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Q140596: INFO: MFC 4.0 No Longer Pre-Registers Window Classes

Article: Q140596
Product(s): Microsoft C Compiler
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): kbcode kbMFC kbVC400 kbWndwClass kbGrpDSMFCATL kbArchitecture kbNoUpdate
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, version 4.0 
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SUMMARY
=======

In previous versions of MFC (prior to MFC 4.0), the framework pre- registered
some Window classes. Existence of these classes allowed applications to use them
to create their own windows. These classes were AfxWnd, AfxFrameOrView,
AfxMDIFrame, and AfxControlBar. The MFC classes shipped with MFC 4.0 no longer
register them, so they are no longer available to be used in an application.

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

In any Windows-based application, every window is created based on a registered
window class. The window class identifies several characteristics of the windows
based on it, including the icon, cursor, background color, and the window
procedure.

In versions prior to and including Visual C++ 2.2, to facilitate creating windows
in an MFC application, the framework pre-registered some classes. It would then
use these classes to create the essential windows in a typical MFC application.
These included the main window, view, and the control bars.

Pre-registration of certain window classes gave rise to several problems like
multiple versions of MFC running in the same process space. As a consequence,
MFC 4.0 does not register them and their usage is illegal unless an application
itself registers window classes with these names. If an MFC 4.0 application goes
ahead and uses any of these classes, the subsequent call to ::CreateWindow fails
and GetLastError returns the message "Cannot find Window class."

To use Window classes that are similar to the ones mentioned here or to port MFC
applications that use these classes to MFC 4.0, an application must register
these classes. Note that even though MFC does not pre- register these classes
any more, it takes care of registering new window classes when it needs them. So
applications that do not use these classes in any custom code do not need to
make any changes.

For example, one typical use of these pre-registered window classes was in
applications that registered their own window class for the main window to help
detect multiple instances of the same application. These applications usually
called ::GetClassInfo first to retrieve the information from the pre-registered
MFC window class before registering a new class under a different name. Now, in
MFC 4.0, ::GetClassInfo will fail to fill out necessary members of the WNDCLASS
structure, and any subsequent attempts to create a window using that window
class will fail.

Technical Note #1, "Window Class Registration." gives the attributes of these
window classes. You can use these attributes to register the necessary window
classes. The following sample code demonstrates how to register a new window
class based on the characteristics of the AfxFrameOrView class:

Sample Code
-----------

  BOOL CYourWinApp::InitInstance()
  {
      // Add the following code to your override of InitInstance.
      // Make sure that this code run before you create any
      // windows that use this new window class

      WNDCLASS wndclass;

      wndclass.style = CS_DBLCLKS | CS_HREDRAW | CS_VREDRAW;
      wndclass.lpfnWndProc = ::DefWindowProc;
      wndclass.cbClsExtra = 0;
      wndclass.cbWndExtra = 0;
      wndclass.hInstance = AfxGetInstanceHandle();
      wndclass.hIcon = LoadIcon(IDR_MAINFRAME);
                       // or load a different icon
      wndclass.hCursor = LoadCursor( IDC_ARROW );
      wndclass.hbrBackground = (HBRUSH) (COLOR_WINDOW + 1);
      wndclass.lpszMenuName = NULL;
      wndclass.lpszClassName = "MyAfxFrameOrView";

      if( !AfxRegisterClass( &wndclass ))
        {
          TRACE("Window class MyAfxFrameOrView could not be
                 registered.\n");
          return FALSE;
        }

  }

Now the MyAfxFrameOrView window class can be used anywhere a need for the
AfxFrameOrView class exists.

REFERENCES
==========

MFC Technical Note #1. For information on attributes of other window classes,
see Technical Note #1. It does not mention the lpfnWndProc member of the
WNDCLASS structure. You should always make it ::DefWindowProc.

Additional query words: AfxWnd AfxFrameOrView AfxMDIFrame AfxControlBar kbusage

======================================================================
Keywords          : kbcode kbMFC kbVC400 kbWndwClass kbGrpDSMFCATL kbArchitecture kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.0
Issue type        : kbinfo

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