Added back removed tests.

src/Avalonia.Controls/ListBox.cs

@@ -2,6 +2,7 @@ using System.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Input;
 using Avalonia.VisualTree;
@@ -31,6 +32,12 @@ namespace Avalonia.Controls
         public static readonly new DirectProperty<SelectingItemsControl, IList> SelectedItemsProperty =
             SelectingItemsControl.SelectedItemsProperty;
 
+        /// <summary>
+        /// Defines the <see cref="Selection"/> property.
+        /// </summary>
+        public static readonly new DirectProperty<SelectingItemsControl, ISelectionModel> SelectionProperty =
+            SelectingItemsControl.SelectionProperty;
+
         /// <summary>
         /// Defines the <see cref="SelectionMode"/> property.
         /// </summary>
@@ -70,6 +77,13 @@ namespace Avalonia.Controls
             set => base.SelectedItems = value;
         }
 
+        /// <inheritdoc/>
+        public new ISelectionModel Selection
+        {
+            get => base.Selection;
+            set => base.Selection = value;
+        }
+
         /// <summary>
         /// Gets or sets the selection mode.
         /// </summary>

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_Multiple.cs

@@ -6,6 +6,7 @@ using System.Linq;
 using Avalonia.Collections;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Primitives;
+using Avalonia.Controls.Selection;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
 using Avalonia.Input;
@@ -1264,6 +1265,178 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(1, target.SelectedIndex);
         }
 
+        [Fact]
+        public void Assigning_Null_To_Selection_Should_Create_New_SelectionModel()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var oldSelection = target.Selection;
+
+            target.Selection = null;
+
+            Assert.NotNull(target.Selection);
+            Assert.NotSame(oldSelection, target.Selection);
+        }
+
+        [Fact]
+        public void Assigning_SelectionModel_With_Different_Source_To_Selection_Should_Fail()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var selection = new SelectionModel<string> { Source = new[] { "baz" } };
+            Assert.Throws<ArgumentException>(() => target.Selection = selection);
+        }
+
+        [Fact]
+        public void Assigning_SelectionModel_With_Null_Source_To_Selection_Should_Set_Source()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            var selection = new SelectionModel<string>();
+            target.Selection = selection;
+
+            Assert.Same(target.Items, selection.Source);
+        }
+
+        [Fact]
+        public void Assigning_Single_Selected_Item_To_Selection_Should_Set_SelectedIndex()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel<string> { SingleSelect = false };
+            selection.Select(1);
+            target.Selection = selection;
+
+            Assert.Equal(1, target.SelectedIndex);
+            Assert.Equal(new[] { "bar" }, target.Selection.SelectedItems);
+            Assert.Equal(new[] { 1 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Assigning_Multiple_Selected_Items_To_Selection_Should_Set_SelectedIndex()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar", "baz" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel<string> { SingleSelect = false };
+            selection.SelectRange(0, 2);
+            target.Selection = selection;
+
+            Assert.Equal(0, target.SelectedIndex);
+            Assert.Equal(new[] { "foo", "bar", "baz" }, target.Selection.SelectedItems);
+            Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
+        }
+
+        [Fact]
+        public void Reassigning_Selection_Should_Clear_Selection()
+        {
+            var target = new TestSelector
+            {
+                Items = new[] { "foo", "bar" },
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Selection.Select(1);
+            target.Selection = new SelectionModel<string>();
+
+            Assert.Equal(-1, target.SelectedIndex);
+            Assert.Null(target.SelectedItem);
+        }
+
+        [Fact]
+        public void Assigning_Selection_Should_Set_Item_IsSelected()
+        {
+            var items = new[]
+            {
+                new ListBoxItem(),
+                new ListBoxItem(),
+                new ListBoxItem(),
+            };
+
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel<object> { SingleSelect = false };
+            selection.SelectRange(0, 1);
+            target.Selection = selection;
+
+            Assert.True(items[0].IsSelected);
+            Assert.True(items[1].IsSelected);
+            Assert.False(items[2].IsSelected);
+        }
+
+        [Fact]
+        public void Assigning_Selection_Should_Raise_SelectionChanged()
+        {
+            var items = new[] { "foo", "bar", "baz" };
+
+            var target = new TestSelector
+            {
+                Items = items,
+                Template = Template(),
+                SelectedItem = "bar",
+            };
+
+            var raised = 0;
+
+            target.SelectionChanged += (s, e) =>
+            {
+                if (raised == 0)
+                {
+                    Assert.Empty(e.AddedItems.Cast<object>());
+                    Assert.Equal(new[] { "bar" }, e.RemovedItems.Cast<object>());
+                }
+                else
+                {
+                    Assert.Equal(new[] { "foo", "baz" }, e.AddedItems.Cast<object>());
+                    Assert.Empty(e.RemovedItems.Cast<object>());
+                }
+
+                ++raised;
+            };
+
+            target.ApplyTemplate();
+            target.Presenter.ApplyTemplate();
+
+            var selection = new SelectionModel<string> { Source = items, SingleSelect = false };
+            selection.Select(0);
+            selection.Select(2);
+            target.Selection = selection;
+
+            Assert.Equal(2, raised);
+        }
         private IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
             return target.Presenter.Panel.Children
@@ -1298,6 +1471,12 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 set { base.SelectedItems = value; }
             }
 
+            public new ISelectionModel Selection
+            {
+                get => base.Selection;
+                set => base.Selection = value;
+            }
+
             public new SelectionMode SelectionMode
             {
                 get { return base.SelectionMode; }