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