int GetCurSel() const;
Retrieves the zero-based index of the currently selected item, if any, in a single-selection list box.
GetCurSel should not be called for a multiple-selection list box.
The zero-based index of the currently selected item. It is LB_ERR if no item is currently selected or if the list box is a multiple-selection list box.
LB_GETCURSEL, CListBox::SetCurSel