private void Arrow_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
 {
     CombBox.IsDropDownOpen = true;
     if (CxOnOpen != null)
     {
         CxOnOpen.Invoke(this, e);
     }
 }
 public event EventHandler CxOnOpen; //addEvent
 private void CombBox_DropDownOpened(object sender, EventArgs e)
 {
     try
     {
         OldItem = CombBox.Items[CombBox.SelectedIndex];
     }
     catch (ArgumentOutOfRangeException)
     {
         OldItem = null;
     }
     if (CxOnOpen != null)
     {
         CxOnOpen.Invoke(this, e);
     }
 }