An application sends a CB_SHOWDROPDOWN message to show or hide the list box of a combo box that has the CBS_DROPDOWN or CBS_DROPDOWNLIST style.
CB_SHOWDROPDOWN wParam = (WPARAM) (BOOL) fShow; // the show/hide flag lParam = 0; // not used; must be zero
The return value is always TRUE.
This message has no effect on a combo box created with the CBS_SIMPLE style.