MFC, EndDialog, repeated dialog creation causes unexpected behavior - c++

Question

I need to adjust the dialog window dynamically based on its size. To do so I employ the following technique:
I load it up and get its size from the CDialog::OnInitDialog() handler.
If the size is too big, I end the dialog by calling CDialog::EndDialog
And then update global variable and reinit the dialog-derived class again with the size adjustment.
What happens is that on the second pass, some APIs start acting strangely. For instance, MessageBox does not show (thus all ASSERT macros stop working) and some SetWindowText APIs crash the app. Any idea why?
Here're the code snippets:
#define SPECIAL_VALUE -1
//From CWinApp-derived class
BOOL CWinAppDerivedClass::InitInstance()
{
//...
for(;;)
{
CDialogDerivedClass dlg(&nGlobalCounter);
m_pMainWnd = &dlg;
if(dlg.DoModal() != SPECIAL_VALUE)
break;
}
//...
}
And then from the dialog class itself:
//From CDialogDerivedClass
BOOL CDialogDerivedClass::OnInitDialog()
{
//The following API shows message box only on the 1st pass, why?
::MessageBox(NULL, L"1", L"2", MB_OK);
//...
if(checkedDialogSizeIndicatesReload)
{
this->EndDialog(SPECIAL_VALUE);
return FALSE;
}
//Continue loading dialog as usual
...
}
EDIT: I noticed by chance that if I comment out the following line it seems to work. Any idea why?
//m_pMainWnd = &dlg;

Answer 1

Variable dlg is not yet a window at the place where you are setting m_pMainWnd (the dialog box is displayed only after OnInitInstance returns TRUE); the Following code should work:
for(;;)
{
CDialogDerivedClass dlg(&nGlobalCounter);
// m_pMainWnd = &dlg;
if(dlg.DoModal() != SPECIAL_VALUE)
break;
}
m_pMainWnd = &dlg;

Answer 2

InitDialog is the last message processed before the dialog window appears on the screen - you can detect and adjust the size in place and not have the kind of funky global variable thing you are doing.
if(checkedDialogSizeIndicatesReload)
{
// look up SetWindowPos -
// I am nt sure if there is another parameter or not that is optional
int x,y,cx,cy;
WINDOWPLACEMENT wp;
GetWindowPlacement(&wp);
// calc new size here
SetWindowPos(this,x,y,cx,cy);
}
// window appears when the message handler returns