Add IReadOnlyObservableGroup<out TKey>

CommunityToolkit.Mvvm/Collections/IReadOnlyObservableGroup{TKey}.cs

@@ -0,0 +1,18 @@
+// 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 CommunityToolkit.Mvvm.Collections;
+
+/// <summary>
+/// An interface for a grouped collection of items.
+/// </summary>
+/// <typeparam name="TKey">The type of the group key.</typeparam>
+public interface IReadOnlyObservableGroup<out TKey> : IReadOnlyObservableGroup
+    where TKey : notnull
+{
+    /// <summary>
+    /// Gets the key for the current collection.
+    /// </summary>
+    new TKey Key { get; }
+}

CommunityToolkit.Mvvm/Collections/ObservableGroup{TKey,TValue}.cs

@@ -17,7 +17,7 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// <typeparam name="TKey">The type of the group key.</typeparam>
 /// <typeparam name="TValue">The type of the items in the collection.</typeparam>
 [DebuggerDisplay("Key = {Key}, Count = {Count}")]
-public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
+public class ObservableGroup<TKey, TValue> : ObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup<TKey>
     where TKey : notnull
 {
     /// <summary>

CommunityToolkit.Mvvm/Collections/ReadOnlyObservableGroup{TKey,TValue}.cs

@@ -13,7 +13,7 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// </summary>
 /// <typeparam name="TKey">The type of the group key.</typeparam>
 /// <typeparam name="TValue">The type of the items in the collection.</typeparam>
-public sealed class ReadOnlyObservableGroup<TKey, TValue> : ReadOnlyObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup
+public sealed class ReadOnlyObservableGroup<TKey, TValue> : ReadOnlyObservableCollection<TValue>, IGrouping<TKey, TValue>, IReadOnlyObservableGroup<TKey>
     where TKey : notnull
 {
     /// <summary>