Tip: WM_SYSCOMMAND "gotcha"

[Eric Pearson]

If your program's callback function processes the WM_SYSCOMMAND message, here is an important Windows "quirk" to keep in mind.

Here's a typical block of DDT code, but exactly the same thing applies to API-based windows too...

IF CBMSG = %WM_SYSCOMMAND THEN
    IF CBWPARAM = %SC_CLOSE THEN
        'do something here
    END IF
END IF

The problem is that the last byte of the wParam value (CBWPARAM in DDT programs) is not always predictable when WM_SYSCOMMAND is detected. For example, if you click the Restore button on a dialog's title bar the wParam value will be %SC_RESTORE, or &HF120&. But if you double-click the dialog's title bar, %SC_RESTORE+2 (&HF122&) will be generated by Windows!.

So you always need to do some extra processing of a WM_SYSCOMMAND's wParam...

IF CBMSG = %WMSYSCOMMAND THEN
    IF [b](CBWPARAM AND &hFFF0)[/b] = %SC_CLOSE THEN
        'do something here
    END IF
END IF

For more information about this, see WM_SYSCOMMAND in the Win32.HLP file.

-- Eric


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[This message has been edited by Eric Pearson (edited April 02, 2001).]