An application sends a BM_SETCHECK message to set the check state of a radio button or check box.
Result = SendMessage( // returns LRESULT in lResult (HWND) hWndControl, // handle to destination control (UINT) BM_SETCHECK, // message ID (WPARAM) wParam, // = (WPARAM) () wParam; (LPARAM) lParam // = 0; not used, must be zero );