Added ItemsControl.ItemsSource.

.ncrunch/Avalonia.Generators.Tests.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <IgnoreThisComponentCompletely>True</IgnoreThisComponentCompletely>
+  </Settings>
+</ProjectConfiguration>

.ncrunch/Generators.Sandbox.v3.ncrunchproject

@@ -0,0 +1,5 @@
+<ProjectConfiguration>
+  <Settings>
+    <IgnoreThisComponentCompletely>True</IgnoreThisComponentCompletely>
+  </Settings>
+</ProjectConfiguration>

samples/ControlCatalog/Pages/ComboBoxPage.xaml.cs

@@ -18,7 +18,7 @@ private void InitializeComponent()
         {
             AvaloniaXamlLoader.Load(this);
             var fontComboBox = this.Get<ComboBox>("fontComboBox");
-            fontComboBox.Items = FontManager.Current.SystemFonts;
+            fontComboBox.ItemsSource = FontManager.Current.SystemFonts;
             fontComboBox.SelectedIndex = 0;
         }
     }

src/Avalonia.Base/Metadata/InheritDataTypeFromItemsAttribute.cs

@@ -9,7 +9,7 @@ namespace Avalonia.Metadata;
 /// A typical usage example is a ListBox control, where <see cref="InheritDataTypeFromItemsAttribute"/> is defined on the ItemTemplate property,
 /// allowing the template to inherit the data type from the Items collection binding. 
 /// </remarks>
-[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
+[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
 public sealed class InheritDataTypeFromItemsAttribute : Attribute
 {
     /// <summary>

src/Avalonia.Controls.ItemsRepeater/Controls/ItemsRepeater.cs

@@ -39,7 +39,10 @@ public class ItemsRepeater : Panel, IChildIndexProvider
         /// Defines the <see cref="Items"/> property.
         /// </summary>
         public static readonly DirectProperty<ItemsRepeater, IEnumerable?> ItemsProperty =
-            ItemsControl.ItemsProperty.AddOwner<ItemsRepeater>(o => o.Items, (o, v) => o.Items = v);
+            AvaloniaProperty.RegisterDirect<ItemsRepeater, IEnumerable?>(
+                nameof(Items),
+                o => o.Items,
+                (o, v) => o.Items = v);
 
         /// <summary>
         /// Defines the <see cref="Layout"/> property.

src/Avalonia.Controls/Flyouts/MenuFlyout.cs

@@ -19,7 +19,9 @@ public MenuFlyout()
         /// Defines the <see cref="Items"/> property
         /// </summary>
         public static readonly DirectProperty<MenuFlyout, IEnumerable?> ItemsProperty =
-            ItemsControl.ItemsProperty.AddOwner<MenuFlyout>(x => x.Items,
+            AvaloniaProperty.RegisterDirect<MenuFlyout, IEnumerable?>(
+                nameof(Items),
+                x => x.Items,
                 (x, v) => x.Items = v);
 
         /// <summary>

src/Avalonia.Controls/ItemCollection.cs

@@ -0,0 +1,165 @@
+using System;
+using System.Collections;
+using System.Collections.Specialized;
+using System.Diagnostics.CodeAnalysis;
+using Avalonia.Collections;
+
+namespace Avalonia.Controls
+{
+    /// <summary>
+    /// Holds the list of items that constitute the content of an <see cref="ItemsControl"/>.
+    /// </summary>
+    public class ItemCollection : ItemsSourceView, IList
+    {
+// Suppress "Avoid zero-length array allocations": This is a sentinel value and must be unique.
+#pragma warning disable CA1825
+        private static readonly object?[] s_uninitialized = new object?[0];
+#pragma warning restore CA1825
+
+        private Mode _mode;
+
+        internal ItemCollection()
+            : base(s_uninitialized)
+        {
+        }
+
+        public new object? this[int index]
+        {
+            get => base[index];
+            set => WritableSource[index] = value;
+        }
+
+        public bool IsReadOnly => _mode == Mode.ItemsSource;
+
+        internal event EventHandler? SourceChanged;
+
+        /// <summary>
+        /// Adds an item to the <see cref="ItemsControl"/>.
+        /// </summary>
+        /// <param name="value">The item to add to the collection.</param>
+        /// <returns>
+        /// The position into which the new element was inserted, or -1 to indicate that
+        /// the item was not inserted into the collection.
+        /// </returns>
+        /// <exception cref="InvalidOperationException">
+        /// The collection is in ItemsSource mode.
+        /// </exception>
+        public int Add(object? value) => WritableSource.Add(value);
+
+        /// <summary>
+        /// Clears the collection and releases the references on all items currently in the
+        /// collection.
+        /// </summary>
+        /// <exception cref="InvalidOperationException">
+        /// The collection is in ItemsSource mode.
+        /// </exception>
+        public void Clear() => WritableSource.Clear();
+
+        /// <summary>
+        /// Inserts an element into the collection at the specified index.
+        /// </summary>
+        /// <param name="index">The zero-based index at which to insert the item.</param>
+        /// <param name="value">The item to insert.</param>
+        /// <exception cref="InvalidOperationException">
+        /// The collection is in ItemsSource mode.
+        /// </exception>
+        public void Insert(int index, object? value) => WritableSource.Insert(index, value);
+
+        /// <summary>
+        /// Removes the item at the specified index of the collection or view.
+        /// </summary>
+        /// <param name="index">The zero-based index of the item to remove.</param>
+        /// <exception cref="InvalidOperationException">
+        /// The collection is in ItemsSource mode.
+        /// </exception>
+        public void RemoveAt(int index) => WritableSource.RemoveAt(index);
+
+        /// <summary>
+        /// Removes the specified item reference from the collection or view.
+        /// </summary>
+        /// <param name="value">The object to remove.</param>
+        /// <returns>True if the item was removed; otherwise false.</returns>
+        /// <exception cref="InvalidOperationException">
+        /// The collection is in ItemsSource mode.
+        /// </exception>
+        public bool Remove(object? value)
+        {
+            var c = Count;
+            WritableSource.Remove(value);
+            return Count < c;
+        }
+
+        int IList.Add(object? value) => Add(value);
+        void IList.Clear() => Clear();
+        void IList.Insert(int index, object? value) => Insert(index, value);
+        void IList.RemoveAt(int index) => RemoveAt(index);
+
+        private IList WritableSource
+        {
+            get
+            {
+                if (IsReadOnly)
+                    ThrowIsItemsSource();
+                if (Source == s_uninitialized)
+                    SetSource(CreateDefaultCollection());
+                return Source;
+            }
+        }
+
+        internal IList? GetItemsPropertyValue()
+        {
+            if (_mode == Mode.ObsoleteItemsSetter)
+                return Source == s_uninitialized ? null : Source;
+            return this;
+        }
+
+        internal void SetItems(IList? items)
+        {
+            _mode = Mode.ObsoleteItemsSetter;
+            SetSource(items ?? s_uninitialized);
+        }
+
+        internal void SetItemsSource(IEnumerable? value)
+        {
+            if (_mode != Mode.ItemsSource && Count > 0)
+                throw new InvalidOperationException(
+                    "Items collection must be empty before using ItemsSource.");
+
+            _mode = value is not null ? Mode.ItemsSource : Mode.Items;
+            SetSource(value ?? CreateDefaultCollection());
+        }
+
+        private new void SetSource(IEnumerable source)
+        {
+            var oldSource = Source;
+
+            base.SetSource(source);
+
+            if (oldSource.Count > 0)
+                RaiseCollectionChanged(new(NotifyCollectionChangedAction.Remove, oldSource, 0));
+            if (Source.Count > 0)
+                RaiseCollectionChanged(new(NotifyCollectionChangedAction.Add, Source, 0));
+            SourceChanged?.Invoke(this, EventArgs.Empty);
+        }
+
+        private static AvaloniaList<object?> CreateDefaultCollection()
+        {
+            return new() { ResetBehavior = ResetBehavior.Remove };
+        }
+
+        [DoesNotReturn]
+        private static void ThrowIsItemsSource()
+        {
+            throw new InvalidOperationException(
+                "Operation is not valid while ItemsSource is in use." +
+                "Access and modify elements with ItemsControl.ItemsSource instead.");
+        }
+
+        private enum Mode
+        {
+            Items,
+            ItemsSource,
+            ObsoleteItemsSetter,
+        }
+    }
+}