This example sends an MM_SETITEMATTR message to set the IDM_LARGE menu item's state to checked, and then sends another MM_SETITEMATTR message to set the IDM_MEDIUM menu item's state to unchecked.

WinSendMsg(hwndActionBar, MM_SETITEMATTR,
    MPFROM2SHORT(IDM_LARGE, TRUE),
    MPFROM2SHORT(MIA_CHECKED, MIA_CHECKED));
WinSendMsg(hwndActionBar, MM_SETITEMATTR,
    MPFROM2SHORT(IDM_MEDIUM, TRUE),
    MPFROM2SHORT(MIA_CHECKED, FALSE));