'SendMessage函数在VB中的函数说明如下:
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
Private Const CB_SHOWDROPDOWN = &H14F

Private Sub Combo1_Change()
SendMessage Combo1.hwnd, CB_SHOWDROPDOWN, 1, 0 ' 展开
End Sub

Private Sub Command2_Click()
  SendMessage Combo1.hwnd, CB_SHOWDROPDOWN, 0, 0  '收起
End Sub

Private Sub Form_Load()
    Combo1.AddItem "AA00001"
    Combo1.AddItem "AA00002"
    Combo1.AddItem "AA00003"
   
    Combo1.AddItem "AB00001"
    Combo1.AddItem "AB00002"
    Combo1.AddItem "AB00003"
   
    Combo1.AddItem "AC00001"
    Combo1.AddItem "AC00002"
    Combo1.AddItem "AC00003"
   
End Sub