An application sends a CB_GETITEMDATA message to a combo box to retrieve the application-supplied 32-bit value associated with the specified item in the combo box.
CB_GETITEMDATA wParam = (WPARAM) index; // item index lParam = 0; // not used; must be zero
The return value is the 32-bit value associated with the item. If an error occurs, it is CB_ERR.
If the item is in an owner-drawn combo box created without the CBS_HASSTRINGS style, the return value is the 32-bit value contained in the lParam parameter of the CB_ADDSTRING or CB_INSERTSTRING message that added the item to the combo box. If the CBS_HASSTRINGS style was not used, the return value is the lParam parameter contained in a CB_SETITEMDATA message.