An application sends a CB_GETCURSEL message to retrieve the index of the currently selected item, if any, in the list box of a combo box.
CB_GETCURSEL wParam = 0; // not used; must be zero lParam = 0; // not used; must be zero
This message has no parameters.
The return value is the zero-based index of the currently selected item. If no item is selected, it is CB_ERR.