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const
  WM_ChangeType  = WM_USER + 10001;

type

TMainMenu2 = class(TMainMenu)
private
protected
      procedure WMChangeType(var Message);    message WM_ChangeType;
public
published
 published
end;

TForm2 = class(TForm)
   MainMenu21: TMainMenu2;
   procedure FormClick(Sender: TObject);
private
   procedure WMChangeType(var Message);    message WM_ChangeType;
public
end;

implementation 

procedure TForm2.FormClick(Sender: TObject);
begin
   sendMessage (MainMenu21.Items.Handle,WM_ChangeType,integer(0),integer(Self)); //macht nichts !!!
   sendMessage (MainMenu21.Handle,WM_ChangeType,integer(0),integer(Self)); //macht nichts !!!
   sendMessage (Self.Handle,WM_ChangeType,integer(0),integer(Self)); //hier geht's!!!
end;