// 1) open App.xaml.cs
// 2) Insert the code below

 // http://madprops.org/blog/wpf-textbox-selectall-on-focus/

  protected override void OnStartup(StartupEventArgs e) {
     EventManager.RegisterClassHandler(typeof(TextBox),
        TextBox.GotFocusEvent, new RoutedEventHandler(TextBox_GotFocus));
            base.OnStartup(e);
  }

  private void TextBox_GotFocus(object sender, RoutedEventArgs e) {
         (sender as TextBox).SelectAll();
  }