How do you programmatically set focus to the SelectedItem in a WPF ListBox that already has focus?

It’s a couple lines of code. If you didn’t want it in code-behind, I sure it could be packaged in a attached behaviour.

private void Button_Click(object sender, RoutedEventArgs e)
{
    MainListBox.SelectedItem = MainListBox.Items[3];
    MainListBox.UpdateLayout(); // Pre-generates item containers 

    var listBoxItem = (ListBoxItem) MainListBox
        .ItemContainerGenerator
        .ContainerFromItem(MainListBox.SelectedItem);

    listBoxItem.Focus();
}