{"id":26783,"date":"2007-05-21T10:00:00","date_gmt":"2007-05-21T10:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2007\/05\/21\/dont-be-helpless-i-dont-know-anything-about-mfc-modal-loops-but-unlike-some-people-im-not-afraid-to-find-out\/"},"modified":"2007-05-21T10:00:00","modified_gmt":"2007-05-21T10:00:00","slug":"dont-be-helpless-i-dont-know-anything-about-mfc-modal-loops-but-unlike-some-people-im-not-afraid-to-find-out","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20070521-00\/?p=26783","title":{"rendered":"Don't be helpless: I don't know anything about MFC modal loops, but unlike some people, I'm not afraid to find out"},"content":{"rendered":"

\n\nCommenter Tom Grelinger asks via the Suggestion Box<\/a>:\n<\/p>\n

\nIf I have a modal CDialog that is visible and usable to the user.\nLet’s say I receive an event somewhere else in the program\nand I call DestroyWindow on the modal CDialog from within the event.\nI notice that the OnDestroy is called on the CDialog,\nbut DoModal never exits until a WM_QUIT is posted to the modal’s message pump.\nWhat are the pitfalls to this?\nUnfortunately, there is really no way to avoid this situation.\n<\/p><\/blockquote>\n

\nI’m not sure what the question is, actually.\nThe question as stated is “What are the pitfalls to this?”\nbut he answered that in his own question:\nThe pitfall is that “DoModal never exits until a WM_QUIT<\/code> is\nposted to the modal dialog’s message pump.”\n<\/p>\n

\nI’m going to assume that the question really is,\n“Why doesn’t destroying the window work?”\nwith the follow-up question,\n“What is the correct way to dismiss a modal dialog?”\n<\/p>\n

\nThe first problem with this question is that it assumes that I know\nwhat a CDialog<\/code> is.\nFrom its name, I’m going to assume that this\nis an MFC class for managing a dialog box.\nBut you don’t even have to know that to answer the first\nreformulated question operating only from Win32 principles:\nDestroyWindow<\/code> is not how you exit a modal dialog.\nYou exit a modal dialog with EndDialog<\/code>.\nThe DestroyWindow<\/code> technique is for modeless<\/i>\ndialogs.\n<\/p>\n

\nBut let’s look at the question another way,\nwhich is my point for today:\nYou have the MFC source code.\nDon’t be afraid to read it.\nEspecially since I don’t use MFC personally;\nI don’t even know the basic principles of application design with MFC.\nI work in straight Win32.\nAs a result,\nI don’t know the answer off the top of my\nhead, but fifteen minutes reading the MFC source code quickly reveals\nthe reason why destroying the window doesn’t work.\n<\/p>\n

\nWatch me as I go and find out the answer.\nIt’s nothing you can’t already do yourself.\n<\/p>\n

\nThe\nCDialog::DoModal<\/code> method\ncalls CWnd::RunModalLoop<\/code> to run the dialog loop.\nIf you look at CWnd::RunModalLoop<\/code>,\nyou can see the conditions under which it will exit the modal loop.\nHere’s the code with irrelevant details deleted.\n(They’re irrelevant because they have nothing to do with how the\nmodal loop exits.)\n<\/p>\n

\nint CWnd::RunModalLoop(DWORD dwFlags)\n{\n    ... preparatory work ...\n    \/\/ acquire and dispatch messages until the modal state is done\n    for (;;)\n    {\n        ... code that doesn't break out of the loop ...\n        \/\/ phase2: pump messages while available\n        do\n        {\n            \/\/ pump message, but quit on WM_QUIT\n            if (!AfxGetThread()->PumpMessage())\n            {\n                AfxPostQuitMessage(0);\n                return -1;\n            }\n            ... other code that doesn't break out of the loop ...\n            if (!ContinueModal())\n                goto ExitModal;\n            ... other code that doesn't break the loop ...\n        }  while (::PeekMessage(pMsg, NULL, NULL, NULL, PM_NOREMOVE))\n    }\nExitModal:\n    m_nFlags &= ~(WF_MODALLOOP|WF_CONTINUEMODAL);\n    return m_nModalResult;\n}\n<\/pre>\n
\nThere are only two ways out of this loop.\nThe first is the receipt of a WM_QUIT<\/code> message.\nThe second is if CWnd::ContinueModal<\/code> decides that\nthe modal loop is finished.\nThe commenter already mentioned the quit message aspect to the\nmodal loop, so that just leaves CWnd::ContinueModal<\/code>.\n<\/p>\n
\nThe CWnd::ContinueModal<\/code> method is very simple:\n<\/p>\n
\nBOOL CWnd::ContinueModal()\n{\n    return m_nFlags & WF_CONTINUEMODAL;\n}\n<\/pre>\n
\nTherefore, the only other way the loop can exit is if somebody\nclears the WF_CONTINUEMODAL<\/code> flag.\nA little grepping shows that there are only three places where this\nflag is cleared.\nOne is in CPropertyPage<\/code>, which is a derived class\nof CDialog<\/code> and therefore isn’t relevant here.\n(I’ll ignore CPropertyPage<\/code> in future searches.)\nThe second is in the line above right after the label\nExitModal<\/code>.\nAnd the third is this method:\n<\/p>\n
\nvoid CWnd::EndModalLoop(int nResult)\n{\n    \/\/ this result will be returned from CWnd::RunModalLoop\n    m_nModalResult = nResult;\n    \/\/ make sure a message goes through to exit the modal loop\n    if (m_nFlags & WF_CONTINUEMODAL)\n    {\n        m_nFlags &= ~WF_CONTINUEMODAL;\n        PostMessage(WM_NULL);\n    }\n}\n<\/pre>\n
\nThis method is called in only one place:\n<\/p>\n
\nvoid CDialog::EndDialog(int nResult)\n{\n    if (m_nFlags & (WF_MODALLOOP|WF_CONTINUEMODAL))\n        EndModalLoop(nResult);\n    ::EndDialog(m_hWnd, nResult);\n}\n<\/pre>\n
\nFollowing the money one last step,\nthe CDialog::EndDialog<\/code> method is called\nfrom four places in CDialog<\/code>.\nIt’s called from CDialog::HandleInitDialog<\/code> and\nCDialog::InitDialog<\/cODE> if some catastrophic error\noccurs during dialog initialization.\nAnd it's called from CDialog::OnOK<\/code>\nand CDialog::OnCancel<\/code> in response to the\nuser clicking the OK or Cancel buttons.\n<\/p>\n
\nNotice that the CDialog::EndDialog<\/code> method is not\ncalled when somebody forcibly destroys the dialog from\nthe outside.\n<\/p>\n
\nThat's why destroying the dialog window doesn't break the modal loop.\nIf you want to break out of the modal loop, your only choices are\nto post a quit message or call CWnd::EndModalLoop<\/code>,\neither directly or indirectly (via CDialog::EndDialog<\/code>,\nfor example).\n<\/p>\n
\nNotice that the MFC modal loop obeys the convention on quit messages\nby re-posting the quit message when it breaks out of the modal loop.\n(Though it really should have posted the wParam<\/code> from\nthe quit message rather than just posting zero.)\n<\/p>\n
\nThe workaround therefore is not to destroy the dialog with\nDestroyWindow<\/code> (something you should have known\nnot to do a priori<\/i> since that's not how you exit\nmodal dialog boxes) but rather by calling\nCDialog::EndDialog<\/code>, passing a result code that\nlets the caller of CDialog::DoModal<\/code> know that\nthe dialog box exited under unusual circumstances.\n<\/p>\n
\nThis took me fifteen minutes to research and a little over an hour to\nwrite up.\nAll this work to answer a question that you should have been able\nto answer yourself with a little elbow grease.\nYou're a smart person.\nHave confidence in yourself.\nYou can do it.\nI know you can.<\/p>\n","protected":false},"excerpt":{"rendered":"
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, […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-26783","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
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, […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/26783","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=26783"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/26783\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=26783"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=26783"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=26783"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}