Private blnOpen As Boolean
   Public Function PreFilterMessage(ByRef m As System.Windows.Forms.Message) As _
     Boolean Implements System.Windows.Forms.IMessageFilter.PreFilterMessage
      If m.Msg = &H202 Then
         If Me.DroppedDown Then
            If blnOpen Then
               Debug.WriteLine("DropDown offen halten")
            Else
               blnOpen = True
               Debug.WriteLine("DropDown geöffnet")
            End If
         Else
            blnOpen = False
            Debug.WriteLine("DropDown geschlossen")
         End If
      End If
   End Function