An application sends an LB_GETITEMDATA message to retrieve the application-defined 32-bit value associated with the specified list box item.
LB_GETITEMDATA wParam = (WPARAM) index; // item index lParam = 0; // not used; must be zero
Windows 95: The wParam parameter is limited to 16-bit values. This means list boxes cannot contain more than 32,767 items. Although the number of items is restricted, the total size in bytes of the items in a listbox is limited only by available memory.
The return value is the 32-bit value associated with the item, or LB_ERR if an error occurs. If the item is in an owner-drawn list box and was created without the LBS_HASSTRINGS style, this 32-bit value was in the lParam parameter of the LB_ADDSTRING or LB_INSERTSTRING message that added the item to the list box. Otherwise, it is the value in the lParam of an LB_SETITEMDATA message.