LB_GETSELCOUNT
wParam = 0; /* not used, must be zero */
lParam = 0L; /* not used, must be zero */
An application sends an LB_GETSELCOUNT message to retrieve the total number of selected items in a multiple-selection list box.
This message has no parameters.
The return value is the count of selected items in a list box. The return value is LB_ERR if the list box is a single-selection list box.