Added the IDictionary<in TKey> interface

Microsoft.Toolkit.Mvvm/Microsoft.Collections.Extensions/DictionarySlim{TKey,TValue}.cs

@@ -29,7 +29,7 @@ namespace Microsoft.Collections.Extensions
     /// 3) This means it can avoid storing a comparer, and avoid the likely virtual call to a comparer.
     /// </remarks>
     [DebuggerDisplay("Count = {Count}")]
-    internal sealed class DictionarySlim<TKey, TValue>
+    internal sealed class DictionarySlim<TKey, TValue> : IDictionary<TKey>
         where TKey : struct, IEquatable<TKey>
     {
         // See info in CoreFX labs for how this works
@@ -71,7 +71,7 @@ namespace Microsoft.Collections.Extensions
             _entries = InitialEntries;
         }
 
-        /// <inheritdoc cref="Dictionary{TKey,TValue}.Remove"/>
+        /// <inheritdoc/>
         public bool Remove(TKey key)
         {
             Entry[] entries = _entries;

Microsoft.Toolkit.Mvvm/Microsoft.Collections.Extensions/IDictionary{TKey}.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.Collections.Extensions
+{
+    /// <summary>
+    /// An interface providing value-invariant access to a <see cref="DictionarySlim{TKey,TValue}"/> instance.
+    /// </summary>
+    /// <typeparam name="TKey">The type of keys in the dictionary.</typeparam>
+    internal interface IDictionary<in TKey>
+        where TKey : struct, IEquatable<TKey>
+    {
+        /// <inheritdoc cref="Dictionary{TKey,TValue}.Remove"/>
+        bool Remove(TKey key);
+    }
+}