Una de las preguntas mas hechas en los foros, aqui les paso un codigo cortesia del amigo Vichenzo Vinç

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Private Declare Function SendMessageLong Lib “user32.dll” Alias “SendMessageA” ( _
ByVal hwnd As Long, _
ByVal wMsg As Long, _
ByVal wParam As Long, _
ByRef lParam As Long) As Long
Private Const CB_SHOWDROPDOWN As Long = &H14F

Private Sub Combo1_GotFocus()
DesplegarCombo ActiveControl.hwnd
End Sub

Public Sub DesplegarCombo(ByVal hWndCombo As Long)
SendMessageLong hWndCombo, CB_SHOWDROPDOWN, 1, 0&
End Sub
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