Avalonia/samples/ControlCatalog/Pages/TreeViewPage.xaml

<UserControl xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="ControlCatalog.Pages.TreeViewPage">
  <StackPanel Orientation="Vertical" Spacing="4">
    <TextBlock Classes="h1">TreeView</TextBlock>
    <TextBlock Classes="h2">Displays a hierachical tree of data.</TextBlock>

    <StackPanel Orientation="Horizontal"
                Margin="0,16,0,0"
                HorizontalAlignment="Center"
                Spacing="16">
      <StackPanel Orientation="Vertical" Spacing="8">
        <TreeView Items="{Binding Items}" SelectedItems="{Binding SelectedItems}" SelectionMode="{Binding SelectionMode}" Width="250" Height="350">
          <TreeView.ItemTemplate>
            <TreeDataTemplate ItemsSource="{Binding Children}">
              <TextBlock Text="{Binding Header}"/>
            </TreeDataTemplate>
          </TreeView.ItemTemplate>
        </TreeView>

        <Button Command="{Binding AddItemCommand}">Add</Button>

        <Button Command="{Binding RemoveItemCommand}">Remove</Button>

        <ComboBox SelectedIndex="{Binding SelectionMode, Mode=TwoWay}">
          <ComboBoxItem>Single</ComboBoxItem>
          <ComboBoxItem>Multiple</ComboBoxItem>
          <ComboBoxItem>Toggle</ComboBoxItem>
          <ComboBoxItem>AlwaysSelected</ComboBoxItem>
        </ComboBox>
      </StackPanel>
    </StackPanel>
  </StackPanel>
</UserControl>