This example responds to a select menu message (WM_MENUSELECT) by querying (via WinIsMenuItemChecked) the check attribute and then setting the check state of the menu item that was selected.

#define INCL_WINWINDOWMGR       /* Window Manager Functions     */
#include <os2.h>

USHORT  usItemId;       /* menu item id                         */
HWND    hwndMenu;       /* menu handle                          */
BOOL    usChkstate;     /* new checked state                    */
BOOL    fSuccess;       /* success indicator                    */
MPARAM  mp1;            /* Parameter 1 (menu item id)           */
MPARAM  mp2;            /* Parameter 2 (menu handle)            */

case WM_MENUSELECT:
     usItemId = SHORT1FROMMP(mp1);
     hwndMenu = HWNDFROMMP(mp2);

     /* query current check state */
     usChkstate = WinIsMenuItemChecked(hwndMenu, usItemId);

     /* set menu item check state */
     fSuccess = WinCheckMenuItem(hwndMenu, usItemId, ! usChkstate);