WPF中TextBox控件对于鼠标单击获取焦点后的全选

void OnLostFocus(object sender, RoutedEventArgs e)
         {
             TextBox tb = e.Source as TextBox;
             tb.PreviewMouseDown += new MouseButtonEventHandler(OnPreviewMouseDown);
         }
 
         void OnPreviewMouseDown(object sender, MouseButtonEventArgs e)
         {
             TextBox tb = e.Source as TextBox;
             tb.Focus();
             e.Handled = true;
         }
 
         void OnGotFocus(object sender, RoutedEventArgs e)
         {
             TextBox tb = e.Source as TextBox;
             tb.SelectAll();
             tb.PreviewMouseDown -= new MouseButtonEventHandler(OnPreviewMouseDown);
         }

TextBox Name="searchTextBox" Background="DarkOrange" HorizontalAlignment="Stretch" MaxLength="15" TextAlignment ="Center"
     TextChanged="OnSearchTextChange" GotFocus="OnGotFocus" PreviewMouseDown="OnPreviewMouseDown" LostFocus="OnLostFocus">

tb.PreviewMouseDown -= new MouseButtonEventHandler(OnPreviewMouseDown);