This message fills a buffer with an array of integers specifying the item numbers of selected items in a multiselection list box.
wParam
Specifies the maximum number of selected items whose item numbers are to be placed in the buffer.
lParam
Contains a long pointer to a buffer large enough for the number of integers specified by the wParam parameter.
The return value is the actual number of items placed in the buffer. If the list box is a single-selection list box, the return value is LB_ERR.