Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.ServiceModel/System/ServiceModel/IExtensionCollection.cs

@@ -0,0 +1,17 @@
+//-----------------------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+//-----------------------------------------------------------------------------
+
+namespace System.ServiceModel
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Collections.ObjectModel;
+
+    public interface IExtensionCollection<T> : ICollection<IExtension<T>>
+    where T : IExtensibleObject<T>
+    {
+        E Find<E>();
+        Collection<E> FindAll<E>();
+    }
+}