Fix working with collections of value types.

`IEnumerable<T>` is not covariant for value types, so we need to use the non-generic `IEnumerable` everywhere under the hood to ensure we can work with both value and reference types.

src/Avalonia.Controls/ItemsSourceView.cs

@@ -7,6 +7,7 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Avalonia.Controls.Utils;
 
@@ -15,29 +16,30 @@ using Avalonia.Controls.Utils;
 namespace Avalonia.Controls
 {
     /// <summary>
-    /// Represents a standardized view of the supported interactions between a given
-    /// <see cref="ItemsControl"/> or <see cref="ItemsRepeater"/> and its items.
+    /// Represents a standardized view of the supported interactions between a given ItemsSource
+    /// object and an <see cref="ItemsRepeater"/> control.
     /// </summary>
-    public class ItemsSourceView<T> : INotifyCollectionChanged, IDisposable, IReadOnlyList<T>
+    /// <remarks>
+    /// Components written to work with ItemsRepeater should consume the
+    /// <see cref="ItemsRepeater.Items"/> via ItemsSourceView since this provides a normalized
+    /// view of the Items. That way, each component does not need to know if the source is an
+    /// IEnumerable, an IList, or something else.
+    /// </remarks>
+    public class ItemsSourceView : INotifyCollectionChanged, IDisposable
     {
         /// <summary>
-        ///  Gets an empty <see cref="ItemsSourceView{T}"/>
+        ///  Gets an empty <see cref="ItemsSourceView"/>
         /// </summary>
-        public static ItemsSourceView<T> Empty { get; } = new ItemsSourceView<T>(Array.Empty<T>());
+        public static ItemsSourceView Empty { get; } = new ItemsSourceView(Array.Empty<object>());
 
-        private readonly IList _inner;
+        private protected readonly IList _inner;
         private INotifyCollectionChanged? _notifyCollectionChanged;
 
         /// <summary>
         /// Initializes a new instance of the ItemsSourceView class for the specified data source.
         /// </summary>
         /// <param name="source">The data source.</param>
-        public ItemsSourceView(IEnumerable<T> source)
-            : this((IEnumerable)source)
-        {
-        }
-
-        private protected ItemsSourceView(IEnumerable source)
+        public ItemsSourceView(IEnumerable source)
         {
             source = source ?? throw new ArgumentNullException(nameof(source));
 
@@ -45,13 +47,13 @@ namespace Avalonia.Controls
             {
                 _inner = list;
             }
-            else if (source is IEnumerable<object?> enumerable)
+            else if (source is IEnumerable<object> objectEnumerable)
             {
-                _inner = new List<object?>(enumerable);
+                _inner = new List<object>(objectEnumerable);
             }
             else
             {
-                _inner = new List<object?>(source.Cast<object?>());
+                _inner = new List<object>(source.Cast<object>());
             }
 
             ListenToCollectionChanges();
@@ -75,7 +77,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="index">The index.</param>
         /// <returns>The item.</returns>
-        public T this[int index] => GetAt(index);
+        public object? this[int index] => GetAt(index);
 
         /// <summary>
         /// Occurs when the collection has changed to indicate the reason for the change and which items changed.
@@ -96,13 +98,13 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="index">The index.</param>
         /// <returns>The item.</returns>
-        public T GetAt(int index) => _inner is IList<T> typed ? typed[index] : (T)_inner[index];
+        public object? GetAt(int index) => _inner[index];
 
         public int IndexOf(object? item) => _inner.IndexOf(item);
 
-        public static ItemsSourceView<T> GetOrCreate(IEnumerable<T>? items)
+        public static ItemsSourceView GetOrCreate(IEnumerable? items)
         {
-            if (items is ItemsSourceView<T> isv)
+            if (items is ItemsSourceView isv)
             {
                 return isv;
             }
@@ -112,7 +114,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                return new ItemsSourceView<T>(items);
+                return new ItemsSourceView(items);
             }
         }
 
@@ -142,11 +144,6 @@ namespace Avalonia.Controls
             throw new NotImplementedException();
         }
 
-        public IEnumerator<T> GetEnumerator() => _inner is IList<T> typed ?
-            typed.GetEnumerator() : _inner.Cast<T>().GetEnumerator();
-
-        IEnumerator IEnumerable.GetEnumerator() => _inner.GetEnumerator();
-
         internal void AddListener(ICollectionChangedListener listener)
         {
             if (_inner is INotifyCollectionChanged incc)
@@ -183,11 +180,61 @@ namespace Avalonia.Controls
         }
     }
 
-    public class ItemsSourceView : ItemsSourceView<object?>
+    public class ItemsSourceView<T> : ItemsSourceView, IReadOnlyList<T>
     {
-        public ItemsSourceView(IEnumerable source)
+        /// <summary>
+        ///  Gets an empty <see cref="ItemsSourceView"/>
+        /// </summary>
+        public new static ItemsSourceView<T> Empty { get; } = new ItemsSourceView<T>(Array.Empty<T>());
+
+        /// <summary>
+        /// Initializes a new instance of the ItemsSourceView class for the specified data source.
+        /// </summary>
+        /// <param name="source">The data source.</param>
+        public ItemsSourceView(IEnumerable<T> source)
             : base(source)
         {
         }
+
+        private ItemsSourceView(IEnumerable source)
+            : base(source)
+        {
+        }
+
+        /// <summary>
+        /// Retrieves the item at the specified index.
+        /// </summary>
+        /// <param name="index">The index.</param>
+        /// <returns>The item.</returns>
+#pragma warning disable CS8603
+        public new T this[int index] => GetAt(index);
+#pragma warning restore CS8603
+
+        /// <summary>
+        /// Retrieves the item at the specified index.
+        /// </summary>
+        /// <param name="index">The index.</param>
+        /// <returns>The item.</returns>
+        [return: MaybeNull]
+        public new T GetAt(int index) => (T)_inner[index];
+
+        public IEnumerator<T> GetEnumerator() => _inner.Cast<T>().GetEnumerator();
+        IEnumerator IEnumerable.GetEnumerator() => _inner.GetEnumerator();
+
+        public static new ItemsSourceView<T> GetOrCreate(IEnumerable? items)
+        {
+            if (items is ItemsSourceView<T> isv)
+            {
+                return isv;
+            }
+            else if (items is null)
+            {
+                return Empty;
+            }
+            else
+            {
+                return new ItemsSourceView<T>(items);
+            }
+        }
     }
 }

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -32,46 +32,10 @@ namespace Avalonia.Controls.Selection
             Source = source;
         }
 
-        public override IEnumerable<T>? Source
+        public new IEnumerable<T>? Source
         {
-            get => base.Source;
-            set
-            {
-                if (base.Source != value)
-                {
-                    if (_operation is object)
-                    {
-                        throw new InvalidOperationException("Cannot change source while update is in progress.");
-                    }
-
-                    if (base.Source is object && value is object)
-                    {
-                        using var update = BatchUpdate();
-                        update.Operation.SkipLostSelection = true;
-                        Clear();
-                    }
-
-                    base.Source = value;
-
-                    using (var update = BatchUpdate())
-                    {
-                        update.Operation.IsSourceUpdate = true;
-
-                        if (_hasInitSelectedItem)
-                        {
-                            SelectedItem = _initSelectedItem;
-                            _initSelectedItem = default;
-                            _hasInitSelectedItem = false;
-                        }
-                        else
-                        {
-                            TrimInvalidSelections(update.Operation);
-                        }
-
-                        RaisePropertyChanged(nameof(Source));
-                    }
-                }
-            }
+            get => base.Source as IEnumerable<T>;
+            set => SetSource(value);
         }
 
         public bool SingleSelect 
@@ -168,7 +132,7 @@ namespace Avalonia.Controls.Selection
         IEnumerable? ISelectionModel.Source 
         {
             get => Source;
-            set => Source = (IEnumerable<T>?)value;
+            set => SetSource(value);
         }
 
         object? ISelectionModel.SelectedItem
@@ -298,6 +262,44 @@ namespace Avalonia.Controls.Selection
             PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
         }
 
+        private void SetSource(IEnumerable? value)
+        {
+            if (base.Source != value)
+            {
+                if (_operation is object)
+                {
+                    throw new InvalidOperationException("Cannot change source while update is in progress.");
+                }
+
+                if (base.Source is object && value is object)
+                {
+                    using var update = BatchUpdate();
+                    update.Operation.SkipLostSelection = true;
+                    Clear();
+                }
+
+                base.Source = value;
+
+                using (var update = BatchUpdate())
+                {
+                    update.Operation.IsSourceUpdate = true;
+
+                    if (_hasInitSelectedItem)
+                    {
+                        SelectedItem = _initSelectedItem;
+                        _initSelectedItem = default;
+                        _hasInitSelectedItem = false;
+                    }
+                    else
+                    {
+                        TrimInvalidSelections(update.Operation);
+                    }
+
+                    RaisePropertyChanged(nameof(Source));
+                }
+            }
+        }
+
         private protected override void OnIndexesChanged(int shiftIndex, int shiftDelta)
         {
             IndexesChanged?.Invoke(this, new SelectionModelIndexesChangedEventArgs(shiftIndex, shiftDelta));
@@ -511,7 +513,7 @@ namespace Avalonia.Controls.Selection
                 return default;
             }
 
-            return ItemsView.GetAt(index);
+            return ItemsView[index];
         }
 
         private int CoerceIndex(int index)
@@ -710,7 +712,7 @@ namespace Avalonia.Controls.Selection
             public int SelectedIndex { get; set; }
             public List<IndexRange>? SelectedRanges { get; set; }
             public List<IndexRange>? DeselectedRanges { get; set; }
-            public IReadOnlyList<T> DeselectedItems { get; set; }
+            public IReadOnlyList<T>? DeselectedItems { get; set; }
         }
     }
 }

src/Avalonia.Controls/Selection/SelectionNodeBase.cs

@@ -10,12 +10,12 @@ namespace Avalonia.Controls.Selection
 {
     public abstract class SelectionNodeBase<T> : ICollectionChangedListener
     {
-        private IEnumerable<T>? _source;
+        private IEnumerable? _source;
         private bool _rangesEnabled;
         private List<IndexRange>? _ranges;
         private int _collectionChanging;
 
-        public virtual IEnumerable<T>? Source
+        protected IEnumerable? Source
         {
             get => _source;
             set

tests/Avalonia.Controls.UnitTests/Selection/SelectionModelTests_Single.cs

@@ -237,6 +237,14 @@ namespace Avalonia.Controls.UnitTests.Selection
 
                 Assert.Equal(1, raised);
             }
+
+            [Fact]
+            public void Can_Assign_ValueType_Collection_To_SelectionModel_Of_Object()
+            {
+                var target = (ISelectionModel)new SelectionModel<object>();
+
+                target.Source = new[] { 1, 2, 3 };
+            }
         }
 
         public class SelectedIndex