MFC OLE bug: CoFreeUnusedLibraries & CFileDialog

mzinner@berlin.snafu.de
Tuesday, March 25, 1997


Environment:  Win 95, NT 4.0, VC++ 4.2b

(using ControlWizard and MFC OLE control container support)

I'm writing an MFC OLE control container and came across a bug
which is reproducible with just a few lines of code. It shows up only 
on W95, not on NT. It happens if CoFreeUnusedLibraries() is 
called while a CFileDialog is open.

Here are the steps to get the problem:
(1) using ControlWizard, create a dummy control NULLCTRL which does
nothing. 
(2) using AppWizard, create a SDI application; check 'support
for OLE controls' and 'OLE automation', default settings otherwise
(3) insert a menu command 'createDeleteControl'
(4) insert this menu handler code which will create and destroy a
NULLCTRL:

CMyView::OnCreateDeleteControl()
{
    CLSID clsID = // insert your NULLCTRL's clsid here
     {0xd607c82b, 0xa508, 0x11d0, 0x86, 0xab, 0x44, 0x45, 0x53, 0x54, 0, 0};

    CWnd *wnd = new CWnd;

    if(!wnd->CreateControl(clsID, NULL, WS_VISIBLE, CRect(...), this, 1, NULL)) {
        AfxMessageBox("error creating control");
        // ... handle error
    }
    wnd->DestroyWindow();
    delete wnd;

    // this will fix it - see note below
    // CoFreeUnusedLibraries();
}

(5) start; invoke 'createDeleteControl'; then, invoke 'File/Open' and
keep the dialog open for more than 1 minute, changing directories from time to time.

After less than 60 seconds, the CFileDialog will freeze and the application
hangs.

If I have TRACE output on, I can see a line 
'NULLCTRL: Info: AfxDllCanUnloadNow returns S_OK'
just when the dialog freezes. Obviously, CoFreeUnusedLibraries is
called at this moment.

I can fix this problem by calling CoFreeUnusedLibraries explicitely
after deleting the control (and, therefore, before invoking
CFileDialog).

This thing might not show up too often because CoFreeUnusedLibraries()
will be called on Idle time every 60 seconds (see
AfxOleTermOrFreeLib(), called by CThread::OnIdle())

What is going on? CoFreeUnusedLibraries() seems to free some dll that
the (still visible) CFileDialog needs.

Thanks for any comments,

-manfred

p.s.: on NT, the line 
'NULLCTRL: Info: AfxDllCanUnloadNow returns S_OK'
always shows up after I close CFileDialog.

---

Mike Blaszczak -- mikeblas@nwlink.com
Thursday, March 27, 1997

At 16:25 3/25/97 +0000, mzinner@berlin.snafu.de wrote:
>
>Environment:  Win 95, NT 4.0, VC++ 4.2b
>(using ControlWizard and MFC OLE control container support)
>
>I'm writing an MFC OLE control container and came across a bug
>which is reproducible with just a few lines of code. It shows up only 
>on W95, not on NT. It happens if CoFreeUnusedLibraries() is 
>called while a CFileDialog is open.
>
>Here are the steps to get the problem:
>(1) using ControlWizard, create a dummy control NULLCTRL which does
>nothing. 
>(2) using AppWizard, create a SDI application; check 'support
>for OLE controls' and 'OLE automation', default settings otherwise
>(3) insert a menu command 'createDeleteControl'
>(4) insert this menu handler code which will create and destroy a
>NULLCTRL:

Well, that's an interesting one.

'Course, nothing you've posted indicates that it's really an MFC bug--it
looks to me, in fact, that it is actually far more likely to be a bug
in Windows.

I'll take it into work and poke around with it.


.B ekiM
http://www.nwlink.com/~mikeblas/
These words are my own. I do not speak on behalf of Microsoft.
       One is too many and a million is not enough.

---

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