Implement changing selection mode.

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -423,6 +423,11 @@ namespace Avalonia.Controls.Primitives
                     _selection.Clear();
                 }
             }
+            else if (change.Property == SelectionModeProperty && _selection is object)
+            {
+                var newValue = change.NewValue.GetValueOrDefault<SelectionMode>();
+                _selection.SingleSelect = !newValue.HasFlagCustom(SelectionMode.Multiple);
+            }
         }
 
         /// <summary>

src/Avalonia.Controls/Selection/SelectionModel.cs

@@ -45,6 +45,14 @@ namespace Avalonia.Controls.Selection
             {
                 if (_singleSelect != value)
                 {
+                    if (value == true)
+                    {
+                        using var update = BatchUpdate();
+                        var selectedIndex = SelectedIndex;
+                        Clear();
+                        SelectedIndex = selectedIndex;
+                    }
+
                     _singleSelect = value;
                     RangesEnabled = !value;
 

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

@@ -7,6 +7,7 @@ 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;
@@ -1321,6 +1322,19 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(1, target.SelectedIndex);
         }
 
+        [Fact]
+        public void Setting_SelectionMode_Should_Update_SelectionModel()
+        {
+            var target = new TestSelector();
+            var model = target.Selection;
+
+            Assert.True(model.SingleSelect);
+
+            target.SelectionMode = SelectionMode.Multiple;
+
+            Assert.False(model.SingleSelect);
+        }
+
         private static void Prepare(SelectingItemsControl target)
         {
             var root = new TestRoot
@@ -1401,6 +1415,18 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 SelectionMode = selectionMode;
             }
 
+            public new ISelectionModel Selection
+            {
+                get => base.Selection;
+                set => base.Selection = value;
+            }
+
+            public new SelectionMode SelectionMode
+            {
+                get => base.SelectionMode;
+                set => base.SelectionMode = value;
+            }
+
             public new bool MoveSelection(NavigationDirection direction, bool wrap)
             {
                 return base.MoveSelection(direction, wrap);

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

@@ -712,6 +712,54 @@ namespace Avalonia.Controls.UnitTests.Selection
             }
         }
 
+        public class SingleSelect
+        {
+            [Fact]
+            public void Converting_To_Single_Selection_Removes_Multiple_Selection()
+            {
+                var target = CreateTarget();
+                var raised = 0;
+
+                target.SelectRange(1, 3);
+
+                target.SelectionChanged += (s, e) =>
+                {
+                    Assert.Equal(new[] { 2, 3 }, e.DeselectedIndexes);
+                    Assert.Equal(new[] { "baz", "qux" }, e.DeselectedItems);
+                    Assert.Empty(e.SelectedIndexes);
+                    Assert.Empty(e.SelectedItems);
+                    ++raised;
+                };
+
+                target.SingleSelect = true;
+
+                Assert.Equal(1, target.SelectedIndex);
+                Assert.Equal(new[] { 1 }, target.SelectedIndexes);
+                Assert.Equal("bar", target.SelectedItem);
+                Assert.Equal(new[] { "bar" }, target.SelectedItems);
+                Assert.Equal(1, raised);
+            }
+
+            [Fact]
+            public void Raises_PropertyChanged()
+            {
+                var target = CreateTarget();
+                var raised = 0;
+
+                target.PropertyChanged += (s, e) =>
+                {
+                    if (e.PropertyName == nameof(target.SingleSelect))
+                    {
+                        ++raised;
+                    }
+                };
+
+                target.SingleSelect = true;
+
+                Assert.Equal(1, raised);
+            }
+        }
+
         public class CollectionChanges
         {
             [Fact]