Use SelectionModel in samples.

samples/VirtualizationDemo/MainWindow.xaml

@@ -44,8 +44,8 @@
         </StackPanel>
 
         <ListBox Name="listBox" 
-                 Items="{Binding Items}"
-                 SelectedItems="{Binding SelectedItems}"
+                 Items="{Binding Items}" 
+                 Selection="{Binding Selection}"
                  SelectionMode="Multiple"
                  VirtualizationMode="{Binding VirtualizationMode}"
                  ScrollViewer.HorizontalScrollBarVisibility="{Binding HorizontalScrollBarVisibility, Mode=TwoWay}"

samples/VirtualizationDemo/ViewModels/MainWindowViewModel.cs

@@ -7,6 +7,7 @@ using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
 using ReactiveUI;
 using Avalonia.Layout;
+using Avalonia.Controls.Selection;
 
 namespace VirtualizationDemo.ViewModels
 {
@@ -48,8 +49,7 @@ namespace VirtualizationDemo.ViewModels
             set { this.RaiseAndSetIfChanged(ref _itemCount, value); }
         }
 
-        public AvaloniaList<ItemViewModel> SelectedItems { get; }
-            = new AvaloniaList<ItemViewModel>();
+        public SelectionModel<ItemViewModel> Selection { get; } = new SelectionModel<ItemViewModel>();
 
         public AvaloniaList<ItemViewModel> Items
         {
@@ -138,9 +138,9 @@ namespace VirtualizationDemo.ViewModels
         {
             var index = Items.Count;
 
-            if (SelectedItems.Count > 0)
+            if (Selection.SelectedItems.Count > 0)
             {
-                index = Items.IndexOf(SelectedItems[0]);
+                index = Selection.SelectedIndex;
             }
 
             Items.Insert(index, new ItemViewModel(_newItemIndex++, NewItemString));
@@ -148,9 +148,9 @@ namespace VirtualizationDemo.ViewModels
 
         private void Remove()
         {
-            if (SelectedItems.Count > 0)
+            if (Selection.SelectedItems.Count > 0)
             {
-                Items.RemoveAll(SelectedItems);
+                Items.RemoveAll(Selection.SelectedItems.ToList());
             }
         }
 
@@ -164,8 +164,7 @@ namespace VirtualizationDemo.ViewModels
 
         private void SelectItem(int index)
         {
-            SelectedItems.Clear();
-            SelectedItems.Add(Items[index]);
+            Selection.SelectedIndex = index;
         }
     }
 }