An application sends an LB_SETANCHORINDEX message to set the anchor item¾that is, the item from which a multiple selection starts. A multiple selection spans all items from the anchor item to the caret item.
LB_SETANCHORINDEX wParam = (WPARAM) index; // index to set as anchor 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.
If the message succeeds, the return value is zero.
If the message fails, the return value is LB_ERR.