(C#7) Use pattern matching `is` rather than `as` with null check (#21828)

* Use pattern matching `is` rather than `as` with null check

41 files changed, 86 insertions, 194 deletions

diff --git a/src/System.Private.CoreLib/shared/System/AggregateException.cs b/src/System.Private.CoreLib/shared/System/AggregateException.cs
index 022a1eae39..048005c479 100644
--- a/src/System.Private.CoreLib/shared/System/AggregateException.cs
+++ b/src/System.Private.CoreLib/shared/System/AggregateException.cs
@@ -399,11 +399,9 @@ namespace System
                         continue;
                     }
 
-                    AggregateException currentInnerAsAggregate = currentInnerException as AggregateException;
-
                     // If this exception is an aggregate, keep it around for later.  Otherwise,
                     // simply add it to the list of flattened exceptions to be returned.
-                    if (currentInnerAsAggregate != null)
+                    if (currentInnerException is AggregateException currentInnerAsAggregate)
                     {
                         exceptionsToFlatten.Add(currentInnerAsAggregate);
                     }
diff --git a/src/System.Private.CoreLib/shared/System/AppContext.cs b/src/System.Private.CoreLib/shared/System/AppContext.cs
index 9df7c8dc8e..c3994ea351 100644
--- a/src/System.Private.CoreLib/shared/System/AppContext.cs
+++ b/src/System.Private.CoreLib/shared/System/AppContext.cs
@@ -99,8 +99,7 @@ namespace System
                 }
             }
 
-            string value = GetData(switchName) as string;
-            if (value != null && bool.TryParse(value, out isEnabled))
+            if (GetData(switchName) is string value && bool.TryParse(value, out isEnabled))
             {
                return true;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
index e22fb5bb91..24a6cbff62 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Comparer.cs
@@ -59,16 +59,13 @@ namespace System.Collections
             if (b == null) return 1;
 
             string sa = a as string;
-            string sb = b as string;
-            if (sa != null && sb != null)
+            if (sa != null && b is string sb)
                 return _compareInfo.Compare(sa, sb);
 
-            IComparable ia = a as IComparable;
-            if (ia != null)
+            if (a is IComparable ia)
                 return ia.CompareTo(b);
 
-            IComparable ib = b as IComparable;
-            if (ib != null)
+            if (b is IComparable ib)
                 return -ib.CompareTo(a);
 
             throw new ArgumentException(SR.Argument_ImplementIComparable);
diff --git a/src/System.Private.CoreLib/shared/System/Collections/CompatibleComparer.cs b/src/System.Private.CoreLib/shared/System/Collections/CompatibleComparer.cs
index 587fd68391..5342dcd6fb 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/CompatibleComparer.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/CompatibleComparer.cs
@@ -37,8 +37,7 @@ namespace System.Collections
                 return _comparer.Compare(a, b);
             }
 
-            IComparable ia = a as IComparable;
-            if (ia != null)
+            if (a is IComparable ia)
             {
                 return ia.CompareTo(b);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
index f3634e8864..17e7fd5b77 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs
@@ -80,8 +80,7 @@ namespace System.Collections.Concurrent
             // case we round its length up to a power of 2, as all segments must
             // be a power of 2 in length.
             int length = InitialSegmentLength;
-            var c = collection as ICollection<T>;
-            if (c != null)
+            if (collection is ICollection<T> c)
             {
                 int count = c.Count;
                 if (count > length)
@@ -143,8 +142,7 @@ namespace System.Collections.Concurrent
         void ICollection.CopyTo(Array array, int index)
         {
             // Special-case when the Array is actually a T[], taking a faster path
-            T[] szArray = array as T[];
-            if (szArray != null)
+            if (array is T[] szArray)
             {
                 CopyTo(szArray, index);
                 return;
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
index 9efd671275..c96a1576f3 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/Collection.cs
@@ -215,8 +215,7 @@ namespace System.Collections.ObjectModel
                 ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
             }
 
-            T[] tArray = array as T[];
-            if (tArray != null)
+            if (array is T[] tArray)
             {
                 items.CopyTo(tArray, index);
             }
@@ -294,8 +293,7 @@ namespace System.Collections.ObjectModel
                 // readonly collections are fixed size, if our internal item 
                 // collection does not implement IList.  Note that Array implements
                 // IList, and therefore T[] and U[] will be fixed-size.
-                IList list = items as IList;
-                if (list != null)
+                if (items is IList list)
                 {
                     return list.IsFixedSize;
                 }
diff --git a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
index 584b4cc711..9d463da751 100644
--- a/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
+++ b/src/System.Private.CoreLib/shared/System/Collections/ObjectModel/ReadOnlyCollection.cs
@@ -147,8 +147,7 @@ namespace System.Collections.ObjectModel
                 ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall);
             }
 
-            T[] items = array as T[];
-            if (items != null)
+            if (array is T[] items)
             {
                 list.CopyTo(items, index);
             }
diff --git a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
index 5f7108a527..d506ffae4c 100644
--- a/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/ComponentModel/DefaultValueAttribute.cs
@@ -225,9 +225,7 @@ namespace System.ComponentModel
                 return true;
             }
 
-            DefaultValueAttribute other = obj as DefaultValueAttribute;
-
-            if (other != null)
+            if (obj is DefaultValueAttribute other)
             {
                 if (Value != null)
                 {
diff --git a/src/System.Private.CoreLib/shared/System/ComponentModel/EditorBrowsableAttribute.cs b/src/System.Private.CoreLib/shared/System/ComponentModel/EditorBrowsableAttribute.cs
index 9b4d6e626e..a59ee839bc 100644
--- a/src/System.Private.CoreLib/shared/System/ComponentModel/EditorBrowsableAttribute.cs
+++ b/src/System.Private.CoreLib/shared/System/ComponentModel/EditorBrowsableAttribute.cs
@@ -28,9 +28,7 @@ namespace System.ComponentModel
                 return true;
             }
 
-            EditorBrowsableAttribute other = obj as EditorBrowsableAttribute;
-
-            return (other != null) && other.browsableState == browsableState;
+            return (obj is EditorBrowsableAttribute other) && other.browsableState == browsableState;
         }
 
         public override int GetHashCode()
diff --git a/src/System.Private.CoreLib/shared/System/Convert.cs b/src/System.Private.CoreLib/shared/System/Convert.cs
index 077dda9d4f..2bc01794e4 100644
--- a/src/System.Private.CoreLib/shared/System/Convert.cs
+++ b/src/System.Private.CoreLib/shared/System/Convert.cs
@@ -160,8 +160,7 @@ namespace System
         public static TypeCode GetTypeCode(object value)
         {
             if (value == null) return TypeCode.Empty;
-            IConvertible temp = value as IConvertible;
-            if (temp != null)
+            if (value is IConvertible temp)
             {
                 return temp.GetTypeCode();
             }
@@ -173,8 +172,7 @@ namespace System
         public static bool IsDBNull(object value)
         {
             if (value == System.DBNull.Value) return true;
-            IConvertible convertible = value as IConvertible;
-            return convertible != null ? convertible.GetTypeCode() == TypeCode.DBNull : false;
+            return value is IConvertible convertible ? convertible.GetTypeCode() == TypeCode.DBNull : false;
         }
 
         // Converts the given object to the given type. In general, this method is
@@ -201,8 +199,7 @@ namespace System
                 return null;
             }
 
-            IConvertible v = value as IConvertible;
-            if (v == null)
+            if (!(value is IConvertible v))
             {
                 throw new InvalidCastException(SR.InvalidCast_IConvertible);
             }
@@ -330,8 +327,7 @@ namespace System
                 return null;
             }
 
-            IConvertible ic = value as IConvertible;
-            if (ic == null)
+            if (!(value is IConvertible ic))
             {
                 if (value.GetType() == conversionType)
                 {
@@ -2002,11 +1998,9 @@ namespace System
 
         public static string ToString(object value, IFormatProvider provider)
         {
-            IConvertible ic = value as IConvertible;
-            if (ic != null)
+            if (value is IConvertible ic)
                 return ic.ToString(provider);
-            IFormattable formattable = value as IFormattable;
-            if (formattable != null)
+            if (value is IFormattable formattable)
                 return formattable.ToString(null, provider);
             return value == null ? string.Empty : value.ToString();
         }
diff --git a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
index 3dc1cb050b..8159bad6b2 100644
--- a/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
+++ b/src/System.Private.CoreLib/shared/System/Diagnostics/Tracing/EventSource.cs
@@ -403,8 +403,7 @@ namespace System.Diagnostics.Tracing
             {
                 foreach (WeakReference eventSourceRef in EventListener.s_EventSources)
                 {
-                    EventSource eventSource = eventSourceRef.Target as EventSource;
-                    if (eventSource != null && !eventSource.IsDisposed)
+                    if (eventSourceRef.Target is EventSource eventSource && !eventSource.IsDisposed)
                         ret.Add(eventSource);
                 }
             }
@@ -2715,8 +2714,7 @@ namespace System.Diagnostics.Tracing
                 // TODO Enforce singleton pattern 
                 foreach (WeakReference eventSourceRef in EventListener.s_EventSources)
                 {
-                    EventSource eventSource = eventSourceRef.Target as EventSource;
-                    if (eventSource != null && eventSource.Guid == m_guid && !eventSource.IsDisposed)
+                    if (eventSourceRef.Target is EventSource eventSource && eventSource.Guid == m_guid && !eventSource.IsDisposed)
                     {
                         if (eventSource != this)
                         {
@@ -4160,8 +4158,7 @@ namespace System.Diagnostics.Tracing
             {
                 foreach (var esRef in s_EventSources)
                 {
-                    EventSource es = esRef.Target as EventSource;
-                    if (es != null)
+                    if (esRef.Target is EventSource es)
                         es.Dispose();
                 }
             }
@@ -4181,8 +4178,7 @@ namespace System.Diagnostics.Tracing
             // Foreach existing EventSource in the appdomain
             foreach (WeakReference eventSourceRef in s_EventSources)
             {
-                EventSource eventSource = eventSourceRef.Target as EventSource;
-                if (eventSource != null)
+                if (eventSourceRef.Target is EventSource eventSource)
                 {
                     // Is the first output dispatcher the dispatcher we are removing?
                     if (eventSource.m_Dispatchers.m_Listener == listenerToRemove)
@@ -4246,8 +4242,7 @@ namespace System.Diagnostics.Tracing
                 foreach (WeakReference eventSourceRef in s_EventSources)
                 {
                     id++;
-                    EventSource eventSource = eventSourceRef.Target as EventSource;
-                    if (eventSource == null)
+                    if (!(eventSourceRef.Target is EventSource eventSource))
                         continue;
                     Debug.Assert(eventSource.m_id == id, "Unexpected event source ID.");
 
@@ -4328,8 +4323,7 @@ namespace System.Diagnostics.Tracing
                         for (int i = 0; i < eventSourcesSnapshot.Length; i++)
                         {
                             WeakReference eventSourceRef = eventSourcesSnapshot[i];
-                            EventSource eventSource = eventSourceRef.Target as EventSource;
-                            if (eventSource != null)
+                            if (eventSourceRef.Target is EventSource eventSource)
                             {
                                 EventSourceCreatedEventArgs args = new EventSourceCreatedEventArgs();
                                 args.EventSource = eventSource;
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/CompareInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/CompareInfo.cs
index 21fd7212a8..9f7669edcc 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/CompareInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/CompareInfo.cs
@@ -1371,9 +1371,7 @@ namespace System.Globalization
 
         public override bool Equals(object value)
         {
-            CompareInfo that = value as CompareInfo;
-
-            if (that != null)
+            if (value is CompareInfo that)
             {
                 return this.Name == that.Name;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
index f3f56dda3e..153512639a 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/CultureInfo.cs
@@ -872,9 +872,7 @@ namespace System.Globalization
             if (object.ReferenceEquals(this, value))
                 return true;
 
-            CultureInfo that = value as CultureInfo;
-
-            if (that != null)
+            if (value is CultureInfo that)
             {
                 // using CompareInfo to verify the data passed through the constructor
                 // CultureInfo(String cultureName, String textAndCompareCultureName)
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs b/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
index 849602abcb..56f38ff628 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/IdnMapping.cs
@@ -145,9 +145,8 @@ namespace System.Globalization
 
         public override bool Equals(object obj)
         {
-            IdnMapping that = obj as IdnMapping;
             return
-                that != null &&
+                obj is IdnMapping that &&
                 _allowUnassigned == that._allowUnassigned &&
                 _useStd3AsciiRules == that._useStd3AsciiRules;
         }
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/RegionInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/RegionInfo.cs
index 8416257d91..576672b222 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/RegionInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/RegionInfo.cs
@@ -360,8 +360,7 @@ namespace System.Globalization
         ////////////////////////////////////////////////////////////////////////
         public override bool Equals(object value)
         {
-            RegionInfo that = value as RegionInfo;
-            if (that != null)
+            if (value is RegionInfo that)
             {
                 return this.Name.Equals(that.Name);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/SortKey.cs b/src/System.Private.CoreLib/shared/System/Globalization/SortKey.cs
index 9624946257..f9c7f68f5e 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/SortKey.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/SortKey.cs
@@ -135,9 +135,7 @@ namespace System.Globalization
         ////////////////////////////////////////////////////////////////////////
         public override bool Equals(object value)
         {
-            SortKey that = value as SortKey;
-
-            if (that != null)
+            if (value is SortKey that)
             {
                 return Compare(this, that) == 0;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/StringInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/StringInfo.cs
index 936a3cb9c1..425e6f2379 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/StringInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/StringInfo.cs
@@ -34,8 +34,7 @@ namespace System.Globalization
 
         public override bool Equals(object value)
         {
-            StringInfo that = value as StringInfo;
-            if (that != null)
+            if (value is StringInfo that)
             {
                 return (_str.Equals(that._str));
             }
diff --git a/src/System.Private.CoreLib/shared/System/Globalization/TextInfo.cs b/src/System.Private.CoreLib/shared/System/Globalization/TextInfo.cs
index 61e2160a62..77964fb642 100644
--- a/src/System.Private.CoreLib/shared/System/Globalization/TextInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Globalization/TextInfo.cs
@@ -654,9 +654,7 @@ namespace System.Globalization
         ////////////////////////////////////////////////////////////////////////
         public override bool Equals(object obj)
         {
-            TextInfo that = obj as TextInfo;
-
-            if (that != null)
+            if (obj is TextInfo that)
             {
                 return CultureName.Equals(that.CultureName);
             }
diff --git a/src/System.Private.CoreLib/shared/System/IO/FileStreamCompletionSource.Win32.cs b/src/System.Private.CoreLib/shared/System/IO/FileStreamCompletionSource.Win32.cs
index ec499d8456..a7996fd042 100644
--- a/src/System.Private.CoreLib/shared/System/IO/FileStreamCompletionSource.Win32.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/FileStreamCompletionSource.Win32.cs
@@ -137,8 +137,7 @@ namespace System.IO
                 // be directly the FileStreamCompletion that's completing (in the case where the preallocated
                 // overlapped was already in use by another operation).
                 object state = ThreadPoolBoundHandle.GetNativeOverlappedState(pOverlapped);
-                FileStream fs = state as FileStream;
-                FileStreamCompletionSource completionSource = fs != null ?
+                FileStreamCompletionSource completionSource = state is FileStream fs ?
                     fs._currentOverlappedOwner :
                     (FileStreamCompletionSource)state;
                 Debug.Assert(completionSource._overlapped == pOverlapped, "Overlaps don't match");
diff --git a/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs b/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
index 8ca162b57c..b9f2b5e35f 100644
--- a/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/MemoryStream.cs
@@ -527,8 +527,7 @@ namespace System.IO
                 return Task.CompletedTask;
 
             // If destination is not a memory stream, write there asynchronously:
-            MemoryStream memStrDest = destination as MemoryStream;
-            if (memStrDest == null)
+            if (!(destination is MemoryStream memStrDest))
                 return destination.WriteAsync(_buffer, pos, n, cancellationToken);
 
             try
diff --git a/src/System.Private.CoreLib/shared/System/IO/Stream.cs b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
index fe9f9fe24e..537d28ca8a 100644
--- a/src/System.Private.CoreLib/shared/System/IO/Stream.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/Stream.cs
@@ -1078,8 +1078,7 @@ namespace System.IO
 
             internal static int EndRead(IAsyncResult asyncResult)
             {
-                SynchronousAsyncResult ar = asyncResult as SynchronousAsyncResult;
-                if (ar == null || ar._isWrite)
+                if (!(asyncResult is SynchronousAsyncResult ar) || ar._isWrite)
                     throw new ArgumentException(SR.Arg_WrongAsyncResult);
 
                 if (ar._endXxxCalled)
@@ -1093,8 +1092,7 @@ namespace System.IO
 
             internal static void EndWrite(IAsyncResult asyncResult)
             {
-                SynchronousAsyncResult ar = asyncResult as SynchronousAsyncResult;
-                if (ar == null || !ar._isWrite)
+                if (!(asyncResult is SynchronousAsyncResult ar) || !ar._isWrite)
                     throw new ArgumentException(SR.Arg_WrongAsyncResult);
 
                 if (ar._endXxxCalled)
diff --git a/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs b/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
index e470517a7b..030a015e3c 100644
--- a/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
+++ b/src/System.Private.CoreLib/shared/System/IO/TextWriter.cs
@@ -286,8 +286,7 @@ namespace System.IO
         {
             if (value != null)
             {
-                IFormattable f = value as IFormattable;
-                if (f != null)
+                if (value is IFormattable f)
                 {
                     Write(f.ToString(null, FormatProvider));
                 }
@@ -497,8 +496,7 @@ namespace System.IO
             {
                 // Call WriteLine(value.ToString), not Write(Object), WriteLine().
                 // This makes calls to WriteLine(Object) atomic.
-                IFormattable f = value as IFormattable;
-                if (f != null)
+                if (value is IFormattable f)
                 {
                     WriteLine(f.ToString(null, FormatProvider));
                 }
diff --git a/src/System.Private.CoreLib/shared/System/String.Comparison.cs b/src/System.Private.CoreLib/shared/System/String.Comparison.cs
index 360167df40..5bae67be0e 100644
--- a/src/System.Private.CoreLib/shared/System/String.Comparison.cs
+++ b/src/System.Private.CoreLib/shared/System/String.Comparison.cs
@@ -511,9 +511,7 @@ namespace System
                 return 1;
             }
 
-            string other = value as string;
-
-            if (other == null)
+            if (!(value is string other))
             {
                 throw new ArgumentException(SR.Arg_MustBeString);
             }
@@ -606,8 +604,7 @@ namespace System
             if (object.ReferenceEquals(this, obj))
                 return true;
 
-            string str = obj as string;
-            if (str == null)
+            if (!(obj is string str))
                 return false;
 
             if (this.Length != str.Length)
diff --git a/src/System.Private.CoreLib/shared/System/StringComparer.cs b/src/System.Private.CoreLib/shared/System/StringComparer.cs
index e4378c5efa..8fab833592 100644
--- a/src/System.Private.CoreLib/shared/System/StringComparer.cs
+++ b/src/System.Private.CoreLib/shared/System/StringComparer.cs
@@ -114,18 +114,15 @@ namespace System
             if (x == null) return -1;
             if (y == null) return 1;
 
-            string sa = x as string;
-            if (sa != null)
+            if (x is string sa)
             {
-                string sb = y as string;
-                if (sb != null)
+                if (y is string sb)
                 {
                     return Compare(sa, sb);
                 }
             }
 
-            IComparable ia = x as IComparable;
-            if (ia != null)
+            if (x is IComparable ia)
             {
                 return ia.CompareTo(y);
             }
@@ -138,11 +135,9 @@ namespace System
             if (x == y) return true;
             if (x == null || y == null) return false;
 
-            string sa = x as string;
-            if (sa != null)
+            if (x is string sa)
             {
-                string sb = y as string;
-                if (sb != null)
+                if (y is string sb)
                 {
                     return Equals(sa, sb);
                 }
@@ -157,8 +152,7 @@ namespace System
                 throw new ArgumentNullException(nameof(obj));
             }
 
-            string s = obj as string;
-            if (s != null)
+            if (obj is string s)
             {
                 return GetHashCode(s);
             }
@@ -232,9 +226,8 @@ namespace System
         // Equals method for the comparer itself.
         public override bool Equals(object obj)
         {
-            CultureAwareComparer comparer = obj as CultureAwareComparer;
             return
-                comparer != null &&
+                obj is CultureAwareComparer comparer &&
                 _options == comparer._options &&
                 _compareInfo.Equals(comparer._compareInfo);
         }
@@ -316,8 +309,7 @@ namespace System
         // Equals method for the comparer itself. 
         public override bool Equals(object obj)
         {
-            OrdinalComparer comparer = obj as OrdinalComparer;
-            if (comparer == null)
+            if (!(obj is OrdinalComparer comparer))
             {
                 return false;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
index 8c62730c26..d94554c6fd 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderBestFitFallback.cs
@@ -40,8 +40,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            InternalDecoderBestFitFallback that = value as InternalDecoderBestFitFallback;
-            if (that != null)
+            if (value is InternalDecoderBestFitFallback that)
             {
                 return (_encoding.CodePage == that._encoding.CodePage);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
index 56c004714f..0525861a5b 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderExceptionFallback.cs
@@ -31,8 +31,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            DecoderExceptionFallback that = value as DecoderExceptionFallback;
-            if (that != null)
+            if (value is DecoderExceptionFallback that)
             {
                 return (true);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs b/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
index a97baf05cb..2af0d12535 100644
--- a/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/DecoderReplacementFallback.cs
@@ -84,8 +84,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            DecoderReplacementFallback that = value as DecoderReplacementFallback;
-            if (that != null)
+            if (value is DecoderReplacementFallback that)
             {
                 return (_strDefault == that._strDefault);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
index 4aab3f62a3..c3d07c980e 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderBestFitFallback.cs
@@ -40,8 +40,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            InternalEncoderBestFitFallback that = value as InternalEncoderBestFitFallback;
-            if (that != null)
+            if (value is InternalEncoderBestFitFallback that)
             {
                 return (_encoding.CodePage == that._encoding.CodePage);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
index 92afcf701a..077d92762f 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderExceptionFallback.cs
@@ -30,8 +30,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            EncoderExceptionFallback that = value as EncoderExceptionFallback;
-            if (that != null)
+            if (value is EncoderExceptionFallback that)
             {
                 return (true);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs b/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
index 760c280fde..cfce4108f5 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncoderReplacementFallback.cs
@@ -87,8 +87,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            EncoderReplacementFallback that = value as EncoderReplacementFallback;
-            if (that != null)
+            if (value is EncoderReplacementFallback that)
             {
                 return (_strDefault == that._strDefault);
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
index c1b64961f7..175e5442fd 100644
--- a/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/Encoding.cs
@@ -1190,8 +1190,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            Encoding that = value as Encoding;
-            if (that != null)
+            if (value is Encoding that)
                 return (_codePage == that._codePage) &&
                        (EncoderFallback.Equals(that.EncoderFallback)) &&
                        (DecoderFallback.Equals(that.DecoderFallback));
diff --git a/src/System.Private.CoreLib/shared/System/Text/EncodingInfo.cs b/src/System.Private.CoreLib/shared/System/Text/EncodingInfo.cs
index b8d634c22e..002d9ef0b5 100644
--- a/src/System.Private.CoreLib/shared/System/Text/EncodingInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/EncodingInfo.cs
@@ -24,8 +24,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            EncodingInfo that = value as EncodingInfo;
-            if (that != null)
+            if (value is EncodingInfo that)
             {
                 return this.CodePage == that.CodePage;
             }
diff --git a/src/System.Private.CoreLib/shared/System/Text/UTF32Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/UTF32Encoding.cs
index d579cc9e5e..02f3167df2 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UTF32Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UTF32Encoding.cs
@@ -1159,8 +1159,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            UTF32Encoding that = value as UTF32Encoding;
-            if (that != null)
+            if (value is UTF32Encoding that)
             {
                 return (_emitUTF32ByteOrderMark == that._emitUTF32ByteOrderMark) &&
                        (_bigEndian == that._bigEndian) &&
diff --git a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
index 5c51c81076..a932778b45 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UTF7Encoding.cs
@@ -99,8 +99,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            UTF7Encoding that = value as UTF7Encoding;
-            if (that != null)
+            if (value is UTF7Encoding that)
             {
                 return (_allowOptionals == that._allowOptionals) &&
                        (EncoderFallback.Equals(that.EncoderFallback)) &&
diff --git a/src/System.Private.CoreLib/shared/System/Text/UTF8Encoding.cs b/src/System.Private.CoreLib/shared/System/Text/UTF8Encoding.cs
index 426835ed53..5e1b9ae9a3 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UTF8Encoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UTF8Encoding.cs
@@ -2555,8 +2555,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            UTF8Encoding that = value as UTF8Encoding;
-            if (that != null)
+            if (value is UTF8Encoding that)
             {
                 return (_emitUTF8Identifier == that._emitUTF8Identifier) &&
                        (EncoderFallback.Equals(that.EncoderFallback)) &&
diff --git a/src/System.Private.CoreLib/shared/System/Text/UnicodeEncoding.cs b/src/System.Private.CoreLib/shared/System/Text/UnicodeEncoding.cs
index d80c229693..870962c6b6 100644
--- a/src/System.Private.CoreLib/shared/System/Text/UnicodeEncoding.cs
+++ b/src/System.Private.CoreLib/shared/System/Text/UnicodeEncoding.cs
@@ -1842,8 +1842,7 @@ namespace System.Text
 
         public override bool Equals(object value)
         {
-            UnicodeEncoding that = value as UnicodeEncoding;
-            if (that != null)
+            if (value is UnicodeEncoding that)
             {
                 //
                 // Big Endian Unicode has different code page (1201) than small Endian one (1200),
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
index e3339efd42..5cc0b6737b 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/Future.cs
@@ -597,14 +597,13 @@ namespace System.Threading.Tasks
         {
             // Invoke the delegate
             Debug.Assert(m_action != null);
-            var func = m_action as Func<TResult>;
-            if (func != null)
+            if (m_action is Func<TResult> func)
             {
                 m_result = func();
                 return;
             }
-            var funcWithState = m_action as Func<object, TResult>;
-            if (funcWithState != null)
+
+            if (m_action is Func<object, TResult> funcWithState)
             {
                 m_result = funcWithState(m_stateObject);
                 return;
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskExceptionHolder.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskExceptionHolder.cs
index 0037923bdb..cdf5c78ea4 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskExceptionHolder.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskExceptionHolder.cs
@@ -123,8 +123,7 @@ namespace System.Threading.Tasks
             // If this changes, make sure to only conditionally mark as handled below.
 
             // Store the cancellation exception
-            var oce = exceptionObject as OperationCanceledException;
-            if (oce != null)
+            if (exceptionObject is OperationCanceledException oce)
             {
                 m_cancellationException = ExceptionDispatchInfo.Capture(oce);
             }
@@ -155,24 +154,21 @@ namespace System.Threading.Tasks
             else Debug.Assert(exceptions.Count > 0, "Expected existing exceptions list to have > 0 exceptions.");
 
             // Handle Exception by capturing it into an ExceptionDispatchInfo and storing that
-            var exception = exceptionObject as Exception;
-            if (exception != null)
+            if (exceptionObject is Exception exception)
             {
                 exceptions.Add(ExceptionDispatchInfo.Capture(exception));
             }
             else
             {
                 // Handle ExceptionDispatchInfo by storing it into the list
-                var edi = exceptionObject as ExceptionDispatchInfo;
-                if (edi != null)
+                if (exceptionObject is ExceptionDispatchInfo edi)
                 {
                     exceptions.Add(edi);
                 }
                 else
                 {
                     // Handle enumerables of exceptions by capturing each of the contained exceptions into an EDI and storing it
-                    var exColl = exceptionObject as IEnumerable<Exception>;
-                    if (exColl != null)
+                    if (exceptionObject is IEnumerable<Exception> exColl)
                     {
 #if DEBUG
                         int numExceptions = 0;
@@ -192,8 +188,7 @@ namespace System.Threading.Tasks
                     else
                     {
                         // Handle enumerables of EDIs by storing them directly
-                        var ediColl = exceptionObject as IEnumerable<ExceptionDispatchInfo>;
-                        if (ediColl != null)
+                        if (exceptionObject is IEnumerable<ExceptionDispatchInfo> ediColl)
                         {
                             exceptions.AddRange(ediColl);
 #if DEBUG
diff --git a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
index c5bf02b9bc..13257b3077 100644
--- a/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
+++ b/src/System.Private.CoreLib/shared/System/Threading/Tasks/TaskScheduler.cs
@@ -500,8 +500,7 @@ namespace System.Threading.Tasks
                 return null;
 
             // If it can be cast to an array, use it directly
-            Task[] activeTasksArray = activeTasksSource as Task[];
-            if (activeTasksArray == null)
+            if (!(activeTasksSource is Task[] activeTasksArray))
             {
                 activeTasksArray = (new List<Task>(activeTasksSource)).ToArray();
             }
diff --git a/src/System.Private.CoreLib/shared/System/TimeZoneInfo.Win32.cs b/src/System.Private.CoreLib/shared/System/TimeZoneInfo.Win32.cs
index a16f6a8764..a7095dee61 100644
--- a/src/System.Private.CoreLib/shared/System/TimeZoneInfo.Win32.cs
+++ b/src/System.Private.CoreLib/shared/System/TimeZoneInfo.Win32.cs
@@ -678,8 +678,7 @@ namespace System
 
         private static unsafe bool TryGetTimeZoneEntryFromRegistry(RegistryKey key, string name, out REG_TZI_FORMAT dtzi)
         {
-            byte[] regValue = key.GetValue(name, null) as byte[];
-            if (regValue == null || regValue.Length != sizeof(REG_TZI_FORMAT))
+            if (!(key.GetValue(name, null) is byte[] regValue) || regValue.Length != sizeof(REG_TZI_FORMAT))
             {
                 dtzi = default;
                 return false;
diff --git a/src/System.Private.CoreLib/shared/System/Tuple.cs b/src/System.Private.CoreLib/shared/System/Tuple.cs
index ebbd156dda..bb72e4f0ec 100644
--- a/src/System.Private.CoreLib/shared/System/Tuple.cs
+++ b/src/System.Private.CoreLib/shared/System/Tuple.cs
@@ -124,9 +124,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1> objTuple = other as Tuple<T1>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1> objTuple))
             {
                 return false;
             }
@@ -143,9 +141,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1> objTuple = other as Tuple<T1>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -227,9 +223,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2> objTuple = other as Tuple<T1, T2>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2> objTuple))
             {
                 return false;
             }
@@ -246,9 +240,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2> objTuple = other as Tuple<T1, T2>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -345,9 +337,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3> objTuple = other as Tuple<T1, T2, T3>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3> objTuple))
             {
                 return false;
             }
@@ -364,9 +354,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3> objTuple = other as Tuple<T1, T2, T3>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -474,9 +462,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3, T4> objTuple = other as Tuple<T1, T2, T3, T4>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4> objTuple))
             {
                 return false;
             }
@@ -493,9 +479,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3, T4> objTuple = other as Tuple<T1, T2, T3, T4>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -614,9 +598,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3, T4, T5> objTuple = other as Tuple<T1, T2, T3, T4, T5>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5> objTuple))
             {
                 return false;
             }
@@ -633,9 +615,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3, T4, T5> objTuple = other as Tuple<T1, T2, T3, T4, T5>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -765,9 +745,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3, T4, T5, T6> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6> objTuple))
             {
                 return false;
             }
@@ -784,9 +762,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3, T4, T5, T6> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -927,9 +903,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3, T4, T5, T6, T7> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6, T7>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6, T7> objTuple))
             {
                 return false;
             }
@@ -946,9 +920,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3, T4, T5, T6, T7> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6, T7>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6, T7> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }
@@ -1105,9 +1077,7 @@ namespace System
         {
             if (other == null) return false;
 
-            Tuple<T1, T2, T3, T4, T5, T6, T7, TRest> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6, T7, TRest>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6, T7, TRest> objTuple))
             {
                 return false;
             }
@@ -1124,9 +1094,7 @@ namespace System
         {
             if (other == null) return 1;
 
-            Tuple<T1, T2, T3, T4, T5, T6, T7, TRest> objTuple = other as Tuple<T1, T2, T3, T4, T5, T6, T7, TRest>;
-
-            if (objTuple == null)
+            if (!(other is Tuple<T1, T2, T3, T4, T5, T6, T7, TRest> objTuple))
             {
                 throw new ArgumentException(SR.Format(SR.ArgumentException_TupleIncorrectType, this.GetType().ToString()), nameof(other));
             }