如何利用程序拉下ComboBox？

除了以鼠标或键盘拉下 ComboBox 的列表框之外， 如何在程序中拉下 ComboBox 的列表框? 方法是对 ComboBox 送出 CB_SHOWDROPDOWN 讯息， 细节如下：

''声明:
Const CB_SHOWDROPDOWN = &H14F
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal _
hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, lParam As Any) As Long

''代码:

SendMessage Combo1.hwnd, CB_SHOWDROPDOWN, True, ByVal 0&
如果收起列表框，则是：
SendMessage Combo1.hwnd, CB_SHOWDROPDOWN, False, ByVal 0&