Re: MessageBox hangs app. AfxMessageBox

JC <>
Tue, 13 Nov 2007 18:36:09 GMT
Hi Joe,
Thanks for the response and all the help.

You mentioned that popping up message boxes in the constructor is dicey. I
am not trying to do that. I am waiting until all the pages are created and
then putting up a message when the OK button is pressed on the
PropertyPage. However, I appreciate the info becuase I really did NOT know
to steer clear of that.

Thanks for all the other info as well. I have fixed all of the points you
mentioned in the previous responses. I did not bother to post the new code
here but I did make the fixes.

By the way, the fix that RainMan offered up worked. I can now put up a
dialog from the PropertyPage. It needed the extended style

Thanks for killer website as well chock full of good info.


Joseph M. Newcomer <> wrote in

Subject: Re: MessageBox hangs app. AfxMessageBox
From: Joseph M. Newcomer <>

For message boxes, popping them up in constructors is VERY dangerous.
This is because a constructor could be called before there is an HWND
object associated with the CWnd object, which can lead to all kinds of
problems. Never pop up message boxes in constructors.

The place to handle them is in OnInitDialog, and even then it is
questionable; if I have an initialization error, I will PostMessage a
notification to the dialog and abort initialization; when I process
the message, I will terminate the dialog. I try to avoid any
situation that would actually result in needing to do this because
dialogs should not spontaneously terminate themselves, they should
only be terminated by the user (special exception for timed-out
dialogs, but that's a rare and exotic situation).

When creating a modal dialog from any CWnd derived class, the parent
of the modal dialog (which includes MessageBoxes) should be the class
issuing the dialog. Otherwise, the world can become very confused.
I've never needed to do any kind of reflection or any special handling
to deal with message boxes from property pages (although I don't do
wizards, just tabbed property sheets), but the anomaly I see here is
that you are going to extra effort to do something very suspicious, so
that's the first thing I'd eliminate.

It is possible to really tricky things, but the more conventional you
try to be, the fewer problems you are likely to have. A lot of MFC
makes assumptions about how the software is designed, and you tread
outside those bounds with great caution, and generally expect to have
weird experiences if you do.

One of the most common errors I see is that someone sees a parameter
specification that says
          SomeAPI(SOMETHING * thing);
and immdialely arrives at the conclusion that the parameter must be a
VARIABLE of type SOMETHING*. This is not true; what it means is the
VALUE must be of type SOMETHING*, so if you declare
          SOMETHING t;
then the call
creates an EXPRESSION whose value is SOMETHING*. Note that no
allocation or deallocation via new/delete is required. I actually
have a sequence of slides in my Systems Programming course which shows
the cases
     SOMETHING * t;
[fails because t is uninitialized]
[silly, and note the delete is missing!]
     ... use t
     delete t;
[works, but gratuitously complex and solves a problem which does not
need to be solved, and requires that all paths after the new SOMETHING
must effect a delete t or there will be a storage leak, so the code is
not robust]
     SOMETHING * t = &s;
[works, but why introduce an unnecessary variable?]

Remember that the parameter specifications state SOLELY the TYPE of
the parameter, and have nothing to do with whether it is a variable!


On Sat, 10 Nov 2007 17:14:21 GMT, JC <>

[posted and mailed]

Joseph M. Newcomer <> wrote in

On Fri, 09 Nov 2007 20:35:10 GMT, JC <>

Subject: Message box hangs application
From: JC <>
Newsgroups: microsoft.public.vstudio.development

- Apologies for posting in 2 groups separately.

I also posted this on microsoft.public.vstudio.development

Will properly cross-post next time. I don't use newsgroups a lot and
am just getting the hang of this Xnews reader.

- Apologies for posting in 2 groups separately.


I am an embedded programmer that is new to Windows programming. So,
that makes me a newbie to Windows (other than a little maintenance
here and there).

I have created a simple application in VS2005. It is a dialog app
(not SDI or MDI). In the application code a CDialog is created; this
was put there by VS2005 code generation.

In the application code a CDialog is created (as mentioned
previously). Then I added a property sheet from the CDialog class.
Then I added property pages from the CPropertySheet class.
When I try to put up an AfxMessageBox or MessageBox, the app. hangs.

I set all dialogs and sheets as "'child" in the dialog editor.

Any ideas? Any help would be greatly appreciated.


Here are the code excerpts.

BOOL CTestCaseApp::InitInstance()
   ... Other wizard generated code...

   CTestCaseDlg dlg;
   m_pMainWnd = &dlg;
   INT_PTR nResponse = dlg.DoModal();
   return FALSE;

BOOL CTestCaseDlg::OnInitDialog()
   CDialog::OnInitDialog(); // wizard generated

   ... Other wizard generated code...

   m_pPSheetCase = new CPSheetCase(m_sWinTitle, (CWnd*)this, 0);
   m_pPSheetCase->Create((CWnd*)this, dwStyle);

There is no reason to cast this to a CWnd*; get rid of the cast.

OK! Got it.


   m_pbIDOK = (CButton*)GetDlgItem(IDOK);

DO NOT USE GetDlgItem!!!!! This code is potentially wrong,
because GetDlgItem creates a temporary object and the value must not
be saved across message pump invocations. But when you write
GetDlgItem, the very first thought you should have is "I am making a
serious mistake".
 After some years of MFC programming, you can use it if you know
exactly what you are doing, but you never, ever want to save it in
other than a local variable. I write two to four GetDlgItem calls
per year. You have nearly written more than that in one subroutine.

Right click on the control and do "Add Variable" to get a control
variable, and then you don't need to use ->; you would just write

I should have put a little more disclaimer in my original post. I have
looked at your site occasionally for over a year even though I didn't
really do any Windows programming except for a little maintenance on
some apps. that violate most of the discipline you explain. I did
actually intend to remove that GetDlgItem(). I read your article on
that a long time ago and am a Newcomer convert. By the way, one of the
apps that I had to maintain calls GetDlgItem for EVERYTHING. Out of
maybe 100 controls, not one has a variable created for it. Of course,
I am not alotted the time to fix it and as a newbie, I may break more
than I am able to fix!


I looked at the documentation of EnableWindow, and it clearly states
the parameter is BOOL. Why are you using an integer value here?
FALSE would work, but 0 makes no sense. Never use an integer for a
BOOL argument.

OK! Got it.


I looked at the documentation of ShowWindow, and not one of the ten
documented values you can use for the argument is 0. I have no idea
what 0 means. You might use SW_SHOW, or SW_HIDE, or any of the
other values, but 0 is nothing but meaningless noise. What possible
meaning could this have? I don't know what this will do. And I
shouldn't have to. Use only documented values. 0 is not a
documented value and must not be used.

Note that I would never enable or disable any control in any dialog
in the OnInitDialog. I establish the state and call a single
updateControls() handler whose responsibility is to
enable/disable/show/hide EVERY control that has changeable state.
Controls are never enabled or disabled anywhere else. Otherwise you
end up with an unmanageable mess. The last call in my OnInitDialog,
right before the return TRUE, is a call on updateControls().

Very good advice. MUCH APPRECIATED! It seems that I do remember that
you have a "tips" article about this, I will go check.

   By default, from the VS project wizard, the "Close button" which
   is the 'X' in the upper-right corner of the dialog does not work
   with the IDCANCEL button disabled. To fix it, WM_ON_CLOSE() was
   added to the message map for this file. Also, the overridden
   funcion OnClose() had to be created. Now the 'X' calls OnClose()
   in this cpp file. JAC
   m_pbIDCANCEL = (CButton*)GetDlgItem(IDCANCEL);


Ditto back

   m_pbID_HELP = (CButton*)GetDlgItem(ID_HELP);


Ditto back

   return TRUE; // return TRUE unless you set the focus to a

CPSheetCase::CPSheetCase(LPCTSTR pszCaption, CWnd* pParentWnd, UINT
   :CPropertySheet(pszCaption, pParentWnd, iSelectPage)
   m_pOsInfo = new (OSVERSIONINFO);
   m_pOsInfo->dwOSVersionInfoSize = sizeof(OSVERSIONINFO);

Why are you allocating a new object and storing it in a pointer to
an object? If you want an OSVERSIONINFO, you would declare a member
There is no justification for using a pointer variable or allocating
something using new. Another good rule in C++ programming is the
reallization that if you write 'new' you had better have a very good
reason for writing it, and in this case you do not.

     m_pOsInfo.dwOSVersionInfoSize = sizeof(OSVERSIONINFO);

I cut and pasted this from one of those other apps that I need to
maintain. I saw that the other application checked for OS level so I
shouls too. I will fix it. Just because I need to check OS level
doesn't mean I need to do it incorrectly!

   //Check if XP Operating System
   bIsWindowsXPorLater = ((m_pOsInfo->dwMajorVersion >5 ) ||
    ( (m_pOsInfo->dwMajorVersion ==5 ) &&
      (m_pOsInfo->dwMinorVersion >=1 ) ) );

   m_pPPage1 = NULL;

Why are you assigning NULL to a variable that the next statement
assigns to? Delete these assignments of NULL!

My thought here was just setting up a convention to insure invalid
pointers were NULL. I may not always be setting the pionter to its
object right after I set it to NULL. I thought maybe I will have to
move the "new CPropertyPage" somewhere else in code eventually and the
constructore will just guarantee they are at a known state. But, your
are right. They are currently not needed. I will remove them.

   m_pPPage1 = new CPPageCase1;

   m_pPPage2 = NULL;
   m_pPPage2 = new CPPageCase2;

   m_pPPage3 = NULL;
   m_pPPage3 = new CPPageCase3;

   m_pPPage4 = NULL;
   m_pPPage4 = new CPPageCase4;

void CPPageCase1::OnBnClickedPpage()
   CPropertySheet* pParent;
   pParent = static_cast<CPropertySheet*>(GetParent());
// pParent->GetParent()->MessageBox("Shit");
// pParent->MessageBox("Msg1");
// pParent = ::GetParent((HWND)this);

OK, what is going on here? This looks like it responds to a button
clicked on property sheet 1. If you want to pop up a MessageBox,
why are you not doing AfxMessageBox? The parent of the message box
should be the dialog which is the property page. I see no reason
the parent should be the property sheet. I have no idea what the
consequences would be, but overall, if you are seeing bad behavior
from a MessageBox, the first thing to do is make sure that the
MessageBox call is correct. If a property page wants to pop up a
message box, then the property page should be the parent; this has
to do with how modal dialogs disable windows associated with them.

Generally, it is considered Best Practice to use AfxMessageBox
rather than CWnd::MessageBox. This is because the standards are
that a MessageBox should have the app name. There is nothing more
dismaying than to come to a computer and see a MessageBox that says
    | Error |
    | You have a ... |
                | [OK] |

It doesn't matter what the text of the message is; WHICH OF MY 35
RUNNING APPS ISSUED IT? This is why the app name must be in the
caption, and AfxMessageBox takes care of that.

Go browse my MVP Tips site. In particular, the essays such as
"Avoding GetDlgItem", "Avoiding UpdateData", "Dialog control
management" and "The n habits of highly defective Windows programs"
are recommended.


Yeah, I was just trying to respond to the OK button on a ProprtySheet
and put up a message box. Just trying stuff and learning. I did
originally try AfxMessageBox. You will see that it is currently
commented out as I was grasping at straws and just trying all kinds of
crap to see if I could it to work. I tried getting a pointer to the
PropertySheet and having the sheet put it up, that didn't work. I
tried MessageBox and AfxMessageBox. They all cause a hang. I can call
AfxMessageBox from the PropertySheet in the constructor and it works.
I just can't get the thing to work from the PropertyPage. A coworker
mentioned that I may have to put "Reflect()" in to get the message to
propogate correctly. **==**==**




Joseph M. Newcomer [MVP]
MVP Tips:

Thanks for all the support to a newbie on this.
I am not sure what proper newsgroup etiquette is. Do I respond to
all individually or address all in one response. I am going to
respond to all individually for now.

Also, Xnews give the options:
"Followup to newsgroup"
"Reply by Mail"
"Follwup and Mail Reply"

I will use "Followup and Mail Reply" to see how that goes. Please
let me know if that is incorrect.
I got the same Xnews error as the others "10049 Address Not Found" so
I just replied to the newgroup.

Joseph M. Newcomer [MVP]
MVP Tips:


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