// 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 {

    using System;
    using System.Reflection;
    using System.Collections.Generic;
    using System.Runtime.InteropServices;
    using System.Globalization;
    using System.Diagnostics;
    using System.Diagnostics.Contracts;
    using System.Security;
    using System.Security.Permissions;

    [Serializable]
    [AttributeUsageAttribute(AttributeTargets.All, Inherited = true, AllowMultiple=false)] 
    [ClassInterface(ClassInterfaceType.None)]
    [ComDefaultInterface(typeof(_Attribute))]
    [System.Runtime.InteropServices.ComVisible(true)]
    public abstract class Attribute : _Attribute
    {
        #region Private Statics

        #region PropertyInfo
        private static Attribute[] InternalGetCustomAttributes(PropertyInfo element, Type type, bool inherit)
        {
            Contract.Requires(element != null);
            Contract.Requires(type != null);
            Contract.Requires(type.IsSubclassOf(typeof(Attribute)) || type == typeof(Attribute));

            // walk up the hierarchy chain
            Attribute[] attributes = (Attribute[])element.GetCustomAttributes(type, inherit);

            if (!inherit)
                return attributes;

            // create the hashtable that keeps track of inherited types
            Dictionary<Type, AttributeUsageAttribute> types = new Dictionary<Type, AttributeUsageAttribute>(11);

            // create an array list to collect all the requested attibutes
            List<Attribute> attributeList = new List<Attribute>();
            CopyToArrayList(attributeList, attributes, types);

            //if this is an index we need to get the parameter types to help disambiguate
            Type[] indexParamTypes = GetIndexParameterTypes(element);
            

            PropertyInfo baseProp = GetParentDefinition(element, indexParamTypes);
            while (baseProp != null)
            {
                attributes = GetCustomAttributes(baseProp, type, false);
                AddAttributesToList(attributeList, attributes, types);
                baseProp = GetParentDefinition(baseProp, indexParamTypes);
            }
            Array array = CreateAttributeArrayHelper(type, attributeList.Count);
            Array.Copy(attributeList.ToArray(), 0, array, 0, attributeList.Count);
            return (Attribute[])array;
        }

        private static bool InternalIsDefined(PropertyInfo element, Type attributeType, bool inherit)
        {
            // walk up the hierarchy chain
            if (element.IsDefined(attributeType, inherit))
                return true;
            
            if (inherit)
            {
                AttributeUsageAttribute usage = InternalGetAttributeUsage(attributeType);

                if (!usage.Inherited) 
                    return false;

                //if this is an index we need to get the parameter types to help disambiguate
                Type[] indexParamTypes = GetIndexParameterTypes(element);

                PropertyInfo baseProp = GetParentDefinition(element, indexParamTypes);

                while (baseProp != null)
                {
                    if (baseProp.IsDefined(attributeType, false))
                        return true;

                    baseProp = GetParentDefinition(baseProp, indexParamTypes);
                }
            }

            return false;
        }

        private static PropertyInfo GetParentDefinition(PropertyInfo property, Type[] propertyParameters)
        {
            Contract.Requires(property != null);

            // for the current property get the base class of the getter and the setter, they might be different
            // note that this only works for RuntimeMethodInfo
            MethodInfo propAccessor = property.GetGetMethod(true); 

            if (propAccessor == null) 
                propAccessor = property.GetSetMethod(true);

            RuntimeMethodInfo rtPropAccessor = propAccessor as RuntimeMethodInfo;

            if (rtPropAccessor != null)
            {
                rtPropAccessor = rtPropAccessor.GetParentDefinition();

                if (rtPropAccessor != null)
                {
                    // There is a public overload of Type.GetProperty that takes both a BingingFlags enum and a return type.
                    // However, we cannot use that because it doesn't accept null for "types".
                    return rtPropAccessor.DeclaringType.GetProperty(
                        property.Name,
                        BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.DeclaredOnly,
                        null, //will use default binder
                        property.PropertyType,
                        propertyParameters, //used for index properties
                        null);
                }
            }

            return null;
        }

        #endregion

        #region EventInfo
        private static Attribute[] InternalGetCustomAttributes(EventInfo element, Type type, bool inherit)
        {
            Contract.Requires(element != null);
            Contract.Requires(type != null);
            Contract.Requires(type.IsSubclassOf(typeof(Attribute)) || type == typeof(Attribute));

            // walk up the hierarchy chain
            Attribute[] attributes = (Attribute[])element.GetCustomAttributes(type, inherit);
            if (inherit)
            {
                // create the hashtable that keeps track of inherited types
                Dictionary<Type, AttributeUsageAttribute> types = new Dictionary<Type, AttributeUsageAttribute>(11);
                // create an array list to collect all the requested attibutes
                List<Attribute> attributeList = new List<Attribute>();
                CopyToArrayList(attributeList, attributes, types);

                EventInfo baseEvent = GetParentDefinition(element);
                while (baseEvent != null)
                {
                    attributes = GetCustomAttributes(baseEvent, type, false);
                    AddAttributesToList(attributeList, attributes, types);
                    baseEvent = GetParentDefinition(baseEvent);
                }
                Array array = CreateAttributeArrayHelper(type, attributeList.Count);
                Array.Copy(attributeList.ToArray(), 0, array, 0, attributeList.Count);
                return (Attribute[])array;
            }
            else
                return attributes;
        }

        private static EventInfo GetParentDefinition(EventInfo ev)
        {
            Contract.Requires(ev != null);

            // note that this only works for RuntimeMethodInfo
            MethodInfo add = ev.GetAddMethod(true);

            RuntimeMethodInfo rtAdd = add as RuntimeMethodInfo;

            if (rtAdd != null)
            {
                rtAdd = rtAdd.GetParentDefinition();
                if (rtAdd != null) 
                    return rtAdd.DeclaringType.GetEvent(ev.Name);
            }
            return null;
        }

        private static bool InternalIsDefined (EventInfo element, Type attributeType, bool inherit)
        {
            Contract.Requires(element != null);

            // walk up the hierarchy chain
            if (element.IsDefined(attributeType, inherit))
                return true;
            
            if (inherit)
            {
                AttributeUsageAttribute usage = InternalGetAttributeUsage(attributeType);

                if (!usage.Inherited) 
                    return false;

                EventInfo baseEvent = GetParentDefinition(element);

                while (baseEvent != null)
                {
                    if (baseEvent.IsDefined(attributeType, false))
                        return true;
                    baseEvent = GetParentDefinition(baseEvent);
                }
            }

            return false;
        }

        #endregion

        #region ParameterInfo
        private static ParameterInfo GetParentDefinition(ParameterInfo param)
        {
            Contract.Requires(param != null);

            // note that this only works for RuntimeMethodInfo
            RuntimeMethodInfo rtMethod = param.Member as RuntimeMethodInfo;

            if (rtMethod != null)
            {
                rtMethod = rtMethod.GetParentDefinition();

                if (rtMethod != null)
                {
                    // Find the ParameterInfo on this method
                    int position = param.Position;
                    if (position == -1)
                    {
                        return rtMethod.ReturnParameter;
                    }
                    else
                    {
                        ParameterInfo[] parameters = rtMethod.GetParameters();
                        return parameters[position]; // Point to the correct ParameterInfo of the method
                    }
                }
            }
            return null;
        }

        private static Attribute[] InternalParamGetCustomAttributes(ParameterInfo param, Type type, bool inherit)
        {
            Contract.Requires(param != null);

            // For ParameterInfo's we need to make sure that we chain through all the MethodInfo's in the inheritance chain that
            // have this ParameterInfo defined. .We pick up all the CustomAttributes for the starting ParameterInfo. We need to pick up only attributes 
            // that are marked inherited from the remainder of the MethodInfo's in the inheritance chain.
            // For MethodInfo's on an interface we do not do an inheritance walk so the default ParameterInfo attributes are returned.
            // For MethodInfo's on a class we walk up the inheritance chain but do not look at the MethodInfo's on the interfaces that the
            // class inherits from and return the respective ParameterInfo attributes

            List<Type> disAllowMultiple = new List<Type>();
            Object [] objAttr;

            if (type == null)
                type = typeof(Attribute);

            objAttr = param.GetCustomAttributes(type, false); 
                
            for (int i =0;i < objAttr.Length;i++)
            {
                Type objType = objAttr[i].GetType();
                AttributeUsageAttribute attribUsage = InternalGetAttributeUsage(objType);
                if (attribUsage.AllowMultiple == false)
                    disAllowMultiple.Add(objType);
            }

            // Get all the attributes that have Attribute as the base class
            Attribute [] ret = null;
            if (objAttr.Length == 0) 
                ret = CreateAttributeArrayHelper(type,0);
            else 
                ret = (Attribute[])objAttr;
            
            if (param.Member.DeclaringType == null) // This is an interface so we are done.
                return ret;
            
            if (!inherit) 
                return ret;

            ParameterInfo baseParam = GetParentDefinition(param);

            while (baseParam != null)
            {
                objAttr = baseParam.GetCustomAttributes(type, false); 
                
                int count = 0;
                for (int i =0;i < objAttr.Length;i++)
                {
                    Type objType = objAttr[i].GetType();
                    AttributeUsageAttribute attribUsage = InternalGetAttributeUsage(objType);

                    if ((attribUsage.Inherited) && (disAllowMultiple.Contains(objType) == false))
                    {
                        if (attribUsage.AllowMultiple == false)
                            disAllowMultiple.Add(objType);
                        count++;
                    }
                    else
                        objAttr[i] = null;
                }

                // Get all the attributes that have Attribute as the base class
                Attribute [] attributes = CreateAttributeArrayHelper(type,count);
                
                count = 0;
                for (int i =0;i < objAttr.Length;i++)
                {
                    if (objAttr[i] != null)
                    {
                        attributes[count] = (Attribute)objAttr[i];
                        count++;
                    }
                }
                
                Attribute [] temp = ret;
                ret = CreateAttributeArrayHelper(type,temp.Length + count);
                Array.Copy(temp,ret,temp.Length);
                
                int offset = temp.Length;

                for (int i =0;i < attributes.Length;i++) 
                    ret[offset + i] = attributes[i];

                baseParam = GetParentDefinition(baseParam);
            } 

            return ret;
        }

        private static bool InternalParamIsDefined(ParameterInfo param, Type type, bool inherit)
        {
            Contract.Requires(param != null);
            Contract.Requires(type != null);

            // For ParameterInfo's we need to make sure that we chain through all the MethodInfo's in the inheritance chain.
            // We pick up all the CustomAttributes for the starting ParameterInfo. We need to pick up only attributes 
            // that are marked inherited from the remainder of the ParameterInfo's in the inheritance chain.
            // For MethodInfo's on an interface we do not do an inheritance walk. For ParameterInfo's on a
            // Class we walk up the inheritance chain but do not look at the MethodInfo's on the interfaces that the class inherits from.

            if (param.IsDefined(type, false))
                return true;
            
            if (param.Member.DeclaringType == null || !inherit) // This is an interface so we are done.
                return false;

            ParameterInfo baseParam = GetParentDefinition(param);

            while (baseParam != null)
            {
                Object[] objAttr = baseParam.GetCustomAttributes(type, false); 
                                
                for (int i =0; i < objAttr.Length; i++)
                {
                    Type objType = objAttr[i].GetType();
                    AttributeUsageAttribute attribUsage = InternalGetAttributeUsage(objType);

                    if ((objAttr[i] is Attribute) && (attribUsage.Inherited))
                        return true;
                }

                baseParam = GetParentDefinition(baseParam);
            } 

            return false;
        }

        #endregion

        #region Utility
        private static void CopyToArrayList(List<Attribute> attributeList,Attribute[] attributes,Dictionary<Type, AttributeUsageAttribute> types) 
        {
            for (int i = 0; i < attributes.Length; i++) 
            {
                attributeList.Add(attributes[i]);

                Type attrType = attributes[i].GetType();

                if (!types.ContainsKey(attrType)) 
                    types[attrType] = InternalGetAttributeUsage(attrType);
            }
        }

        private static Type[] GetIndexParameterTypes(PropertyInfo element)
        {
            ParameterInfo[] indexParams = element.GetIndexParameters();

            if (indexParams.Length > 0)
            {
                Type[] indexParamTypes = new Type[indexParams.Length];
                for (int i = 0; i < indexParams.Length; i++)
                {
                    indexParamTypes[i] = indexParams[i].ParameterType;
                }
                return indexParamTypes;
            }

            return Array.Empty<Type>();
        }

        private static void AddAttributesToList(List<Attribute> attributeList, Attribute[] attributes, Dictionary<Type, AttributeUsageAttribute> types) 
        {
            for (int i = 0; i < attributes.Length; i++) 
            {
                Type attrType = attributes[i].GetType();
                AttributeUsageAttribute usage = null;
                types.TryGetValue(attrType, out usage);

                if (usage == null) 
                {
                    // the type has never been seen before if it's inheritable add it to the list
                    usage = InternalGetAttributeUsage(attrType);
                    types[attrType] = usage;

                    if (usage.Inherited) 
                        attributeList.Add(attributes[i]);
                }
                else if (usage.Inherited && usage.AllowMultiple)
                {
                    // we saw this type already add it only if it is inheritable and it does allow multiple 
                    attributeList.Add(attributes[i]);
                }
            }
        }

        private static AttributeUsageAttribute InternalGetAttributeUsage(Type type)
        {
            // Check if the custom attributes is Inheritable
            Object [] obj = type.GetCustomAttributes(typeof(AttributeUsageAttribute), false); 

            if (obj.Length == 1)
                return (AttributeUsageAttribute)obj[0];

            if (obj.Length == 0)
                return AttributeUsageAttribute.Default;

            throw new FormatException(
                Environment.GetResourceString("Format_AttributeUsage", type));
        }

        private static Attribute[] CreateAttributeArrayHelper(Type elementType, int elementCount)
        {
            return (Attribute[])Array.UnsafeCreateInstance(elementType, elementCount);
        }
        #endregion

        #endregion

        #region Public Statics

        #region MemberInfo
        public static Attribute[] GetCustomAttributes(MemberInfo element, Type type)
        {
            return GetCustomAttributes(element, type, true);
        }
        
        public static Attribute[] GetCustomAttributes(MemberInfo element, Type type, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (type == null)
                throw new ArgumentNullException(nameof(type));
            
            if (!type.IsSubclassOf(typeof(Attribute)) && type != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            switch (element.MemberType)
            {
                case MemberTypes.Property:  
                    return InternalGetCustomAttributes((PropertyInfo)element, type, inherit);

                case MemberTypes.Event: 
                    return InternalGetCustomAttributes((EventInfo)element, type, inherit);

                default:
                    return element.GetCustomAttributes(type, inherit) as Attribute[];
            }
        }

        public static Attribute[] GetCustomAttributes(MemberInfo element)
        {
            return GetCustomAttributes(element, true);
        }

        public static Attribute[] GetCustomAttributes(MemberInfo element, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));
            Contract.EndContractBlock();

            switch (element.MemberType)
            {
                case MemberTypes.Property:  
                    return InternalGetCustomAttributes((PropertyInfo)element, typeof(Attribute), inherit);

                case MemberTypes.Event: 
                    return InternalGetCustomAttributes((EventInfo)element, typeof(Attribute), inherit);

                default:
                    return element.GetCustomAttributes(typeof(Attribute), inherit) as Attribute[];
            }
        }
        
        public static bool IsDefined(MemberInfo element, Type attributeType)
        {
            return IsDefined(element, attributeType, true);
        }

        public static bool IsDefined(MemberInfo element, Type attributeType, bool inherit)
        {
            // Returns true if a custom attribute subclass of attributeType class/interface with inheritance walk
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));
            
            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            switch(element.MemberType)
            {
                case MemberTypes.Property:  
                    return InternalIsDefined((PropertyInfo)element, attributeType, inherit);

                case MemberTypes.Event: 
                    return InternalIsDefined((EventInfo)element, attributeType, inherit);

                default:
                    return element.IsDefined(attributeType, inherit);
            }

        }

        public static Attribute GetCustomAttribute(MemberInfo element, Type attributeType)
        {
            return GetCustomAttribute(element, attributeType, true);
        }

        public static Attribute GetCustomAttribute(MemberInfo element, Type attributeType, bool inherit)
        {
            Attribute[] attrib = GetCustomAttributes(element, attributeType, inherit);

            if (attrib == null || attrib.Length == 0)
                return null;

            if (attrib.Length == 1)
                return attrib[0];

            throw new AmbiguousMatchException(Environment.GetResourceString("RFLCT.AmbigCust"));
        }

        #endregion

        #region ParameterInfo
        public static Attribute[] GetCustomAttributes(ParameterInfo element)
        {
            return GetCustomAttributes(element, true);
        }
        
        public static Attribute[] GetCustomAttributes(ParameterInfo element, Type attributeType)
        {
            return (Attribute[])GetCustomAttributes(element, attributeType, true);
        }

        public static Attribute[] GetCustomAttributes(ParameterInfo element, Type attributeType, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));
            
            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));

            if (element.Member == null)
                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidParameterInfo"), nameof(element));

            Contract.EndContractBlock();

            MemberInfo member = element.Member;
            if (member.MemberType == MemberTypes.Method && inherit) 
                return InternalParamGetCustomAttributes(element, attributeType, inherit) as Attribute[];

            return element.GetCustomAttributes(attributeType, inherit) as Attribute[];
        }

        public static Attribute[] GetCustomAttributes(ParameterInfo element, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (element.Member == null)
                throw new ArgumentException(Environment.GetResourceString("Argument_InvalidParameterInfo"), nameof(element));

            Contract.EndContractBlock();

            MemberInfo member = element.Member;
            if (member.MemberType == MemberTypes.Method && inherit) 
                return InternalParamGetCustomAttributes(element, null, inherit) as Attribute[];
            
            return element.GetCustomAttributes(typeof(Attribute), inherit) as Attribute[];
        }

        public static bool IsDefined(ParameterInfo element, Type attributeType)
        {
            return IsDefined(element, attributeType, true);
        }

        public static bool IsDefined(ParameterInfo element, Type attributeType, bool inherit)
        {
            // Returns true is a custom attribute subclass of attributeType class/interface with inheritance walk
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));
            
            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            MemberInfo member = element.Member;

            switch(member.MemberType)
            {
                case MemberTypes.Method: // We need to climb up the member hierarchy            
                    return InternalParamIsDefined(element, attributeType, inherit);

                case MemberTypes.Constructor:
                    return element.IsDefined(attributeType, false);

                case MemberTypes.Property:
                    return element.IsDefined(attributeType, false);

                default: 
                    Debug.Assert(false, "Invalid type for ParameterInfo member in Attribute class");
                    throw new ArgumentException(Environment.GetResourceString("Argument_InvalidParamInfo"));
            }
        }

        public static Attribute GetCustomAttribute(ParameterInfo element, Type attributeType)
        {
            return GetCustomAttribute(element, attributeType, true);
        }

        public static Attribute GetCustomAttribute(ParameterInfo element, Type attributeType, bool inherit)
        {
            // Returns an Attribute of base class/inteface attributeType on the ParameterInfo or null if none exists.
            // throws an AmbiguousMatchException if there are more than one defined.
            Attribute[] attrib = GetCustomAttributes(element, attributeType, inherit);

            if (attrib == null || attrib.Length == 0)
                return null;

            if (attrib.Length == 0)
                return null;

            if (attrib.Length == 1)
                return attrib[0];

            throw new AmbiguousMatchException(Environment.GetResourceString("RFLCT.AmbigCust"));
        }

        #endregion

        #region Module
        public static Attribute[] GetCustomAttributes(Module element, Type attributeType)
        {
            return GetCustomAttributes (element, attributeType, true);
        }

        public static Attribute[] GetCustomAttributes(Module element)
        {
            return GetCustomAttributes(element, true);
        }

        public static Attribute[] GetCustomAttributes(Module element, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));
            Contract.EndContractBlock();

            return (Attribute[])element.GetCustomAttributes(typeof(Attribute), inherit);
        }

        public static Attribute[] GetCustomAttributes(Module element, Type attributeType, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));

            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            return (Attribute[])element.GetCustomAttributes(attributeType, inherit);
        }

        public static bool IsDefined(Module element, Type attributeType)
        {
            return IsDefined(element, attributeType, false);
        }

        public static bool IsDefined(Module element, Type attributeType, bool inherit)
        {
            // Returns true is a custom attribute subclass of attributeType class/interface with no inheritance walk
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));

            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            return element.IsDefined(attributeType,false);
        }

        public static Attribute GetCustomAttribute(Module element, Type attributeType)
        {
            return GetCustomAttribute(element, attributeType, true);
        }

        public static Attribute GetCustomAttribute(Module element, Type attributeType, bool inherit)
        {
            // Returns an Attribute of base class/inteface attributeType on the Module or null if none exists.
            // throws an AmbiguousMatchException if there are more than one defined.
            Attribute[] attrib = GetCustomAttributes(element,attributeType,inherit);

            if (attrib == null || attrib.Length == 0)
                return null;

            if (attrib.Length == 1)
                return attrib[0];

            throw new AmbiguousMatchException(Environment.GetResourceString("RFLCT.AmbigCust"));
        }

        #endregion

        #region Assembly
        public static Attribute[] GetCustomAttributes(Assembly element, Type attributeType)
        {
            return GetCustomAttributes(element, attributeType, true);
        }

        public static Attribute[] GetCustomAttributes(Assembly element, Type attributeType, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));

            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            return (Attribute[])element.GetCustomAttributes(attributeType, inherit);
        }

        public static Attribute[] GetCustomAttributes(Assembly element)
        {
            return GetCustomAttributes(element, true);
        }

        public static Attribute[] GetCustomAttributes(Assembly element, bool inherit)
        {
            if (element == null)
                throw new ArgumentNullException(nameof(element));
            Contract.EndContractBlock();

            return (Attribute[])element.GetCustomAttributes(typeof(Attribute), inherit);
        }

        public static bool IsDefined (Assembly element, Type attributeType)
        {
            return IsDefined (element, attributeType, true);
        }

        public static bool IsDefined (Assembly element, Type attributeType, bool inherit)
        {
            // Returns true is a custom attribute subclass of attributeType class/interface with no inheritance walk
            if (element == null)
                throw new ArgumentNullException(nameof(element));

            if (attributeType == null)
                throw new ArgumentNullException(nameof(attributeType));

            if (!attributeType.IsSubclassOf(typeof(Attribute)) && attributeType != typeof(Attribute))
                throw new ArgumentException(Environment.GetResourceString("Argument_MustHaveAttributeBaseClass"));
            Contract.EndContractBlock();

            return element.IsDefined(attributeType, false);
        }

        public static Attribute GetCustomAttribute(Assembly element, Type attributeType)
        {
            return GetCustomAttribute (element, attributeType, true);
        }

        public static Attribute GetCustomAttribute(Assembly element, Type attributeType, bool inherit)
        {
            // Returns an Attribute of base class/inteface attributeType on the Assembly or null if none exists.
            // throws an AmbiguousMatchException if there are more than one defined.
            Attribute[] attrib = GetCustomAttributes(element,attributeType,inherit);

            if (attrib == null || attrib.Length == 0)
                return null;

            if (attrib.Length == 1)
                return attrib[0];

            throw new AmbiguousMatchException(Environment.GetResourceString("RFLCT.AmbigCust"));
        }

        #endregion

        #endregion

        #region Constructor
        protected Attribute() { }
        #endregion

        #region Object Overrides
        public override bool Equals(Object obj)
        {
            if (obj == null)
                return false;

            Type thisType = this.GetType();
            Type thatType = obj.GetType();

            if (thatType != thisType)
                return false;

            Object thisObj = this;
            Object thisResult, thatResult;

            while (thisType != typeof(Attribute))
            {
                FieldInfo[] thisFields = thisType.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.DeclaredOnly);

                for (int i = 0; i < thisFields.Length; i++)
                {
                    // Visibility check and consistency check are not necessary.
                    thisResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(thisObj);
                    thatResult = ((RtFieldInfo)thisFields[i]).UnsafeGetValue(obj);

                    if (!AreFieldValuesEqual(thisResult, thatResult))
                    {
                        return false;
                    }
                }
                thisType = thisType.BaseType;
            }

            return true;
        }

        // Compares values of custom-attribute fields.    
        private static bool AreFieldValuesEqual(Object thisValue, Object thatValue)
        {
            if (thisValue == null && thatValue == null)
                return true;
            if (thisValue == null || thatValue == null)
                return false;

            if (thisValue.GetType().IsArray)
            {
                // Ensure both are arrays of the same type.
                if (!thisValue.GetType().Equals(thatValue.GetType()))
                {
                    return false;
                }

                Array thisValueArray = thisValue as Array;
                Array thatValueArray = thatValue as Array;
                if (thisValueArray.Length != thatValueArray.Length)
                {
                    return false;
                }

                // Attributes can only contain single-dimension arrays, so we don't need to worry about 
                // multidimensional arrays.
                Debug.Assert(thisValueArray.Rank == 1 && thatValueArray.Rank == 1);
                for (int j = 0; j < thisValueArray.Length; j++)
                {
                    if (!AreFieldValuesEqual(thisValueArray.GetValue(j), thatValueArray.GetValue(j)))
                    {
                        return false;
                    }
                }
            }
            else
            {
                // An object of type Attribute will cause a stack overflow. 
                // However, this should never happen because custom attributes cannot contain values other than
                // constants, single-dimensional arrays and typeof expressions.
                Debug.Assert(!(thisValue is Attribute));
                if (!thisValue.Equals(thatValue))
                    return false;
            }

            return true;
        }

        public override int GetHashCode()
        {
            Type type = GetType();

            while (type != typeof(Attribute))
            {
                FieldInfo[] fields = type.GetFields(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.DeclaredOnly);
                Object vThis = null;

                for (int i = 0; i < fields.Length; i++)
                {
                    // Visibility check and consistency check are not necessary.
                    Object fieldValue = ((RtFieldInfo)fields[i]).UnsafeGetValue(this);

                    // The hashcode of an array ignores the contents of the array, so it can produce 
                    // different hashcodes for arrays with the same contents.
                    // Since we do deep comparisons of arrays in Equals(), this means Equals and GetHashCode will
                    // be inconsistent for arrays. Therefore, we ignore hashes of arrays.
                    if (fieldValue != null && !fieldValue.GetType().IsArray)
                        vThis = fieldValue;

                    if (vThis != null)
                        break;
                }

                if (vThis != null)
                    return vThis.GetHashCode();

                type = type.BaseType;
            }

            return type.GetHashCode();
        }
        #endregion

        #region Public Virtual Members
        public virtual Object TypeId { get { return GetType(); } }
        
        public virtual bool Match(Object obj) { return Equals(obj); }
        #endregion

        #region Public Members
        public virtual bool IsDefaultAttribute() { return false; }
        #endregion
    }
}