Add ILookup<,> to observable grouped collections

Author: Sergio0694

CommunityToolkit.Mvvm/Collections/ObservableGroupedCollectionExtensions.cs

@@ -69,7 +69,7 @@ static void ThrowArgumentExceptionForKeyNotFound()
         [MethodImpl(MethodImplOptions.NoInlining)]
         static ObservableGroup<TKey, TValue>? FirstOrDefaultWithLinq(ObservableGroupedCollection<TKey, TValue> source, TKey key)
         {
-            return source.FirstOrDefault(group => EqualityComparer<TKey>.Default.Equals(group.Key, key));
+            return Enumerable.FirstOrDefault<ObservableGroup<TKey, TValue>>(source, group => EqualityComparer<TKey>.Default.Equals(group.Key, key));
         }
 
         return FirstOrDefaultWithLinq(source, key);

CommunityToolkit.Mvvm/Collections/ObservableGroupedCollection{TKey,TElement}.cs

@@ -15,7 +15,7 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// </summary>
 /// <typeparam name="TKey">The type of the group keys.</typeparam>
 /// <typeparam name="TElement">The type of elements in the collection.</typeparam>
-public sealed class ObservableGroupedCollection<TKey, TElement> : ObservableCollection<ObservableGroup<TKey, TElement>>
+public sealed class ObservableGroupedCollection<TKey, TElement> : ObservableCollection<ObservableGroup<TKey, TElement>>, ILookup<TKey, TElement>
     where TKey : notnull
 {
     /// <summary>
@@ -30,10 +30,17 @@ public ObservableGroupedCollection()
     /// </summary>
     /// <param name="collection">The initial data to add in the grouped collection.</param>
     public ObservableGroupedCollection(IEnumerable<IGrouping<TKey, TElement>> collection)
-        : base(collection.Select(static c => new ObservableGroup<TKey, TElement>(c)))
+        : base(collection.Select(static group => new ObservableGroup<TKey, TElement>(group)))
     {
     }
 
+    /// <inheritdoc/>
+    IEnumerable<TElement> ILookup<TKey, TElement>.this[TKey key]
+    {
+        // TODO: optimize this
+        get => Enumerable.FirstOrDefault<ObservableGroup<TKey, TElement>>(this, item => EqualityComparer<TKey>.Default.Equals(item.Key, key)) ?? Enumerable.Empty<TElement>();
+    }
+
     /// <summary>
     /// Tries to get the underlying <see cref="List{T}"/> instance, if present.
     /// </summary>
@@ -46,4 +53,17 @@ internal bool TryGetList([NotNullWhen(true)] out List<ObservableGroup<TKey, TEle
 
         return list is not null;
     }
+
+    /// <inheritdoc/>
+    bool ILookup<TKey, TElement>.Contains(TKey key)
+    {
+        // TODO: optimize this
+        return Enumerable.Any<ObservableGroup<TKey, TElement>>(this, item => EqualityComparer<TKey>.Default.Equals(item.Key, key));
+    }
+
+    /// <inheritdoc/>
+    IEnumerator<IGrouping<TKey, TElement>> IEnumerable<IGrouping<TKey, TElement>>.GetEnumerator()
+    {
+        return GetEnumerator();
+    }
 }

CommunityToolkit.Mvvm/Collections/ReadOnlyObservableGroupedCollection{TKey,TElement}.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Collections.Specialized;
 using System.Diagnostics.CodeAnalysis;
@@ -15,7 +16,7 @@ namespace CommunityToolkit.Mvvm.Collections;
 /// </summary>
 /// <typeparam name="TKey">The type of the group keys.</typeparam>
 /// <typeparam name="TElement">The type of elements in the collection.</typeparam>
-public sealed class ReadOnlyObservableGroupedCollection<TKey, TElement> : ReadOnlyObservableCollection<ReadOnlyObservableGroup<TKey, TElement>>
+public sealed class ReadOnlyObservableGroupedCollection<TKey, TElement> : ReadOnlyObservableCollection<ReadOnlyObservableGroup<TKey, TElement>>, ILookup<TKey, TElement>
     where TKey : notnull
 {
     /// <summary>
@@ -37,6 +38,26 @@ public ReadOnlyObservableGroupedCollection(ObservableCollection<ReadOnlyObservab
     {
     }
 
+    /// <inheritdoc/>
+    IEnumerable<TElement> ILookup<TKey, TElement>.this[TKey key]
+    {
+        // TODO: optimize this
+        get => Enumerable.FirstOrDefault<ReadOnlyObservableGroup<TKey, TElement>>(this, item => EqualityComparer<TKey>.Default.Equals(item.Key, key)) ?? Enumerable.Empty<TElement>();
+    }
+
+    /// <inheritdoc/>
+    bool ILookup<TKey, TElement>.Contains(TKey key)
+    {
+        // TODO: optimize this
+        return Enumerable.Any<ReadOnlyObservableGroup<TKey, TElement>>(this, item => EqualityComparer<TKey>.Default.Equals(item.Key, key));
+    }
+
+    /// <inheritdoc/>
+    IEnumerator<IGrouping<TKey, TElement>> IEnumerable<IGrouping<TKey, TElement>>.GetEnumerator()
+    {
+        return GetEnumerator();
+    }
+
     /// <summary>
     /// Forwards the <see cref="INotifyCollectionChanged.CollectionChanged"/> event whenever it is raised by the wrapped collection.
     /// </summary>