Don’t be helpless: I don’t know anything about MFC modal loops, but unlike some people, I’m not afraid to find out

Commenter Tom Grelinger asks via the Suggestion Box:

If I have a modal CDialog that is visible and usable to the user. Let's say I receive an event somewhere else in the program and I call DestroyWindow on the modal CDialog from within the event. I notice that the OnDestroy is called on the CDialog, but DoModal never exits until a WM_QUIT is posted to the modal's message pump. What are the pitfalls to this? Unfortunately, there is really no way to avoid this situation.

I'm not sure what the question is, actually. The question as stated is "What are the pitfalls to this?" but he answered that in his own question: The pitfall is that "DoModal never exits until a WM_QUIT is posted to the modal dialog's message pump."

I'm going to assume that the question really is, "Why doesn't destroying the window work?" with the follow-up question, "What is the correct way to dismiss a modal dialog?"

The first problem with this question is that it assumes that I know what a CDialog is. From its name, I'm going to assume that this is an MFC class for managing a dialog box. But you don't even have to know that to answer the first reformulated question operating only from Win32 principles: DestroyWindow is not how you exit a modal dialog. You exit a modal dialog with EndDialog. The DestroyWindow technique is for modeless dialogs.

But let's look at the question another way, which is my point for today: You have the MFC source code. Don't be afraid to read it. Especially since I don't use MFC personally; I don't even know the basic principles of application design with MFC. I work in straight Win32. As a result, I don't know the answer off the top of my head, but fifteen minutes reading the MFC source code quickly reveals the reason why destroying the window doesn't work.

Watch me as I go and find out the answer. It's nothing you can't already do yourself.

The CDialog::DoModal method calls CWnd::RunModalLoop to run the dialog loop. If you look at CWnd::RunModalLoop, you can see the conditions under which it will exit the modal loop. Here's the code with irrelevant details deleted. (They're irrelevant because they have nothing to do with how the modal loop exits.)

int CWnd::RunModalLoop(DWORD dwFlags)
    ... preparatory work ...

    // acquire and dispatch messages until the modal state is done
    for (;;)
        ... code that doesn't break out of the loop ...

        // phase2: pump messages while available
            // pump message, but quit on WM_QUIT
            if (!AfxGetThread()->PumpMessage())
                return -1;

            ... other code that doesn't break out of the loop ...

            if (!ContinueModal())
                goto ExitModal;

            ... other code that doesn't break the loop ...
        }  while (::PeekMessage(pMsg, NULL, NULL, NULL, PM_NOREMOVE))

    return m_nModalResult;

There are only two ways out of this loop. The first is the receipt of a WM_QUIT message. The second is if CWnd::ContinueModal decides that the modal loop is finished. The commenter already mentioned the quit message aspect to the modal loop, so that just leaves CWnd::ContinueModal.

The CWnd::ContinueModal method is very simple:

BOOL CWnd::ContinueModal()
    return m_nFlags & WF_CONTINUEMODAL;

Therefore, the only other way the loop can exit is if somebody clears the WF_CONTINUEMODAL flag. A little grepping shows that there are only three places where this flag is cleared. One is in CPropertyPage, which is a derived class of CDialog and therefore isn't relevant here. (I'll ignore CPropertyPage in future searches.) The second is in the line above right after the label ExitModal. And the third is this method:

void CWnd::EndModalLoop(int nResult)
    // this result will be returned from CWnd::RunModalLoop
    m_nModalResult = nResult;

    // make sure a message goes through to exit the modal loop
    if (m_nFlags & WF_CONTINUEMODAL)
        m_nFlags &= ~WF_CONTINUEMODAL;

This method is called in only one place:

void CDialog::EndDialog(int nResult)

    ::EndDialog(m_hWnd, nResult);

Following the money one last step, the CDialog::EndDialog method is called from four places in CDialog. It's called from CDialog::HandleInitDialog and CDialog::InitDialog if some catastrophic error occurs during dialog initialization. And it's called from CDialog::OnOK and CDialog::OnCancel in response to the user clicking the OK or Cancel buttons.

Notice that the CDialog::EndDialog method is not called when somebody forcibly destroys the dialog from the outside.

That's why destroying the dialog window doesn't break the modal loop. If you want to break out of the modal loop, your only choices are to post a quit message or call CWnd::EndModalLoop, either directly or indirectly (via CDialog::EndDialog, for example).

Notice that the MFC modal loop obeys the convention on quit messages by re-posting the quit message when it breaks out of the modal loop. (Though it really should have posted the wParam from the quit message rather than just posting zero.)

The workaround therefore is not to destroy the dialog with DestroyWindow (something you should have known not to do a priori since that's not how you exit modal dialog boxes) but rather by calling CDialog::EndDialog, passing a result code that lets the caller of CDialog::DoModal know that the dialog box exited under unusual circumstances.

This took me fifteen minutes to research and a little over an hour to write up. All this work to answer a question that you should have been able to answer yourself with a little elbow grease. You're a smart person. Have confidence in yourself. You can do it. I know you can.

Comments (24)
  1. Neal says:

    One might add that the MFC Reference (help) for the CDialog class states, "A modal dialog box closes automatically when the user presses the OK or Cancel buttons or when your code calls the EndDialog member function."

  2. Neal says:

    Oops, point being that reading the help answers the question without the need to dive into the source code.  It not only tells you to call EndDialog but also directs you toward the OnOk and OnCancel member functions with a whack from a pretty strong cluestick.

  3. johnf says:

    Your story beautifully illustrates the need for Microsoft to release the .NET source to Windows Forms and the BCL.

    Makes debugging and troubleshooting a ton more productive.

    [I thought it illustrated the opposite. Even with the source code, this person couldn’t be bothered. -Raymond]
  4. El Guapo says:

    I thought you beautifully illustrated the point that we don’t need to do research when other people are willing to do it for us, and even blog about it.

  5. Greg Beech says:

    It illustrates the point well – so many people just act helpless and immediately want help without even doing the basics to research the problem. People seem to think I have a psychic ability to solve problems and find out the cause of error messages, when in fact a lot of the time I simply have a non-psychic ability to copy and paste the error message into google then read the top few most likely looking results.

  6. Josh says:

    Ok, this is totally unrelated to the topic at hand, but I was wondering: Why "for (;;)" instead of "while (true)"? Because it’s less characters to type? Optimizes differently? Personal preference?

    I see it a fair bit in the code referenced here, and google isn’t exactly helpful in this arena (all the search engines I tried appear to strip out non-alphanumeric text)

  7. Stanlo says:

    while(true) is a constant expression. How is it ever supposed to get out???????? It is a mystery (to the compiler).

    You get a warning sometimes, while for(;;) will not give a warning.

  8. Rob says:

    > so many people just act helpless

    I, for one, just acted helpless back  in the day. I also often  "cargo-culted" (something didn’t work for some bug I didn’t understood, so I don’t use that something anymore because I believe it’s something which simply doesn’t work or things like that).

    What changed me was a sum of experience and gained self-esteem.

    Help from colleagues and boss was a key factor in this, between other factors.

  9. Alex says:

    > Why "for (;;)" instead of "while (true)" ?


    because what you get with

    #define ever_and_ever (;;)

    is better than what you’d get with

    #define the_hell_is_not_cold (true)

    even if the latter is correct on a logical point of view :)

  10. Rick C says:

    <i>while(true) is a constant expression. How is it ever supposed to get out????????</i>

    break or return will do the job admirably.

  11. Mike Dimmick says:

    johnf: Reflector gives you Windows Forms (the Microsoft-supplied .NET Framework DLLs are _not_ obfuscated), while for the BCL and the runtime, there’s the Shared Source CLI (Rotor). You can use Reflector on the BCL as well, of course, but SSCLI has the original comments in.

    You cannot of course copy that code yourself, but you can understand what it’s doing.

    I’m pretty sure the subject of this article is dealt with in every MFC text *ever*. If you’re not comfortable just grepping (or other tool of your choice) the MFC source, there’s a book called "MFC Internals" by Scot Wingo and George Shepherd which will guide you through it. It’s ‘out of date’ in that it targets MFC 4.0, but the architecture hasn’t actually changed all that much in 10 years.

    MFC’s dialogs are actually a little odd in that an MFC ‘modal’ dialog is actually a modeless Win32 dialog. There’s some very good reason for this which I’ve forgotten (and I can’t be bothered to dig out the book I mentioned above to look it up).

  12. Merus says:

    What would be really poetic is if the guy who submitted the question turned out to be a nitpicker.

  13. Norman Diamond says:

    The CDialog::DoModal method calls

    CWnd::RunModalLoop to run the dialog loop. If

    you look at CWnd::RunModalLoop, you can see

    the conditions under which it will exit the

    modal loop.

    Nish half-agrees.  CDialog::DoModal calls CWnd::RunModalLoop to run the dialog loop, and there are conditions under which it will exit this NON-MODAL dialog loop.

    I haven’t done enough testing, but have the impression that the non-modality of MFC pseudo-modal dialogs is the reason why WM_CLOSE can be received by an MFC application where a corresponding plain Win32 application never gets the WM_CLOSE (turning the Win32 app into a "not responding" app).

    Even with the source code, this person

    couldn’t be bothered.

    Maybe that, or maybe he didn’t know that there’s an option to install it during Visual Studio installation.  Meanwhile there are other people who well could be bothered, who have found some parts of the source readable, who agree with johnf’s opinion.

  14. name not required says:

    "I thought it illustrated the opposite. Even with the source code, this person couldn’t be bothered"

    ‘This person’ is a person who could not be bothered. Therefore, since they are persons, no other persons can be bothered either.

  15. AndyB says:

    “I thought it illustrated the opposite. Even with the source code, this person couldn’t be bothered”

    Yes, but that’s one lazy person. If MS hadn’t released the source for MFC, then you’d be getting a lot more questions like this from people who otherwise would have looked for themselves.

    I don’t know if this blog entry is about people who cannot be bothered to help themselves, or about the fact that the information is there so you can help yourself.

    [It’s about people who can’t be bothered. -Raymond]
  16. Dean Harding says:

    The information is there – have you not tried Reflector?

  17. Zooba says:

    Nice to see a real life example of where [url=";]goto is better than break or continue[/url].

    Source code to Windows Forms would be nice, if only so Go To Definition went somewhere interesting. The documentation tends to be good enough for .NET while the source for MFC is basically required to solve many issues. I prefer plain old Win32 API calls to MFC.

  18. Zooba says:

    Huh. So much for BBCode being supported (from the Help page linked at the top)

  19. Bobbie The Programmer says:

    The very first Google result for "Destroy CDialog modal" has this information also.

    Not to slag the person who posted the question, I’ve made similar mistakes and I suspect most of us have.

    But the real point of the article is not "How to use search engines to answer questions" but "How to use source code to answer questions", with the answer, "It’s really not that hard."

  20. AndyAsh says:

    > The documentation tends to be good enough for .NET while the source for MFC is basically required to solve many issues. I prefer plain old Win32 API calls to MFC.

    I agree with you, but I’d have thought that an API where you are called, is always going to be way more complex to program, than one where you do all the calling. It’s all part of the idea of encapsulation, and black vs white boxes.

    Clearly there are practical constraints in what can be achieved if one goes bonkers and says, "I’ll only implement a call based API" or vice versa.

    At the end of the day, you’re a very lucky person, if you never write code that is called in somebody elses context.

    I’ve never used .NET, what is it? :)

    I never liked MFC, because it led me up the path to nowhere so, so, many times. Win32 never, ever, has.

  21. Niki says:

    > The information is there – have you not tried Reflector?

    Part of the information is there, but with all the comments and local variable names gone, it’s a lot harder to understand than it has to be.

    Take the Dialog example above: In MFC, you could just put a breakpoint on DoModal, step into it, and see what it does, in your program, look at the values of all local variables, you can see which branch it takes, see values of local variables and everything. All you have to do is hit F11 a few times. With .NET, this is _a lot_ harder, especially if you have to look into ROTOR.

    BTW: I’d really like to understand why MS *didn’t* release the .NET source code. They’ve released MFC, ATL in the past, so what’s the big deal about .NET? Are they afraid someone might steal their intellectual property? Then why didn’t they obfuscate?

  22. Ulric says:

    My instinct tells me the answer is so simple…. it’s a dialog, send a message that dismisses a dialog the way a user would, like WM_COMMAND and IDOK/IDCANCEL or WM_CLOSE.

    why indeed would DestroyWindow be used to dismiss a modal dialog

  23. Jules says:

    Jumping into source is a last resort for figuring out why something doesn’t work, but I’ll agree it’s a useful approach.   The only way I figured out why my window creation code was crashing and burning when I called CreateWindowEx with an extended style of WS_EX_MDICHILD was by reading the Wine source code, for instance.  I note that the strange behaviour is documented now.  It definitely wasn’t when I was working on this.

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