VCL Delphi

procedure TForm1.Button1Click(Sender: TObject);
begin
// Check whether DropedDown
if SendMessage(ComboBox1.Handle, CB_GETDROPPEDSTATE, 0, 0) <> 1 then
// nop, so drop it
SendMessage(ComboBox1.Handle, CB_SHOWDROPDOWN, 1, 0);
end;