private void OnSelectExp(SelectExpressionNotification cmd)
 {
     if (Descriptors.Count == 0)
     {
         return;
     }
     if (cmd.SelectorIndex == Index)
     {
         if (Descriptors[SelectedIndex].abbr != cmd.ExpKey)
         {
             SelectedIndex = Descriptors.IndexOf(Descriptors.First(d => d.abbr == cmd.ExpKey));
         }
         DisplayMode = ExpDisMode.Visible;
     }
     else if (cmd.UpdateShadow)
     {
         DisplayMode = DisplayMode == ExpDisMode.Visible ? ExpDisMode.Shadow : ExpDisMode.Hidden;
     }
 }