Added IDictionarySlim interface

Microsoft.Toolkit.Mvvm/Messaging/Microsoft.Collections.Extensions/IDictionarySlim.cs

@@ -0,0 +1,22 @@
+// 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 Microsoft.Collections.Extensions
+{
+    /// <summary>
+    /// A base interface masking <see cref="DictionarySlim{TKey,TValue}"/> instances and exposing non-generic functionalities.
+    /// </summary>
+    internal interface IDictionarySlim
+    {
+        /// <summary>
+        /// Gets the count of entries in the dictionary.
+        /// </summary>
+        int Count { get; }
+
+        /// <summary>
+        /// Clears the current dictionary.
+        /// </summary>
+        void Clear();
+    }
+}

Microsoft.Toolkit.Mvvm/Messaging/Microsoft.Collections.Extensions/IDictionarySlim{TKey}.cs

@@ -10,14 +10,9 @@ namespace Microsoft.Collections.Extensions
     /// An interface providing key type contravariant access to a <see cref="DictionarySlim{TKey,TValue}"/> instance.
     /// </summary>
     /// <typeparam name="TKey">The contravariant type of keys in the dictionary.</typeparam>
-    internal interface IDictionarySlim<in TKey>
+    internal interface IDictionarySlim<in TKey> : IDictionarySlim
         where TKey : IEquatable<TKey>
     {
-        /// <summary>
-        /// Gets the count of entries in the dictionary.
-        /// </summary>
-        int Count { get; }
-
         /// <summary>
         /// Tries to remove a value with a specified key.
         /// </summary>
@@ -32,10 +27,5 @@ internal interface IDictionarySlim<in TKey>
         /// <param name="key">The key of the item to remove.</param>
         /// <returns>Whether or not an item was removed.</returns>
         bool Remove(TKey key);
-
-        /// <summary>
-        /// Clears the current dictionary.
-        /// </summary>
-        void Clear();
     }
 }