Avalonia/samples/ControlCatalog/Pages/TreeViewPage.xaml

<UserControl xmlns="https://github.com/avaloniaui">
  <StackPanel Orientation="Vertical" Gap="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"
              Gap="16">
      <TreeView Items="{Binding}" Width="250" Height="350">
        <TreeView.ItemTemplate>
          <TreeDataTemplate ItemsSource="{Binding Children}">
            <TextBlock Text="{Binding Header}"/>
          </TreeDataTemplate>
        </TreeView.ItemTemplate>
      </TreeView>
    </StackPanel>
  </StackPanel>
</UserControl>