CB_SHOWDROPDOWN
wParam = (WPARAM) (BOOL) fShow; /* the show/hide flag */
lParam = 0; /* not used, must be zero */
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.
fShow
Value of wParam. Specifies whether the drop-down list box is to be shown or hidden. A value of TRUE shows the list box. A value of FALSE hides the list box.
The return value is always TRUE.
This message has no effect on a combo box created with the CBS_SIMPLE style.
CB_GETDROPPEDSTATE