ListBox.SelectedItems プロパティ

[BindableAttribute(true)] 
public IList SelectedItems { get; }

/** @property */
public IList get_SelectedItems ()

public function get SelectedItems () : IList

このプロパティは XAML では使用できません。

private void SelectedItems(object sender, RoutedEventArgs e)
{
    if(lb.SelectedItem != null)
       {
        btn.Content = "Has " + (lb.SelectedItems.Count.ToString()) + " item(s) selected.";
       }
 }