CB_GETITEMDATA
wParam = (WPARAM) index; /* item index */
lParam = 0L; /* not used, must be zero */
An application sends a CB_GETITEMDATA message to a combo box to retrieve the application-supplied doubleword value associated with the specified item in the combo box. (This is the value in the lParam parameter of a CB_SETITEMDATA message.)
index
Value of wParam. Specifies the zero-based index of the item.
The return value is the doubleword value associated with the item, or it is CB_ERR if an error occurs.