Q155224: PRB: Using CEdit SetModify/GetModify Returns Invalid Values

Article: Q155224
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1,4.2
Operating System(s): 
Keyword(s): kbEditCtrl kbMFC KbUIDesign kbVC400 kbVC420 kbGrpDSMFCATL
Last Modified: 06-MAY-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 4.2 
   - Microsoft Visual C++, 32-bit Professional Edition, version 4.2 
-------------------------------------------------------------------------------


SYMPTOMS
========

The CEdit::GetModify() and CEdit::SetModify() methods may be unreliable when the
CEdit object is part of CEditView. Further, using these methods on a CEdit
object that is part of a CEditView object may cause CEditView::LockBuffer(),
CEditView::GetBufferLength(), and other CEditView methods to return invalid
values.

CAUSE
=====

Under Windows 95 and Win32s, CEditView uses CEdit::GetModify() and
CEdit::SetModify() to indicate when an internal buffer--which mirrors the
contents of the edit control--is current. As a result, you can experience
unpredictable behavior if the class you derive from CEditView also uses this
flag, for example:

RESOLUTION
==========

The easiest workaround is to ignore the edit control's internal modified flag
and define your own in the class you derived from CEditView:

     class CMyEditView : public CEditView
     {
     public:
        BOOL GetModify() { return m_bModified; }
        void SetModify(BOOL bModified) { m_bModified = bModified; }
        // Additional public members...
     protected:
        BOOL m_bModified;
        // Additional protected members...
     };

Keep this flag current by overriding OnChange() in the class you derived from
CEditView. OnChange() is called any time the contents of the underlying edit
control changes:

     void CMyEditView::OnChange()
     {
        m_bModified = TRUE;

           //Call base class's EN_CHANGE handler
           CEditView::OnEditChange();
     }

MORE INFORMATION
================

CEdit::GetModify() and CEdit::SetModify() access an internal "dirty" flag that
is maintained by the standard Windows edit control. These methods are useful in
determining if the contents of an edit control have been modified since this
flag was cleared.

Due to limitations under Win32s and Windows 95, the MFC classes set an internal
flag, afxData.bWin32s, to TRUE during initialization under either of these
systems. When this flag is TRUE, CEditView uses a modified method to obtain the
contents of the underlying edit control. Instead of reading data from the edit
control each time you need it, CEditView maintains an internal buffer that
mirrors the contents of the edit control. To ensure this buffer is updated when
the contents of the edit control change, CEditView uses CEdit::GetModify() and
CEdit::SetModify(). This can cause serious conflicts if your application also
uses these methods.

The problem is clearly demonstrated in the source code for
CEditView::LockBuffer(). Instead of accessing the control directly each time,
CEditView::LockBuffer() copies the edit control's contents to a local buffer.
Then CEditView::LockBuffer() calls CEdit::SetModify() with an argument of FALSE
to clear the dirty flag. If your program calls CEditView::LockBuffer() or one of
the many CEditView members that call it, the buffer is updated only if
CEdit::GetModify() returns TRUE (which indicates the edit control's contents
have changed and the local buffer is no longer current).

Because CEditView::LockBuffer() clears the edit control's modified flag, your
program no longer relies on this flag. Please note, if your program clears this
flag, CEditView::LockBuffer() returns the contents of the local buffer even if
the contents of the edit control has changed. CEditView::LockBuffer() is also
called by other CEditView members, such as GetBufferLength() and FindText().

Additional query words: 4.00 4.10 4.20 kbdsd W32s Win95

======================================================================
Keywords          : kbEditCtrl kbMFC KbUIDesign kbVC400 kbVC420 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0,4.1,4.2
Issue type        : kbprb

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.