+                    if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) return i;
+                }
+            }
+            return -1;
+        }
+
+        private void Initialize(int capacity) {
+            int size = HashHelpers.GetPrime(capacity);
+            buckets = new int[size];
+            for (int i = 0; i < buckets.Length; i++) buckets[i] = -1;
+            entries = new Entry[size];
+            freeList = -1;
+        }
+
+        private void Insert(TKey key, TValue value, bool add) {
+        
+            if( key == null ) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
+            }
+
+            if (buckets == null) Initialize(0);
+            int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+            int targetBucket = hashCode % buckets.Length;
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+            int collisionCount = 0;
+#endif
+
+            for (int i = buckets[targetBucket]; i >= 0; i = entries[i].next) {
+                if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) {
+                    if (add) { 
+#if FEATURE_CORECLR
+                        ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException(key);
+#else
+                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_AddingDuplicate);
+#endif
+                    }
+                    entries[i].value = value;
+                    version++;
+                    return;
+                } 
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+                collisionCount++;
+#endif
+            }
+            int index;
+            if (freeCount > 0) {
+                index = freeList;
+                freeList = entries[index].next;
+                freeCount--;
+            }
+            else {
+                if (count == entries.Length)
+                {
+                    Resize();
+                    targetBucket = hashCode % buckets.Length;
+                }
+                index = count;
+                count++;
+            }
+
+            entries[index].hashCode = hashCode;
+            entries[index].next = buckets[targetBucket];
+            entries[index].key = key;
+            entries[index].value = value;
+            buckets[targetBucket] = index;
+            version++;
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+
+#if FEATURE_CORECLR
+            // In case we hit the collision threshold we'll need to switch to the comparer which is using randomized string hashing
+            // in this case will be EqualityComparer<string>.Default.
+            // Note, randomized string hashing is turned on by default on coreclr so EqualityComparer<string>.Default will 
+            // be using randomized string hashing
+
+            if (collisionCount > HashHelpers.HashCollisionThreshold && comparer == NonRandomizedStringEqualityComparer.Default) 
+            {
+                comparer = (IEqualityComparer<TKey>) EqualityComparer<string>.Default;
+                Resize(entries.Length, true);
+            }
+#else
+            if(collisionCount > HashHelpers.HashCollisionThreshold && HashHelpers.IsWellKnownEqualityComparer(comparer)) 
+            {
+                comparer = (IEqualityComparer<TKey>) HashHelpers.GetRandomizedEqualityComparer(comparer);
+                Resize(entries.Length, true);
+            }
+#endif // FEATURE_CORECLR
+
+#endif
+
+        }
+
+        public virtual void OnDeserialization(Object sender) {
+            SerializationInfo siInfo;
+            HashHelpers.SerializationInfoTable.TryGetValue(this, out siInfo);
+            
+            if (siInfo==null) {
+                // It might be necessary to call OnDeserialization from a container if the container object also implements
+                // OnDeserialization. However, remoting will call OnDeserialization again.
+                // We can return immediately if this function is called twice. 
+                // Note we set remove the serialization info from the table at the end of this method.
+                return;
+            }            
+            
+            int realVersion = siInfo.GetInt32(VersionName);
+            int hashsize = siInfo.GetInt32(HashSizeName);
+            comparer   = (IEqualityComparer<TKey>)siInfo.GetValue(ComparerName, typeof(IEqualityComparer<TKey>));
+            
+            if( hashsize != 0) {
+                buckets = new int[hashsize];
+                for (int i = 0; i < buckets.Length; i++) buckets[i] = -1;
+                entries = new Entry[hashsize];
+                freeList = -1;
+
+                KeyValuePair<TKey, TValue>[] array = (KeyValuePair<TKey, TValue>[]) 
+                    siInfo.GetValue(KeyValuePairsName, typeof(KeyValuePair<TKey, TValue>[]));
+
+                if (array==null) {
+                    ThrowHelper.ThrowSerializationException(ExceptionResource.Serialization_MissingKeys);
+                }
+
+                for (int i=0; i<array.Length; i++) {
+                    if ( array[i].Key == null) {
+                        ThrowHelper.ThrowSerializationException(ExceptionResource.Serialization_NullKey);
+                    }
+                    Insert(array[i].Key, array[i].Value, true);
+                }
+            }
+            else {
+                buckets = null;
+            }
+
+            version = realVersion;
+            HashHelpers.SerializationInfoTable.Remove(this);
+        }
+
+        private void Resize() {
+            Resize(HashHelpers.ExpandPrime(count), false);
+        }
+
+        private void Resize(int newSize, bool forceNewHashCodes) {
+            Contract.Assert(newSize >= entries.Length);
+            int[] newBuckets = new int[newSize];
+            for (int i = 0; i < newBuckets.Length; i++) newBuckets[i] = -1;
+            Entry[] newEntries = new Entry[newSize];
+            Array.Copy(entries, 0, newEntries, 0, count);
+            if(forceNewHashCodes) {
+                for (int i = 0; i < count; i++) {
+                    if(newEntries[i].hashCode != -1) {
+                        newEntries[i].hashCode = (comparer.GetHashCode(newEntries[i].key) & 0x7FFFFFFF);
+                    }
+                }
+            }
+            for (int i = 0; i < count; i++) {
+                if (newEntries[i].hashCode >= 0) {
+                    int bucket = newEntries[i].hashCode % newSize;
+                    newEntries[i].next = newBuckets[bucket];
+                    newBuckets[bucket] = i;
+                }
+            }
+            buckets = newBuckets;
+            entries = newEntries;
+        }
+
+        public bool Remove(TKey key) {
+            if(key == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
+            }
+
+            if (buckets != null) {
+                int hashCode = comparer.GetHashCode(key) & 0x7FFFFFFF;
+                int bucket = hashCode % buckets.Length;
+                int last = -1;
+                for (int i = buckets[bucket]; i >= 0; last = i, i = entries[i].next) {
+                    if (entries[i].hashCode == hashCode && comparer.Equals(entries[i].key, key)) {
+                        if (last < 0) {
+                            buckets[bucket] = entries[i].next;
+                        }
+                        else {
+                            entries[last].next = entries[i].next;
+                        }
+                        entries[i].hashCode = -1;
+                        entries[i].next = freeList;
+                        entries[i].key = default(TKey);
+                        entries[i].value = default(TValue);
+                        freeList = i;
+                        freeCount++;
+                        version++;
+                        return true;
+                    }
+                }
+            }
+            return false;
+        }
+
+        public bool TryGetValue(TKey key, out TValue value) {
+            int i = FindEntry(key);
+            if (i >= 0) {
+                value = entries[i].value;
+                return true;
+            }
+            value = default(TValue);
+            return false;
+        }
+
+        // This is a convenience method for the internal callers that were converted from using Hashtable.
+        // Many were combining key doesn't exist and key exists but null value (for non-value types) checks.
+        // This allows them to continue getting that behavior with minimal code delta. This is basically
+        // TryGetValue without the out param
+        internal TValue GetValueOrDefault(TKey key) {
+            int i = FindEntry(key);
+            if (i >= 0) {
+                return entries[i].value;
+            }
+            return default(TValue);
+        }
+
+        bool ICollection<KeyValuePair<TKey,TValue>>.IsReadOnly {
+            get { return false; }
+        }
+
+        void ICollection<KeyValuePair<TKey,TValue>>.CopyTo(KeyValuePair<TKey,TValue>[] array, int index) {
+            CopyTo(array, index);
+        }
+
+        void ICollection.CopyTo(Array array, int index) {
+            if (array == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+            }
+            
+            if (array.Rank != 1) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
+            }
+
+            if( array.GetLowerBound(0) != 0 ) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
+            }
+            
+            if (index < 0 || index > array.Length) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            }
+
+            if (array.Length - index < Count) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+            }
+            
+            KeyValuePair<TKey,TValue>[] pairs = array as KeyValuePair<TKey,TValue>[];
+            if (pairs != null) {
+                CopyTo(pairs, index);
+            }
+            else if( array is DictionaryEntry[]) {
+                DictionaryEntry[] dictEntryArray = array as DictionaryEntry[];
+                Entry[] entries = this.entries;
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0) {
+                        dictEntryArray[index++] = new DictionaryEntry(entries[i].key, entries[i].value);
+                    }
+                }                
+            }
+            else {
+                object[] objects = array as object[];
+                if (objects == null) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                }
+
+                try {
+                    int count = this.count;
+                    Entry[] entries = this.entries;
+                    for (int i = 0; i < count; i++) {
+                        if (entries[i].hashCode >= 0) {
+                            objects[index++] = new KeyValuePair<TKey,TValue>(entries[i].key, entries[i].value);
+                        }
+                    }
+                }
+                catch(ArrayTypeMismatchException) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                }
+            }
+        }
+
+        IEnumerator IEnumerable.GetEnumerator() {
+            return new Enumerator(this, Enumerator.KeyValuePair);
+        }
+    
+        bool ICollection.IsSynchronized {
+            get { return false; }
+        }
+
+        object ICollection.SyncRoot { 
+            get { 
+                if( _syncRoot == null) {
+                    System.Threading.Interlocked.CompareExchange<Object>(ref _syncRoot, new Object(), null);    
+                }
+                return _syncRoot; 
+            }
+        }
+
+        bool IDictionary.IsFixedSize {
+            get { return false; }
+        }
+
+        bool IDictionary.IsReadOnly {
+            get { return false; }
+        }
+
+        ICollection IDictionary.Keys {
+            get { return (ICollection)Keys; }
+        }
+    
+        ICollection IDictionary.Values {
+            get { return (ICollection)Values; }
+        }
+    
+        object IDictionary.this[object key] {
+            get { 
+                if( IsCompatibleKey(key)) {                
+                    int i = FindEntry((TKey)key);
+                    if (i >= 0) { 
+                        return entries[i].value;                
+                    }
+                }
+                return null;
+            }
+            set {                 
+                if (key == null)
+                {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);                          
+                }
+                ThrowHelper.IfNullAndNullsAreIllegalThenThrow<TValue>(value, ExceptionArgument.value);
+
+                try {
+                    TKey tempKey = (TKey)key;
+                    try {
+                        this[tempKey] = (TValue)value; 
+                    }
+                    catch (InvalidCastException) { 
+                        ThrowHelper.ThrowWrongValueTypeArgumentException(value, typeof(TValue));   
+                    }
+                }
+                catch (InvalidCastException) { 
+                    ThrowHelper.ThrowWrongKeyTypeArgumentException(key, typeof(TKey));
+                }
+            }
+        }
+
+        private static bool IsCompatibleKey(object key) {
+            if( key == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);                          
+                }
+            return (key is TKey); 
+        }
+    
+        void IDictionary.Add(object key, object value) {            
+            if (key == null)
+            {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);                          
+            }
+            ThrowHelper.IfNullAndNullsAreIllegalThenThrow<TValue>(value, ExceptionArgument.value);
+
+            try {
+                TKey tempKey = (TKey)key;
+
+                try {
+                    Add(tempKey, (TValue)value);
+                }
+                catch (InvalidCastException) { 
+                    ThrowHelper.ThrowWrongValueTypeArgumentException(value, typeof(TValue));   
+                }
+            }
+            catch (InvalidCastException) { 
+                ThrowHelper.ThrowWrongKeyTypeArgumentException(key, typeof(TKey));
+            }
+        }
+    
+        bool IDictionary.Contains(object key) {    
+            if(IsCompatibleKey(key)) {
+                return ContainsKey((TKey)key);
+            }
+       
+            return false;
+        }
+    
+        IDictionaryEnumerator IDictionary.GetEnumerator() {
+            return new Enumerator(this, Enumerator.DictEntry);
+        }
+    
+        void IDictionary.Remove(object key) {            
+            if(IsCompatibleKey(key)) {
+                Remove((TKey)key);
+            }
+        }
+
+        [Serializable]
+        public struct Enumerator: IEnumerator<KeyValuePair<TKey,TValue>>,
+            IDictionaryEnumerator
+        {
+            private Dictionary<TKey,TValue> dictionary;
+            private int version;
+            private int index;
+            private KeyValuePair<TKey,TValue> current;
+            private int getEnumeratorRetType;  // What should Enumerator.Current return?
+            
+            internal const int DictEntry = 1;
+            internal const int KeyValuePair = 2;
+
+            internal Enumerator(Dictionary<TKey,TValue> dictionary, int getEnumeratorRetType) {
+                this.dictionary = dictionary;
+                version = dictionary.version;
+                index = 0;
+                this.getEnumeratorRetType = getEnumeratorRetType;
+                current = new KeyValuePair<TKey, TValue>();
+            }
+
+            public bool MoveNext() {
+                if (version != dictionary.version) {
+                    ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+                }
+
+                // Use unsigned comparison since we set index to dictionary.count+1 when the enumeration ends.
+                // dictionary.count+1 could be negative if dictionary.count is Int32.MaxValue
+                while ((uint)index < (uint)dictionary.count) {
+                    if (dictionary.entries[index].hashCode >= 0) {
+                        current = new KeyValuePair<TKey, TValue>(dictionary.entries[index].key, dictionary.entries[index].value);
+                        index++;
+                        return true;
+                    }
+                    index++;
+                }
+
+                index = dictionary.count + 1;
+                current = new KeyValuePair<TKey, TValue>();
+                return false;
+            }
+
+            public KeyValuePair<TKey,TValue> Current {
+                get { return current; }
+            }
+
+            public void Dispose() {
+            }
+
+            object IEnumerator.Current {
+                get { 
+                    if( index == 0 || (index == dictionary.count + 1)) {
+                        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                    }      
+
+                    if (getEnumeratorRetType == DictEntry) {
+                        return new System.Collections.DictionaryEntry(current.Key, current.Value);
+                    } else {
+                        return new KeyValuePair<TKey, TValue>(current.Key, current.Value);
+                    }
+                }
+            }
+
+            void IEnumerator.Reset() {
+                if (version != dictionary.version) {
+                    ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+                }
+
+                index = 0;
+                current = new KeyValuePair<TKey, TValue>();    
+            }
+
+            DictionaryEntry IDictionaryEnumerator.Entry {
+                get { 
+                    if( index == 0 || (index == dictionary.count + 1)) {
+                         ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                    }                        
+                    
+                    return new DictionaryEntry(current.Key, current.Value); 
+                }
+            }
+
+            object IDictionaryEnumerator.Key {
+                get { 
+                    if( index == 0 || (index == dictionary.count + 1)) {
+                         ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                    }                        
+                    
+                    return current.Key; 
+                }
+            }
+
+            object IDictionaryEnumerator.Value {
+                get { 
+                    if( index == 0 || (index == dictionary.count + 1)) {
+                         ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                    }                        
+                    
+                    return current.Value; 
+                }
+            }
+        }
+
+        [DebuggerTypeProxy(typeof(Mscorlib_DictionaryKeyCollectionDebugView<,>))]
+        [DebuggerDisplay("Count = {Count}")]        
+        [Serializable]
+        public sealed class KeyCollection: ICollection<TKey>, ICollection, IReadOnlyCollection<TKey>
+        {
+            private Dictionary<TKey,TValue> dictionary;
+
+            public KeyCollection(Dictionary<TKey,TValue> dictionary) {
+                if (dictionary == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
+                }
+                this.dictionary = dictionary;
+            }
+
+            public Enumerator GetEnumerator() {
+                return new Enumerator(dictionary);
+            }
+
+            public void CopyTo(TKey[] array, int index) {
+                if (array == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+                }
+
+                if (index < 0 || index > array.Length) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+                }
+
+                if (array.Length - index < dictionary.Count) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+                }
+                
+                int count = dictionary.count;
+                Entry[] entries = dictionary.entries;
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0) array[index++] = entries[i].key;
+                }
+            }
+
+            public int Count {
+                get { return dictionary.Count; }
+            }
+
+            bool ICollection<TKey>.IsReadOnly {
+                get { return true; }
+            }
+
+            void ICollection<TKey>.Add(TKey item){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_KeyCollectionSet);
+            }
+            
+            void ICollection<TKey>.Clear(){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_KeyCollectionSet);
+            }
+
+            bool ICollection<TKey>.Contains(TKey item){
+                return dictionary.ContainsKey(item);
+            }
+
+            bool ICollection<TKey>.Remove(TKey item){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_KeyCollectionSet);
+                return false;
+            }
+            
+            IEnumerator<TKey> IEnumerable<TKey>.GetEnumerator() {
+                return new Enumerator(dictionary);
+            }
+
+            IEnumerator IEnumerable.GetEnumerator() {
+                return new Enumerator(dictionary);                
+            }
+
+            void ICollection.CopyTo(Array array, int index) {
+                if (array==null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+                }
+
+                if (array.Rank != 1) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
+                }
+
+                if( array.GetLowerBound(0) != 0 ) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
+                }
+
+                if (index < 0 || index > array.Length) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+                }
+
+                if (array.Length - index < dictionary.Count) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+                }
+                
+                TKey[] keys = array as TKey[];
+                if (keys != null) {
+                    CopyTo(keys, index);
+                }
+                else {
+                    object[] objects = array as object[];
+                    if (objects == null) {
+                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                    }
+                                         
+                    int count = dictionary.count;
+                    Entry[] entries = dictionary.entries;
+                    try {
+                        for (int i = 0; i < count; i++) {
+                            if (entries[i].hashCode >= 0) objects[index++] = entries[i].key;
+                        }
+                    }                    
+                    catch(ArrayTypeMismatchException) {
+                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                    }
+                }
+            }
+
+            bool ICollection.IsSynchronized {
+                get { return false; }
+            }
+
+            Object ICollection.SyncRoot { 
+                get { return ((ICollection)dictionary).SyncRoot; }
+            }
+
+            [Serializable]
+            public struct Enumerator : IEnumerator<TKey>, System.Collections.IEnumerator
+            {
+                private Dictionary<TKey, TValue> dictionary;
+                private int index;
+                private int version;
+                private TKey currentKey;
+            
+                internal Enumerator(Dictionary<TKey, TValue> dictionary) {
+                    this.dictionary = dictionary;
+                    version = dictionary.version;
+                    index = 0;
+                    currentKey = default(TKey);                    
+                }
+
+                public void Dispose() {
+                }
+
+                public bool MoveNext() {
+                    if (version != dictionary.version) {
+                        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+                    }
+
+                    while ((uint)index < (uint)dictionary.count) {
+                        if (dictionary.entries[index].hashCode >= 0) {
+                            currentKey = dictionary.entries[index].key;
+                            index++;
+                            return true;
+                        }
+                        index++;
+                    }
+
+                    index = dictionary.count + 1;
+                    currentKey = default(TKey);
+                    return false;
+                }
+                
+                public TKey Current {
+                    get {                        
+                        return currentKey;
+                    }
+                }
+
+                Object System.Collections.IEnumerator.Current {
+                    get {                      
+                        if( index == 0 || (index == dictionary.count + 1)) {
+                             ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                        }                        
+                        
+                        return currentKey;
+                    }
+                }
+                
+                void System.Collections.IEnumerator.Reset() {
+                    if (version != dictionary.version) {
+                        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);                        
+                    }
+
+                    index = 0;                    
+                    currentKey = default(TKey);
+                }
+            }                        
+        }
+
+        [DebuggerTypeProxy(typeof(Mscorlib_DictionaryValueCollectionDebugView<,>))]
+        [DebuggerDisplay("Count = {Count}")]
+        [Serializable]
+        public sealed class ValueCollection: ICollection<TValue>, ICollection, IReadOnlyCollection<TValue>
+        {
+            private Dictionary<TKey,TValue> dictionary;
+
+            public ValueCollection(Dictionary<TKey,TValue> dictionary) {
+                if (dictionary == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.dictionary);
+                }
+                this.dictionary = dictionary;
+            }
+
+            public Enumerator GetEnumerator() {
+                return new Enumerator(dictionary);                
+            }
+
+            public void CopyTo(TValue[] array, int index) {
+                if (array == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+                }
+
+                if (index < 0 || index > array.Length) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+                }
+
+                if (array.Length - index < dictionary.Count) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+                }
+                
+                int count = dictionary.count;
+                Entry[] entries = dictionary.entries;
+                for (int i = 0; i < count; i++) {
+                    if (entries[i].hashCode >= 0) array[index++] = entries[i].value;
+                }
+            }
+
+            public int Count {
+                get { return dictionary.Count; }
+            }
+
+            bool ICollection<TValue>.IsReadOnly {
+                get { return true; }
+            }
+
+            void ICollection<TValue>.Add(TValue item){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_ValueCollectionSet);
+            }
+
+            bool ICollection<TValue>.Remove(TValue item){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_ValueCollectionSet);
+                return false;
+            }
+
+            void ICollection<TValue>.Clear(){
+                ThrowHelper.ThrowNotSupportedException(ExceptionResource.NotSupported_ValueCollectionSet);
+            }
+
+            bool ICollection<TValue>.Contains(TValue item){
+                return dictionary.ContainsValue(item);
+            }
+
+            IEnumerator<TValue> IEnumerable<TValue>.GetEnumerator() {
+                return new Enumerator(dictionary);
+            }
+
+            IEnumerator IEnumerable.GetEnumerator() {
+                return new Enumerator(dictionary);                
+            }
+
+            void ICollection.CopyTo(Array array, int index) {
+                if (array == null) {
+                    ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array);
+                }
+
+                if (array.Rank != 1) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
+                }
+
+                if( array.GetLowerBound(0) != 0 ) {
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_NonZeroLowerBound);
+                }
+
+                if (index < 0 || index > array.Length) { 
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+                }
+
+                if (array.Length - index < dictionary.Count)
+                    ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
+                
+                TValue[] values = array as TValue[];
+                if (values != null) {
+                    CopyTo(values, index);
+                }
+                else {
+                    object[] objects = array as object[];
+                    if (objects == null) {
+                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                    }
+
+                    int count = dictionary.count;
+                    Entry[] entries = dictionary.entries;
+                    try {
+                        for (int i = 0; i < count; i++) {
+                            if (entries[i].hashCode >= 0) objects[index++] = entries[i].value;
+                        }
+                    }
+                    catch(ArrayTypeMismatchException) {
+                        ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+                    }
+                }
+            }
+
+            bool ICollection.IsSynchronized {
+                get { return false; }
+            }
+
+            Object ICollection.SyncRoot { 
+                get { return ((ICollection)dictionary).SyncRoot; }
+            }
+
+            [Serializable]
+            public struct Enumerator : IEnumerator<TValue>, System.Collections.IEnumerator
+            {
+                private Dictionary<TKey, TValue> dictionary;
+                private int index;
+                private int version;
+                private TValue currentValue;
+            
+                internal Enumerator(Dictionary<TKey, TValue> dictionary) {
+                    this.dictionary = dictionary;
+                    version = dictionary.version;
+                    index = 0;
+                    currentValue = default(TValue);
+                }
+
+                public void Dispose() {
+                }
+
+                public bool MoveNext() {                    
+                    if (version != dictionary.version) {
+                        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+                    }
+                    
+                    while ((uint)index < (uint)dictionary.count) {
+                        if (dictionary.entries[index].hashCode >= 0) {
+                            currentValue = dictionary.entries[index].value;
+                            index++;
+                            return true;
+                        }
+                        index++;
+                    }
+                    index = dictionary.count + 1;
+                    currentValue = default(TValue);
+                    return false;
+                }
+                
+                public TValue Current {
+                    get {                        
+                        return currentValue;
+                    }
+                }
+
+                Object System.Collections.IEnumerator.Current {
+                    get {                      
+                        if( index == 0 || (index == dictionary.count + 1)) {
+                             ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumOpCantHappen);                        
+                        }                        
+                        
+                        return currentValue;
+                    }
+                }
+                
+                void System.Collections.IEnumerator.Reset() {
+                    if (version != dictionary.version) {
+                        ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+                    }
+                    index = 0;                    
+                    currentValue = default(TValue);
+                }
+            }
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/EqualityComparer.cs b/src/mscorlib/src/System/Collections/Generic/EqualityComparer.cs
new file mode 100644
index 0000000000..b845d64fed
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/EqualityComparer.cs
@@ -0,0 +1,621 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Security;
+using System.Runtime.Serialization;
+
+namespace System.Collections.Generic
+{
+    using System.Globalization;
+    using System.Runtime;
+    using System.Runtime.CompilerServices;
+    using System.Diagnostics.Contracts;
+    
+    [Serializable]
+    [TypeDependencyAttribute("System.Collections.Generic.ObjectEqualityComparer`1")]
+    public abstract class EqualityComparer<T> : IEqualityComparer, IEqualityComparer<T>
+    {
+        static readonly EqualityComparer<T> defaultComparer = CreateComparer();
+
+        public static EqualityComparer<T> Default {
+            get {
+                Contract.Ensures(Contract.Result<EqualityComparer<T>>() != null);
+                return defaultComparer;
+            }
+        }
+
+        //
+        // Note that logic in this method is replicated in vm\compile.cpp to ensure that NGen
+        // saves the right instantiations
+        //
+        [System.Security.SecuritySafeCritical]  // auto-generated
+        private static EqualityComparer<T> CreateComparer()
+        {
+            Contract.Ensures(Contract.Result<EqualityComparer<T>>() != null);
+            
+            object result = null;
+            RuntimeType t = (RuntimeType)typeof(T);
+            
+            // Specialize type byte for performance reasons
+            if (t == typeof(byte)) {
+                result = new ByteEqualityComparer();
+            }
+            // If T implements IEquatable<T> return a GenericEqualityComparer<T>
+            else if (typeof(IEquatable<T>).IsAssignableFrom(t))
+            {
+                result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericEqualityComparer<int>), t);
+            }
+            else if (default(T) == null) // Reference type/Nullable
+            {
+                // If T is a Nullable<U> where U implements IEquatable<U> return a NullableEqualityComparer<U>
+                if (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>)) {
+                    RuntimeType u = (RuntimeType)t.GetGenericArguments()[0];
+                    if (typeof(IEquatable<>).MakeGenericType(u).IsAssignableFrom(u)) {
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(NullableEqualityComparer<int>), u);
+                    }
+                }
+            }
+            // See the METHOD__JIT_HELPERS__UNSAFE_ENUM_CAST and METHOD__JIT_HELPERS__UNSAFE_ENUM_CAST_LONG cases in getILIntrinsicImplementation
+            else if (t.IsEnum) {
+                TypeCode underlyingTypeCode = Type.GetTypeCode(Enum.GetUnderlyingType(t));
+
+                // Depending on the enum type, we need to special case the comparers so that we avoid boxing
+                // Note: We have different comparers for Short and SByte because for those types we need to make sure we call GetHashCode on the actual underlying type as the 
+                // implementation of GetHashCode is more complex than for the other types.
+                switch (underlyingTypeCode) {
+                    case TypeCode.Int16: // short
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(ShortEnumEqualityComparer<short>), t);
+                        break;
+                    case TypeCode.SByte:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(SByteEnumEqualityComparer<sbyte>), t);
+                        break;
+                    case TypeCode.Int32:
+                    case TypeCode.UInt32:
+                    case TypeCode.Byte:
+                    case TypeCode.UInt16: //ushort
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(EnumEqualityComparer<int>), t);
+                        break;
+                    case TypeCode.Int64:
+                    case TypeCode.UInt64:
+                        result = RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(LongEnumEqualityComparer<long>), t);
+                        break;
+                }
+            }
+            
+            return result != null ?
+                (EqualityComparer<T>)result :
+                new ObjectEqualityComparer<T>(); // Fallback to ObjectEqualityComparer, which uses boxing
+        }
+
+        [Pure]
+        public abstract bool Equals(T x, T y);
+        [Pure]
+        public abstract int GetHashCode(T obj);
+
+        internal virtual int IndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex + count;
+            for (int i = startIndex; i < endIndex; i++) {
+                if (Equals(array[i], value)) return i;
+            }
+            return -1;
+        }
+
+        internal virtual int LastIndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex - count + 1;
+            for (int i = startIndex; i >= endIndex; i--) {
+                if (Equals(array[i], value)) return i;
+            }
+            return -1;
+        }
+
+        int IEqualityComparer.GetHashCode(object obj) {
+            if (obj == null) return 0;
+            if (obj is T) return GetHashCode((T)obj);
+            ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArgumentForComparison);
+            return 0;            
+        }                        
+
+        bool IEqualityComparer.Equals(object x, object y) {
+            if (x == y) return true;
+            if (x == null || y == null) return false;
+            if ((x is T) && (y is T)) return Equals((T)x, (T)y);
+            ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArgumentForComparison);
+            return false;
+        }
+    }
+
+    // The methods in this class look identical to the inherited methods, but the calls
+    // to Equal bind to IEquatable<T>.Equals(T) instead of Object.Equals(Object)
+    [Serializable]
+    internal class GenericEqualityComparer<T>: EqualityComparer<T> where T: IEquatable<T>
+    {
+        [Pure]
+        public override bool Equals(T x, T y) {
+            if (x != null) {
+                if (y != null) return x.Equals(y);
+                return false;
+            }
+            if (y != null) return false;
+            return true;
+        }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            if (obj == null) return 0;
+            return obj.GetHashCode();
+        }
+
+        internal override int IndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex + count;
+            if (value == null) {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (array[i] == null) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (array[i] != null && array[i].Equals(value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        internal override int LastIndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex - count + 1;
+            if (value == null) {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (array[i] == null) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (array[i] != null && array[i].Equals(value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            GenericEqualityComparer<T> comparer = obj as GenericEqualityComparer<T>;
+            return comparer != null;
+        }
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }
+    }
+
+    [Serializable]
+    internal class NullableEqualityComparer<T> : EqualityComparer<Nullable<T>> where T : struct, IEquatable<T>
+    {
+        [Pure]
+        public override bool Equals(Nullable<T> x, Nullable<T> y) {
+            if (x.HasValue) {
+                if (y.HasValue) return x.value.Equals(y.value);
+                return false;
+            }
+            if (y.HasValue) return false;
+            return true;
+        }
+
+        [Pure]
+        public override int GetHashCode(Nullable<T> obj) {
+            return obj.GetHashCode();
+        }
+
+        internal override int IndexOf(Nullable<T>[] array, Nullable<T> value, int startIndex, int count) {
+            int endIndex = startIndex + count;
+            if (!value.HasValue) {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (!array[i].HasValue) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (array[i].HasValue && array[i].value.Equals(value.value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        internal override int LastIndexOf(Nullable<T>[] array, Nullable<T> value, int startIndex, int count) {
+            int endIndex = startIndex - count + 1;
+            if (!value.HasValue) {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (!array[i].HasValue) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (array[i].HasValue && array[i].value.Equals(value.value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            NullableEqualityComparer<T> comparer = obj as NullableEqualityComparer<T>;
+            return comparer != null;
+        }        
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }                                
+    }
+
+    [Serializable]
+    internal class ObjectEqualityComparer<T>: EqualityComparer<T>
+    {
+        [Pure]
+        public override bool Equals(T x, T y) {
+            if (x != null) {
+                if (y != null) return x.Equals(y);
+                return false;
+            }
+            if (y != null) return false;
+            return true;
+        }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            if (obj == null) return 0;
+            return obj.GetHashCode();
+        }
+
+        internal override int IndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex + count;
+            if (value == null) {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (array[i] == null) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i < endIndex; i++) {
+                    if (array[i] != null && array[i].Equals(value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        internal override int LastIndexOf(T[] array, T value, int startIndex, int count) {
+            int endIndex = startIndex - count + 1;
+            if (value == null) {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (array[i] == null) return i;
+                }
+            }
+            else {
+                for (int i = startIndex; i >= endIndex; i--) {
+                    if (array[i] != null && array[i].Equals(value)) return i;
+                }
+            }
+            return -1;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            ObjectEqualityComparer<T> comparer = obj as ObjectEqualityComparer<T>;
+            return comparer != null;
+        }        
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }                                
+    }
+
+#if FEATURE_CORECLR
+    // NonRandomizedStringEqualityComparer is the comparer used by default with the Dictionary<string,...> 
+    // As the randomized string hashing is turned on by default on coreclr, we need to keep the performance not affected 
+    // as much as possible in the main stream scenarios like Dictionary<string,>
+    // We use NonRandomizedStringEqualityComparer as default comparer as it doesnt use the randomized string hashing which 
+    // keep the perofrmance not affected till we hit collision threshold and then we switch to the comparer which is using 
+    // randomized string hashing GenericEqualityComparer<string>
+
+    internal class NonRandomizedStringEqualityComparer : GenericEqualityComparer<string> {
+        static IEqualityComparer<string> s_nonRandomizedComparer;
+        
+        internal static new IEqualityComparer<string> Default {
+            get  {
+                    if (s_nonRandomizedComparer == null) {
+                        s_nonRandomizedComparer = new NonRandomizedStringEqualityComparer();
+                    }
+                    return s_nonRandomizedComparer;
+            }
+        }
+
+        [Pure]
+        public override int GetHashCode(string obj)  {
+            if (obj == null) return 0;
+            return obj.GetLegacyNonRandomizedHashCode();
+        }
+    }
+#endif // FEATURE_CORECLR
+
+    // Performance of IndexOf on byte array is very important for some scenarios.
+    // We will call the C runtime function memchr, which is optimized.
+    [Serializable]
+    internal class ByteEqualityComparer: EqualityComparer<byte>
+    {
+        [Pure]
+        public override bool Equals(byte x, byte y) {
+            return x == y;
+        }
+
+        [Pure]
+        public override int GetHashCode(byte b) {
+            return b.GetHashCode();
+        }
+
+        [System.Security.SecuritySafeCritical]  // auto-generated
+        internal unsafe override int IndexOf(byte[] array, byte value, int startIndex, int count) {
+            if (array==null)
+                throw new ArgumentNullException("array");
+            if (startIndex < 0)
+                throw new ArgumentOutOfRangeException("startIndex", Environment.GetResourceString("ArgumentOutOfRange_Index"));
+            if (count < 0)
+                throw new ArgumentOutOfRangeException("count", Environment.GetResourceString("ArgumentOutOfRange_Count"));
+            if (count > array.Length - startIndex)
+                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidOffLen"));
+            Contract.EndContractBlock();
+            if (count == 0) return -1;
+            fixed (byte* pbytes = array) {
+                return Buffer.IndexOfByte(pbytes, value, startIndex, count);
+            }
+        }
+
+        internal override int LastIndexOf(byte[] array, byte value, int startIndex, int count) {
+            int endIndex = startIndex - count + 1;
+            for (int i = startIndex; i >= endIndex; i--) {
+                if (array[i] == value) return i;
+            }
+            return -1;
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            ByteEqualityComparer comparer = obj as ByteEqualityComparer;
+            return comparer != null;
+        }        
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }                                
+    }
+
+    [Serializable]
+    internal class EnumEqualityComparer<T> : EqualityComparer<T>, ISerializable where T : struct
+    {
+        [Pure]
+        public override bool Equals(T x, T y) {
+            int x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCast(x);
+            int y_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCast(y);
+            return x_final == y_final;
+        }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            int x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCast(obj);
+            return x_final.GetHashCode();
+        }
+
+        public EnumEqualityComparer() { }
+
+        // This is used by the serialization engine.
+        protected EnumEqualityComparer(SerializationInfo information, StreamingContext context) { }
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context) {
+            // For back-compat we need to serialize the comparers for enums with underlying types other than int as ObjectEqualityComparer 
+            if (Type.GetTypeCode(Enum.GetUnderlyingType(typeof(T))) != TypeCode.Int32) {
+                info.SetType(typeof(ObjectEqualityComparer<T>));
+            }
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            EnumEqualityComparer<T> comparer = obj as EnumEqualityComparer<T>;
+            return comparer != null;
+        }
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }
+    }
+
+    [Serializable]
+    internal sealed class SByteEnumEqualityComparer<T> : EnumEqualityComparer<T>, ISerializable where T : struct
+    {
+        public SByteEnumEqualityComparer() { }
+
+        // This is used by the serialization engine.
+        public SByteEnumEqualityComparer(SerializationInfo information, StreamingContext context) { }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            int x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCast(obj);
+            return ((sbyte)x_final).GetHashCode();
+        }
+    }
+
+    [Serializable]
+    internal sealed class ShortEnumEqualityComparer<T> : EnumEqualityComparer<T>, ISerializable where T : struct
+    {
+        public ShortEnumEqualityComparer() { }
+
+        // This is used by the serialization engine.
+        public ShortEnumEqualityComparer(SerializationInfo information, StreamingContext context) { }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            int x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCast(obj);
+            return ((short)x_final).GetHashCode();
+        }
+    }
+
+    [Serializable]
+    internal sealed class LongEnumEqualityComparer<T> : EqualityComparer<T>, ISerializable where T : struct
+    {
+        [Pure]
+        public override bool Equals(T x, T y) {
+            long x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCastLong(x);
+            long y_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCastLong(y);
+            return x_final == y_final;
+        }
+
+        [Pure]
+        public override int GetHashCode(T obj) {
+            long x_final = System.Runtime.CompilerServices.JitHelpers.UnsafeEnumCastLong(obj);
+            return x_final.GetHashCode();
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            LongEnumEqualityComparer<T> comparer = obj as LongEnumEqualityComparer<T>;
+            return comparer != null;
+        }
+
+        public override int GetHashCode() {
+            return this.GetType().Name.GetHashCode();
+        }
+
+        public LongEnumEqualityComparer() { }
+
+        // This is used by the serialization engine.
+        public LongEnumEqualityComparer(SerializationInfo information, StreamingContext context) { }
+
+        [SecurityCritical]
+        public void GetObjectData(SerializationInfo info, StreamingContext context)
+        {
+            // The LongEnumEqualityComparer does not exist on 4.0 so we need to serialize this comparer as ObjectEqualityComparer
+            // to allow for roundtrip between 4.0 and 4.5.
+            info.SetType(typeof(ObjectEqualityComparer<T>));
+        }
+    }
+
+#if FEATURE_RANDOMIZED_STRING_HASHING
+    // This type is not serializeable by design.  It does not exist in previous versions and will be removed 
+    // Once we move the framework to using secure hashing by default.
+    internal sealed class RandomizedStringEqualityComparer : IEqualityComparer<String>, IEqualityComparer, IWellKnownStringEqualityComparer
+    {
+        private long _entropy;
+
+        public RandomizedStringEqualityComparer() {
+            _entropy = HashHelpers.GetEntropy();
+        }
+
+        public new bool Equals(object x, object y) {
+            if (x == y) return true;
+            if (x == null || y == null) return false;
+            if ((x is string) && (y is string)) return Equals((string)x, (string)y);
+            ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArgumentForComparison);
+            return false;
+        }
+
+        [Pure]
+        public bool Equals(string x, string y) {
+            if (x != null) {
+                if (y != null) return x.Equals(y);
+                return false;
+            }
+            if (y != null) return false;
+            return true;
+        }
+
+        [Pure]
+        [SecuritySafeCritical]
+        public int GetHashCode(String obj) {
+            if(obj == null) return 0;
+            return String.InternalMarvin32HashString(obj, obj.Length, _entropy);
+        }
+
+        [Pure]
+        [SecuritySafeCritical]
+        public int GetHashCode(Object obj) {
+            if(obj == null) return 0;
+
+            string sObj = obj as string;
+            if(sObj != null) return  String.InternalMarvin32HashString(sObj, sObj.Length, _entropy);
+
+            return obj.GetHashCode(); 
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj) {
+            RandomizedStringEqualityComparer comparer = obj as RandomizedStringEqualityComparer; 
+            return (comparer != null) && (this._entropy == comparer._entropy);
+        }
+
+        public override int GetHashCode() {
+            return (this.GetType().Name.GetHashCode() ^ ((int) (_entropy & 0x7FFFFFFF))); 
+        }
+
+
+        IEqualityComparer IWellKnownStringEqualityComparer.GetRandomizedEqualityComparer() {
+            return new RandomizedStringEqualityComparer();
+        }
+
+        // We want to serialize the old comparer.
+        IEqualityComparer IWellKnownStringEqualityComparer.GetEqualityComparerForSerialization() {
+            return EqualityComparer<string>.Default;
+        } 
+    }
+
+    // This type is not serializeable by design.  It does not exist in previous versions and will be removed 
+    // Once we move the framework to using secure hashing by default.
+    internal sealed class RandomizedObjectEqualityComparer : IEqualityComparer, IWellKnownStringEqualityComparer
+    {
+        private long _entropy;
+
+        public RandomizedObjectEqualityComparer() {
+            _entropy = HashHelpers.GetEntropy();
+        }
+
+        [Pure]
+        public new bool Equals(Object x, Object y) {
+            if (x != null) {
+                if (y != null) return x.Equals(y);
+                return false;
+            }
+            if (y != null) return false;
+            return true;
+        }
+
+        [Pure]
+        [SecuritySafeCritical]
+        public int GetHashCode(Object obj) {
+            if(obj == null) return 0;
+
+            string sObj = obj as string;
+            if(sObj != null) return  String.InternalMarvin32HashString(sObj, sObj.Length, _entropy);
+
+            return obj.GetHashCode();           
+        }
+
+        // Equals method for the comparer itself. 
+        public override bool Equals(Object obj){
+            RandomizedObjectEqualityComparer comparer = obj as RandomizedObjectEqualityComparer; 
+            return (comparer != null) && (this._entropy == comparer._entropy);
+        }
+
+        public override int GetHashCode() {
+            return (this.GetType().Name.GetHashCode() ^ ((int) (_entropy & 0x7FFFFFFF))); 
+        }
+
+        IEqualityComparer IWellKnownStringEqualityComparer.GetRandomizedEqualityComparer() {
+            return new RandomizedObjectEqualityComparer();
+        }
+
+        // We want to serialize the old comparer, which in this case was null.
+        IEqualityComparer IWellKnownStringEqualityComparer.GetEqualityComparerForSerialization() {
+            return null;
+        }   
+    }
+#endif
+}
+
diff --git a/src/mscorlib/src/System/Collections/Generic/ICollection.cs b/src/mscorlib/src/System/Collections/Generic/ICollection.cs
new file mode 100644
index 0000000000..741e8cc79b
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/ICollection.cs
@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  ICollection
+** 
+** 
+**
+**
+** Purpose: Base interface for all generic collections.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    using System;
+    using System.Runtime.CompilerServices;
+    using System.Diagnostics.Contracts;
+
+    // Base interface for all collections, defining enumerators, size, and 
+    // synchronization methods.
+
+    // Note that T[] : IList<T>, and we want to ensure that if you use
+    // IList<YourValueType>, we ensure a YourValueType[] can be used 
+    // without jitting.  Hence the TypeDependencyAttribute on SZArrayHelper.
+    // This is a special workaround internally though - see VM\compile.cpp.
+    // The same attribute is on IEnumerable<T> and ICollection<T>.
+    [TypeDependencyAttribute("System.SZArrayHelper")]
+    public interface ICollection<T> : IEnumerable<T>
+    {
+        // Number of items in the collections.        
+        int Count { get; }
+
+        bool IsReadOnly { get; }
+
+        void Add(T item);
+
+        void Clear();
+
+        bool Contains(T item); 
+                
+        // CopyTo copies a collection into an Array, starting at a particular
+        // index into the array.
+        // 
+        void CopyTo(T[] array, int arrayIndex);
+                
+        //void CopyTo(int sourceIndex, T[] destinationArray, int destinationIndex, int count);
+
+        bool Remove(T item);
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IComparer.cs b/src/mscorlib/src/System/Collections/Generic/IComparer.cs
new file mode 100644
index 0000000000..7b9e97ff0e
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IComparer.cs
@@ -0,0 +1,30 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IComparer
+** 
+** 
+**
+**
+** Purpose: Interface for comparing two generic Objects.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    
+    using System;
+    // The generic IComparer interface implements a method that compares 
+    // two objects. It is used in conjunction with the Sort and 
+    // BinarySearch methods on the Array, List, and SortedList classes.
+    public interface IComparer<in T>
+    {
+        // Compares two objects. An implementation of this method must return a
+        // value less than zero if x is less than y, zero if x is equal to y, or a
+        // value greater than zero if x is greater than y.
+        // 
+        int Compare(T x, T y);
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IDictionary.cs b/src/mscorlib/src/System/Collections/Generic/IDictionary.cs
new file mode 100644
index 0000000000..2a2da944d3
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IDictionary.cs
@@ -0,0 +1,58 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IDictionary
+** 
+** 
+**
+**
+** Purpose: Base interface for all generic dictionaries.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    using System;
+    using System.Diagnostics.Contracts;
+
+    // An IDictionary is a possibly unordered set of key-value pairs.
+    // Keys can be any non-null object.  Values can be any object.
+    // You can look up a value in an IDictionary via the default indexed
+    // property, Items.
+    public interface IDictionary<TKey, TValue> : ICollection<KeyValuePair<TKey, TValue>>
+    {
+        // Interfaces are not serializable
+        // The Item property provides methods to read and edit entries 
+        // in the Dictionary.
+        TValue this[TKey key] {
+            get;
+            set;
+        }
+    
+        // Returns a collections of the keys in this dictionary.
+        ICollection<TKey> Keys {
+            get;
+        }
+    
+        // Returns a collections of the values in this dictionary.
+        ICollection<TValue> Values {
+            get;
+        }
+    
+        // Returns whether this dictionary contains a particular key.
+        //
+        bool ContainsKey(TKey key);
+    
+        // Adds a key-value pair to the dictionary.
+        // 
+        void Add(TKey key, TValue value);
+    
+        // Removes a particular key from the dictionary.
+        //
+        bool Remove(TKey key);
+
+        bool TryGetValue(TKey key, out TValue value);
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IEnumerable.cs b/src/mscorlib/src/System/Collections/Generic/IEnumerable.cs
new file mode 100644
index 0000000000..67f35ce675
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IEnumerable.cs
@@ -0,0 +1,38 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IEnumerable
+** 
+** 
+**
+**
+** Purpose: Interface for providing generic IEnumerators
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    using System;
+    using System.Collections;
+    using System.Runtime.InteropServices;
+    using System.Runtime.CompilerServices;
+    using System.Diagnostics.Contracts;
+
+    // Implement this interface if you need to support foreach semantics.
+
+    // Note that T[] : IList<T>, and we want to ensure that if you use
+    // IList<YourValueType>, we ensure a YourValueType[] can be used 
+    // without jitting.  Hence the TypeDependencyAttribute on SZArrayHelper.
+    // This is a special workaround internally though - see VM\compile.cpp.
+    // The same attribute is on IList<T> and ICollection<T>.
+    [TypeDependencyAttribute("System.SZArrayHelper")]
+    public interface IEnumerable<out T> : IEnumerable
+    {
+        // Returns an IEnumerator for this enumerable Object.  The enumerator provides
+        // a simple way to access all the contents of a collection.
+        /// <include file='doc\IEnumerable.uex' path='docs/doc[@for="IEnumerable.GetEnumerator"]/*' />
+        new IEnumerator<T> GetEnumerator();
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IEnumerator.cs b/src/mscorlib/src/System/Collections/Generic/IEnumerator.cs
new file mode 100644
index 0000000000..335616757b
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IEnumerator.cs
@@ -0,0 +1,35 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IEnumerator
+** 
+** 
+**
+**
+** Purpose: Base interface for all generic enumerators.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {    
+    using System;
+    using System.Runtime.InteropServices;
+
+    // Base interface for all generic enumerators, providing a simple approach
+    // to iterating over a collection.
+    public interface IEnumerator<out T> : IDisposable, IEnumerator
+    {    
+        // Returns the current element of the enumeration. The returned value is
+        // undefined before the first call to MoveNext and following a
+        // call to MoveNext that returned false. Multiple calls to
+        // GetCurrent with no intervening calls to MoveNext 
+        // will return the same object.
+        // 
+        /// <include file='doc\IEnumerator.uex' path='docs/doc[@for="IEnumerator.Current"]/*' />
+        new T Current {
+            get; 
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IEqualityComparer.cs b/src/mscorlib/src/System/Collections/Generic/IEqualityComparer.cs
new file mode 100644
index 0000000000..b6ac3be006
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IEqualityComparer.cs
@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+// 
+
+namespace System.Collections.Generic {
+    using System;
+
+    // The generic IEqualityComparer interface implements methods to if check two objects are equal
+    // and generate Hashcode for an object.
+    // It is use in Dictionary class.  
+    public interface IEqualityComparer<in T>
+    {
+        bool Equals(T x, T y);
+        int GetHashCode(T obj);                
+    }
+}
+
diff --git a/src/mscorlib/src/System/Collections/Generic/IList.cs b/src/mscorlib/src/System/Collections/Generic/IList.cs
new file mode 100644
index 0000000000..75ca0a9b00
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IList.cs
@@ -0,0 +1,54 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IList
+** 
+** 
+**
+**
+** Purpose: Base interface for all generic lists.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    
+    using System;
+    using System.Collections;
+    using System.Runtime.CompilerServices;
+    using System.Diagnostics.Contracts;
+
+    // An IList is an ordered collection of objects.  The exact ordering
+    // is up to the implementation of the list, ranging from a sorted
+    // order to insertion order.  
+
+    // Note that T[] : IList<T>, and we want to ensure that if you use
+    // IList<YourValueType>, we ensure a YourValueType[] can be used 
+    // without jitting.  Hence the TypeDependencyAttribute on SZArrayHelper.
+    // This is a special workaround internally though - see VM\compile.cpp.
+    // The same attribute is on IEnumerable<T> and ICollection<T>.
+    [TypeDependencyAttribute("System.SZArrayHelper")]
+    public interface IList<T> : ICollection<T>
+    {
+        // The Item property provides methods to read and edit entries in the List.
+        T this[int index] {
+            get;
+            set;
+        }
+    
+        // Returns the index of a particular item, if it is in the list.
+        // Returns -1 if the item isn't in the list.
+        int IndexOf(T item);
+    
+        // Inserts value into the list at position index.
+        // index must be non-negative and less than or equal to the 
+        // number of elements in the list.  If index equals the number
+        // of items in the list, then value is appended to the end.
+        void Insert(int index, T item);
+        
+        // Removes the item at position index.
+        void RemoveAt(int index);
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IReadOnlyCollection.cs b/src/mscorlib/src/System/Collections/Generic/IReadOnlyCollection.cs
new file mode 100644
index 0000000000..13bc718760
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IReadOnlyCollection.cs
@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IReadOnlyCollection<T>
+** 
+** 
+**
+** Purpose: Base interface for read-only generic lists.
+** 
+===========================================================*/
+using System;
+using System.Diagnostics.Contracts;
+using System.Runtime.CompilerServices;
+
+namespace System.Collections.Generic
+{
+
+    // Provides a read-only, covariant view of a generic list.
+
+    // Note that T[] : IReadOnlyList<T>, and we want to ensure that if you use
+    // IList<YourValueType>, we ensure a YourValueType[] can be used 
+    // without jitting.  Hence the TypeDependencyAttribute on SZArrayHelper.
+    // This is a special workaround internally though - see VM\compile.cpp.
+    // The same attribute is on IList<T>, IEnumerable<T>, ICollection<T>, and IReadOnlyList<T>.
+    [TypeDependencyAttribute("System.SZArrayHelper")]
+    // If we ever implement more interfaces on IReadOnlyCollection, we should also update RuntimeTypeCache.PopulateInterfaces() in rttype.cs
+    public interface IReadOnlyCollection<out T> : IEnumerable<T>
+    {
+        int Count { get; }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IReadOnlyDictionary.cs b/src/mscorlib/src/System/Collections/Generic/IReadOnlyDictionary.cs
new file mode 100644
index 0000000000..3603b9a4ea
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IReadOnlyDictionary.cs
@@ -0,0 +1,29 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IReadOnlyDictionary<TKey, TValue>
+** 
+** 
+**
+** Purpose: Base interface for read-only generic dictionaries.
+** 
+===========================================================*/
+using System;
+using System.Diagnostics.Contracts;
+
+namespace System.Collections.Generic
+{
+    // Provides a read-only view of a generic dictionary.
+    public interface IReadOnlyDictionary<TKey, TValue> : IReadOnlyCollection<KeyValuePair<TKey, TValue>>
+    {
+        bool ContainsKey(TKey key);
+        bool TryGetValue(TKey key, out TValue value);
+
+        TValue this[TKey key] { get; }
+        IEnumerable<TKey> Keys { get; }
+        IEnumerable<TValue> Values { get; }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/IReadOnlyList.cs b/src/mscorlib/src/System/Collections/Generic/IReadOnlyList.cs
new file mode 100644
index 0000000000..77366f0b2f
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/IReadOnlyList.cs
@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  IReadOnlyList<T>
+** 
+** 
+**
+** Purpose: Base interface for read-only generic lists.
+** 
+===========================================================*/
+using System;
+using System.Diagnostics.Contracts;
+using System.Runtime.CompilerServices;
+
+namespace System.Collections.Generic
+{
+
+    // Provides a read-only, covariant view of a generic list.
+
+    // Note that T[] : IReadOnlyList<T>, and we want to ensure that if you use
+    // IList<YourValueType>, we ensure a YourValueType[] can be used 
+    // without jitting.  Hence the TypeDependencyAttribute on SZArrayHelper.
+    // This is a special workaround internally though - see VM\compile.cpp.
+    // The same attribute is on IList<T>, IEnumerable<T>, ICollection<T> and IReadOnlyCollection<T>.
+    [TypeDependencyAttribute("System.SZArrayHelper")]
+    // If we ever implement more interfaces on IReadOnlyList, we should also update RuntimeTypeCache.PopulateInterfaces() in rttype.cs
+    public interface IReadOnlyList<out T> : IReadOnlyCollection<T>
+    {
+        T this[int index] { get; }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/KeyNotFoundException.cs b/src/mscorlib/src/System/Collections/Generic/KeyNotFoundException.cs
new file mode 100644
index 0000000000..ffcd0405fd
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/KeyNotFoundException.cs
@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*=============================================================================
+**
+** 
+** 
+**
+**
+** Purpose: Exception class for Hashtable and Dictionary.
+**
+**
+=============================================================================*/
+
+namespace System.Collections.Generic {
+    
+    using System;
+    using System.Runtime.Remoting;
+    using System.Runtime.Serialization;
+
+    [Serializable]
+    [System.Runtime.InteropServices.ComVisible(true)]
+    public class KeyNotFoundException  : SystemException, ISerializable {
+    
+        public KeyNotFoundException () 
+            : base(Environment.GetResourceString("Arg_KeyNotFound")) {
+            SetErrorCode(System.__HResults.COR_E_KEYNOTFOUND);
+        }
+        
+        public KeyNotFoundException(String message) 
+            : base(message) {
+            SetErrorCode(System.__HResults.COR_E_KEYNOTFOUND);
+        }
+        
+        public KeyNotFoundException(String message, Exception innerException) 
+            : base(message, innerException) {
+            SetErrorCode(System.__HResults.COR_E_KEYNOTFOUND);
+        }
+
+
+        protected KeyNotFoundException(SerializationInfo info, StreamingContext context) : base(info, context) {
+        }        
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/KeyValuePair.cs b/src/mscorlib/src/System/Collections/Generic/KeyValuePair.cs
new file mode 100644
index 0000000000..17e1c531f1
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/KeyValuePair.cs
@@ -0,0 +1,56 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** Interface:  KeyValuePair
+** 
+** 
+**
+**
+** Purpose: Generic key-value pair for dictionary enumerators.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+    
+    using System;
+    using System.Text;
+
+    // A KeyValuePair holds a key and a value from a dictionary.
+    // It is used by the IEnumerable<T> implementation for both IDictionary<TKey, TValue>
+    // and IReadOnlyDictionary<TKey, TValue>.
+    [Serializable]
+    public struct KeyValuePair<TKey, TValue> {
+        private TKey key;
+        private TValue value;
+
+        public KeyValuePair(TKey key, TValue value) {
+            this.key = key;
+            this.value = value;
+        }
+
+        public TKey Key {
+            get { return key; }
+        }
+
+        public TValue Value {
+            get { return value; }
+        }
+
+        public override string ToString() {
+            StringBuilder s = StringBuilderCache.Acquire();
+            s.Append('[');
+            if( Key != null) {
+                s.Append(Key.ToString());
+            }
+            s.Append(", ");
+            if( Value != null) {
+               s.Append(Value.ToString());
+            }
+            s.Append(']');
+            return StringBuilderCache.GetStringAndRelease(s);
+        }
+    }
+}
diff --git a/src/mscorlib/src/System/Collections/Generic/List.cs b/src/mscorlib/src/System/Collections/Generic/List.cs
new file mode 100644
index 0000000000..ae3356d372
--- /dev/null
+++ b/src/mscorlib/src/System/Collections/Generic/List.cs
@@ -0,0 +1,1120 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+/*============================================================
+**
+** 
+** 
+**
+** Purpose: Implements a generic, dynamically sized list as an 
+**          array.
+**
+** 
+===========================================================*/
+namespace System.Collections.Generic {
+
+    using System;
+    using System.Runtime;
+    using System.Runtime.Versioning;
+    using System.Diagnostics;
+    using System.Diagnostics.Contracts;
+    using System.Collections.ObjectModel;
+    using System.Security.Permissions;
+
+    // Implements a variable-size List that uses an array of objects to store the
+    // elements. A List has a capacity, which is the allocated length
+    // of the internal array. As elements are added to a List, the capacity
+    // of the List is automatically increased as required by reallocating the
+    // internal array.
+    // 
+    [DebuggerTypeProxy(typeof(Mscorlib_CollectionDebugView<>))]
+    [DebuggerDisplay("Count = {Count}")]
+    [Serializable]
+    public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>
+    {
+        private const int _defaultCapacity = 4;
+
+        private T[] _items;
+        [ContractPublicPropertyName("Count")]
+        private int _size;
+        private int _version;
+        [NonSerialized]
+        private Object _syncRoot;
+        
+        static readonly T[]  _emptyArray = new T[0];        
+            
+        // Constructs a List. The list is initially empty and has a capacity
+        // of zero. Upon adding the first element to the list the capacity is
+        // increased to _defaultCapacity, and then increased in multiples of two
+        // as required.
+        public List() {
+            _items = _emptyArray;
+        }
+    
+        // Constructs a List with a given initial capacity. The list is
+        // initially empty, but will have room for the given number of elements
+        // before any reallocations are required.
+        // 
+        public List(int capacity) {
+            if (capacity < 0) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.capacity, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            Contract.EndContractBlock();
+
+            if (capacity == 0)
+                _items = _emptyArray;
+            else
+                _items = new T[capacity];
+        }
+    
+        // Constructs a List, copying the contents of the given collection. The
+        // size and capacity of the new list will both be equal to the size of the
+        // given collection.
+        // 
+        public List(IEnumerable<T> collection) {
+            if (collection==null)
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
+            Contract.EndContractBlock();
+
+            ICollection<T> c = collection as ICollection<T>;
+            if( c != null) {
+                int count = c.Count;
+                if (count == 0)
+                {
+                    _items = _emptyArray;
+                }
+                else {
+                    _items = new T[count];
+                    c.CopyTo(_items, 0);
+                    _size = count;
+                }
+            }    
+            else {                
+                _size = 0;
+                _items = _emptyArray;
+                // This enumerable could be empty.  Let Add allocate a new array, if needed.
+                // Note it will also go to _defaultCapacity first, not 1, then 2, etc.
+                
+                using(IEnumerator<T> en = collection.GetEnumerator()) {
+                    while(en.MoveNext()) {
+                        Add(en.Current);                                    
+                    }
+                }
+            }
+        }
+        
+        // Gets and sets the capacity of this list.  The capacity is the size of
+        // the internal array used to hold items.  When set, the internal 
+        // array of the list is reallocated to the given capacity.
+        // 
+        public int Capacity {
+            get {
+                Contract.Ensures(Contract.Result<int>() >= 0);
+                return _items.Length;
+            }
+            set {
+                if (value < _size) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.value, ExceptionResource.ArgumentOutOfRange_SmallCapacity);
+                }
+                Contract.EndContractBlock();
+
+                if (value != _items.Length) {
+                    if (value > 0) {
+                        T[] newItems = new T[value];
+                        if (_size > 0) {
+                            Array.Copy(_items, 0, newItems, 0, _size);
+                        }
+                        _items = newItems;
+                    }
+                    else {
+                        _items = _emptyArray;
+                    }
+                }
+            }
+        }
+            
+        // Read-only property describing how many elements are in the List.
+        public int Count {
+            get {
+                Contract.Ensures(Contract.Result<int>() >= 0);
+                return _size; 
+            }
+        }
+
+        bool System.Collections.IList.IsFixedSize {
+            get { return false; }
+        }
+
+            
+        // Is this List read-only?
+        bool ICollection<T>.IsReadOnly {
+            get { return false; }
+        }
+
+        bool System.Collections.IList.IsReadOnly {
+            get { return false; }
+        }
+
+        // Is this List synchronized (thread-safe)?
+        bool System.Collections.ICollection.IsSynchronized {
+            get { return false; }
+        }
+    
+        // Synchronization root for this object.
+        Object System.Collections.ICollection.SyncRoot {
+            get { 
+                if( _syncRoot == null) {
+                    System.Threading.Interlocked.CompareExchange<Object>(ref _syncRoot, new Object(), null);    
+                }
+                return _syncRoot;
+            }
+        }
+        // Sets or Gets the element at the given index.
+        // 
+        public T this[int index] {
+            get {
+                // Following trick can reduce the range check by one
+                if ((uint) index >= (uint)_size) {
+                    ThrowHelper.ThrowArgumentOutOfRange_IndexException();
+                }
+                Contract.EndContractBlock();
+                return _items[index]; 
+            }
+
+            set {
+                if ((uint) index >= (uint)_size) {
+                    ThrowHelper.ThrowArgumentOutOfRange_IndexException();
+                }
+                Contract.EndContractBlock();
+                _items[index] = value;
+                _version++;
+            }
+        }
+
+        private static bool IsCompatibleObject(object value) {
+            // Non-null values are fine.  Only accept nulls if T is a class or Nullable<U>.
+            // Note that default(T) is not equal to null for value types except when T is Nullable<U>. 
+            return ((value is T) || (value == null && default(T) == null));
+        }
+
+        Object System.Collections.IList.this[int index] {
+            get {
+                return this[index];
+            }
+            set {
+                ThrowHelper.IfNullAndNullsAreIllegalThenThrow<T>(value, ExceptionArgument.value);
+
+                try { 
+                    this[index] = (T)value;               
+                }
+                catch (InvalidCastException) { 
+                    ThrowHelper.ThrowWrongValueTypeArgumentException(value, typeof(T));            
+                }
+            }
+        }
+
+        // Adds the given object to the end of this list. The size of the list is
+        // increased by one. If required, the capacity of the list is doubled
+        // before adding the new element.
+        //
+        public void Add(T item) {
+            if (_size == _items.Length) EnsureCapacity(_size + 1);
+            _items[_size++] = item;
+            _version++;
+        }
+
+        int System.Collections.IList.Add(Object item)
+        {
+            ThrowHelper.IfNullAndNullsAreIllegalThenThrow<T>(item, ExceptionArgument.item);
+
+            try { 
+                Add((T) item);            
+            }
+            catch (InvalidCastException) { 
+                ThrowHelper.ThrowWrongValueTypeArgumentException(item, typeof(T));            
+            }
+
+            return Count - 1;
+        }
+
+
+        // Adds the elements of the given collection to the end of this list. If
+        // required, the capacity of the list is increased to twice the previous
+        // capacity or the new size, whichever is larger.
+        //
+        public void AddRange(IEnumerable<T> collection) {
+            Contract.Ensures(Count >= Contract.OldValue(Count));
+
+            InsertRange(_size, collection);
+        }
+
+        public ReadOnlyCollection<T> AsReadOnly() {
+            Contract.Ensures(Contract.Result<ReadOnlyCollection<T>>() != null);
+            return new ReadOnlyCollection<T>(this);
+        }
+           
+        // Searches a section of the list for a given element using a binary search
+        // algorithm. Elements of the list are compared to the search value using
+        // the given IComparer interface. If comparer is null, elements of
+        // the list are compared to the search value using the IComparable
+        // interface, which in that case must be implemented by all elements of the
+        // list and the given search value. This method assumes that the given
+        // section of the list is already sorted; if this is not the case, the
+        // result will be incorrect.
+        //
+        // The method returns the index of the given value in the list. If the
+        // list does not contain the given value, the method returns a negative
+        // integer. The bitwise complement operator (~) can be applied to a
+        // negative result to produce the index of the first element (if any) that
+        // is larger than the given search value. This is also the index at which
+        // the search value should be inserted into the list in order for the list
+        // to remain sorted.
+        // 
+        // The method uses the Array.BinarySearch method to perform the
+        // search.
+        // 
+        public int BinarySearch(int index, int count, T item, IComparer<T> comparer) {
+            if (index < 0)
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            if (count < 0)
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            if (_size - index < count)
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
+            Contract.Ensures(Contract.Result<int>() <= index + count);
+            Contract.EndContractBlock();
+
+            return Array.BinarySearch<T>(_items, index, count, item, comparer);
+        }
+    
+        public int BinarySearch(T item)
+        {
+            Contract.Ensures(Contract.Result<int>() <= Count);
+            return BinarySearch(0, Count, item, null);
+        }
+
+        public int BinarySearch(T item, IComparer<T> comparer)
+        {
+            Contract.Ensures(Contract.Result<int>() <= Count);
+            return BinarySearch(0, Count, item, comparer);
+        }
+
+    
+        // Clears the contents of List.
+        public void Clear() {
+            if (_size > 0)
+            {
+                Array.Clear(_items, 0, _size); // Don't need to doc this but we clear the elements so that the gc can reclaim the references.
+                _size = 0;
+            }
+            _version++;
+        }
+    
+        // Contains returns true if the specified element is in the List.
+        // It does a linear, O(n) search.  Equality is determined by calling
+        // EqualityComparer<T>.Default.Equals().
+
+        public bool Contains(T item)
+        {
+            // PERF: IndexOf calls Array.IndexOf, which internally
+            // calls EqualityComparer<T>.Default.IndexOf, which
+            // is specialized for different types. This
+            // boosts performance since instead of making a
+            // virtual method call each iteration of the loop,
+            // via EqualityComparer<T>.Default.Equals, we
+            // only make one virtual call to EqualityComparer.IndexOf.
+
+            return _size != 0 && IndexOf(item) != -1;
+        }
+
+        bool System.Collections.IList.Contains(Object item)
+        {
+            if(IsCompatibleObject(item)) {            
+                return Contains((T) item);                
+            }
+            return false;
+        }
+
+        public List<TOutput> ConvertAll<TOutput>(Converter<T,TOutput> converter) {
+            if( converter == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.converter);
+            }
+
+            Contract.EndContractBlock();
+
+            List<TOutput> list = new List<TOutput>(_size);
+            for( int i = 0; i< _size; i++) {
+                list._items[i] = converter(_items[i]);
+            }
+            list._size = _size;
+            return list;
+        }
+
+        // Copies this List into array, which must be of a 
+        // compatible array type.  
+        //
+        public void CopyTo(T[] array) {
+            CopyTo(array, 0);
+        }
+
+        // Copies this List into array, which must be of a 
+        // compatible array type.  
+        //
+        void System.Collections.ICollection.CopyTo(Array array, int arrayIndex) {
+            if ((array != null) && (array.Rank != 1)) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_RankMultiDimNotSupported);
+            }
+            Contract.EndContractBlock();
+
+            try {                
+                // Array.Copy will check for NULL.
+                Array.Copy(_items, 0, array, arrayIndex, _size);
+            }
+            catch(ArrayTypeMismatchException){
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidArrayType);
+            }
+        }
+    
+        // Copies a section of this list to the given array at the given index.
+        // 
+        // The method uses the Array.Copy method to copy the elements.
+        // 
+        public void CopyTo(int index, T[] array, int arrayIndex, int count) {
+            if (_size - index < count) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);
+            }
+            Contract.EndContractBlock();
+            
+            // Delegate rest of error checking to Array.Copy.
+            Array.Copy(_items, index, array, arrayIndex, count);
+        }
+
+        public void CopyTo(T[] array, int arrayIndex) {
+            // Delegate rest of error checking to Array.Copy.
+            Array.Copy(_items, 0, array, arrayIndex, _size);
+        }
+
+        // Ensures that the capacity of this list is at least the given minimum
+        // value. If the currect capacity of the list is less than min, the
+        // capacity is increased to twice the current capacity or to min,
+        // whichever is larger.
+        private void EnsureCapacity(int min) {
+            if (_items.Length < min) {
+                int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;
+                // Allow the list to grow to maximum possible capacity (~2G elements) before encountering overflow.
+                // Note that this check works even when _items.Length overflowed thanks to the (uint) cast
+                if ((uint)newCapacity > Array.MaxArrayLength) newCapacity = Array.MaxArrayLength;
+                if (newCapacity < min) newCapacity = min;
+                Capacity = newCapacity;
+            }
+        }
+   
+        public bool Exists(Predicate<T> match) {
+            return FindIndex(match) != -1;
+        }
+
+        public T Find(Predicate<T> match) {
+            if( match == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
+            }
+            Contract.EndContractBlock();
+
+            for(int i = 0 ; i < _size; i++) {
+                if(match(_items[i])) {
+                    return _items[i];
+                }
+            }
+            return default(T);
+        }
+  
+        public List<T> FindAll(Predicate<T> match) { 
+            if( match == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
+            }
+            Contract.EndContractBlock();
+
+            List<T> list = new List<T>(); 
+            for(int i = 0 ; i < _size; i++) {
+                if(match(_items[i])) {
+                    list.Add(_items[i]);
+                }
+            }
+            return list;
+        }
+  
+        public int FindIndex(Predicate<T> match) {
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return FindIndex(0, _size, match);
+        }
+  
+        public int FindIndex(int startIndex, Predicate<T> match) {
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < startIndex + Count);
+            return FindIndex(startIndex, _size - startIndex, match);
+        }
+ 
+        public int FindIndex(int startIndex, int count, Predicate<T> match) {
+            if( (uint)startIndex > (uint)_size ) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.startIndex, ExceptionResource.ArgumentOutOfRange_Index);                
+            }
+
+            if (count < 0 || startIndex > _size - count) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count);
+            }
+
+            if( match == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
+            }
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < startIndex + count);
+            Contract.EndContractBlock();
+
+            int endIndex = startIndex + count;
+            for( int i = startIndex; i < endIndex; i++) {
+                if( match(_items[i])) return i;
+            }
+            return -1;
+        }
+ 
+        public T FindLast(Predicate<T> match) {
+            if( match == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
+            }
+            Contract.EndContractBlock();
+
+            for(int i = _size - 1 ; i >= 0; i--) {
+                if(match(_items[i])) {
+                    return _items[i];
+                }
+            }
+            return default(T);
+        }
+
+        public int FindLastIndex(Predicate<T> match) {
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return FindLastIndex(_size - 1, _size, match);
+        }
+   
+        public int FindLastIndex(int startIndex, Predicate<T> match) {
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() <= startIndex);
+            return FindLastIndex(startIndex, startIndex + 1, match);
+        }
+
+        public int FindLastIndex(int startIndex, int count, Predicate<T> match) {
+            if( match == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
+            }
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() <= startIndex);
+            Contract.EndContractBlock();
+
+            if(_size == 0) {
+                // Special case for 0 length List
+                if( startIndex != -1) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.startIndex, ExceptionResource.ArgumentOutOfRange_Index);
+                }
+            }
+            else {
+                // Make sure we're not out of range            
+                if ( (uint)startIndex >= (uint)_size) {
+                    ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.startIndex, ExceptionResource.ArgumentOutOfRange_Index);
+                }
+            }
+            
+            // 2nd have of this also catches when startIndex == MAXINT, so MAXINT - 0 + 1 == -1, which is < 0.
+            if (count < 0 || startIndex - count + 1 < 0) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count);
+            }
+                        
+            int endIndex = startIndex - count;
+            for( int i = startIndex; i > endIndex; i--) {
+                if( match(_items[i])) {
+                    return i;
+                }
+            }
+            return -1;
+        }
+
+        public void ForEach(Action<T> action) {
+            if( action == null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.action);
+            }
+            Contract.EndContractBlock();
+
+            int version = _version;
+
+            for(int i = 0 ; i < _size; i++) {
+                if (version != _version && BinaryCompatibility.TargetsAtLeast_Desktop_V4_5) {
+                    break;
+                }
+                action(_items[i]);
+            }
+
+            if (version != _version && BinaryCompatibility.TargetsAtLeast_Desktop_V4_5)
+                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
+        }
+
+        // Returns an enumerator for this list with the given
+        // permission for removal of elements. If modifications made to the list 
+        // while an enumeration is in progress, the MoveNext and 
+        // GetObject methods of the enumerator will throw an exception.
+        //
+        public Enumerator GetEnumerator() {
+            return new Enumerator(this);
+        }
+
+        /// <internalonly/>
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() {
+            return new Enumerator(this);
+        }
+
+        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() {
+            return new Enumerator(this);
+        }
+
+        public List<T> GetRange(int index, int count) {
+            if (index < 0) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            }
+
+            if (count < 0) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_NeedNonNegNum);
+            }
+
+            if (_size - index < count) {
+                ThrowHelper.ThrowArgumentException(ExceptionResource.Argument_InvalidOffLen);                
+            }
+            Contract.Ensures(Contract.Result<List<T>>() != null);
+            Contract.EndContractBlock();
+
+            List<T> list = new List<T>(count);
+            Array.Copy(_items, index, list._items, 0, count);            
+            list._size = count;
+            return list;
+        }
+
+
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards from beginning to end.
+        // The elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public int IndexOf(T item) {
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < Count);
+            return Array.IndexOf(_items, item, 0, _size);
+        }
+
+        int System.Collections.IList.IndexOf(Object item)
+        {
+            if(IsCompatibleObject(item)) {            
+                return IndexOf((T)item);
+            }
+            return -1;
+        }
+
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards, starting at index
+        // index and ending at count number of elements. The
+        // elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public int IndexOf(T item, int index) {
+            if (index > _size)
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+            return Array.IndexOf(_items, item, index, _size - index);
+        }
+
+        // Returns the index of the first occurrence of a given value in a range of
+        // this list. The list is searched forwards, starting at index
+        // index and upto count number of elements. The
+        // elements of the list are compared to the given value using the
+        // Object.Equals method.
+        // 
+        // This method uses the Array.IndexOf method to perform the
+        // search.
+        // 
+        public int IndexOf(T item, int index, int count) {
+            if (index > _size)
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
+
+            if (count <0 || index > _size - count) ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.count, ExceptionResource.ArgumentOutOfRange_Count);
+            Contract.Ensures(Contract.Result<int>() >= -1);
+            Contract.Ensures(Contract.Result<int>() < Count);
+            Contract.EndContractBlock();
+
+            return Array.IndexOf(_items, item, index, count);
+        }
+    
+        // Inserts an element into this list at a given index. The size of the list
+        // is increased by one. If required, the capacity of the list is doubled
+        // before inserting the new element.
+        // 
+        public void Insert(int index, T item) {
+            // Note that insertions at the end are legal.
+            if ((uint) index > (uint)_size) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_ListInsert);
+            }
+            Contract.EndContractBlock();
+            if (_size == _items.Length) EnsureCapacity(_size + 1);
+            if (index < _size) {
+                Array.Copy(_items, index, _items, index + 1, _size - index);
+            }
+            _items[index] = item;
+            _size++;            
+            _version++;
+        }
+    
+        void System.Collections.IList.Insert(int index, Object item)
+        {
+            ThrowHelper.IfNullAndNullsAreIllegalThenThrow<T>(item, ExceptionArgument.item);
+
+            try { 
+                Insert(index, (T) item);
+            }
+            catch (InvalidCastException) { 
+                ThrowHelper.ThrowWrongValueTypeArgumentException(item, typeof(T));            
+            }
+        }
+
+        // Inserts the elements of the given collection at a given index. If
+        // required, the capacity of the list is increased to twice the previous
+        // capacity or the new size, whichever is larger.  Ranges may be added
+        // to the end of the list by setting index to the List's size.
+        //
+        public void InsertRange(int index, IEnumerable<T> collection) {
+            if (collection==null) {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection);
+            }
+            
+            if ((uint)index > (uint)_size) {
+                ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index);
+            }
+            Contract.EndContractBlock();
+
+            ICollection<T> c = collection as ICollection<T>;
+            if( c != null ) {    // if collection is ICollection<T>
+                int count = c.Count;
+                if (count > 0) {
+                    EnsureCapacity(_size + count);