PRB: Assertion Failed, WINCORE.CPP--Line 129, 133, 182, or 307 (113421)

SYMPTOMS

Opening a dialog box in an application that subclasses a control of the dialog box using CWnd::SubclassDlgItem() or CWnd::SubclassWindow() causes an "Assertion Failed!" error in WINCORE.CPP. The line number reported depends on the version of the Microsoft Foundation Classes (MFC) being used:

    Version 2.0 for Windows    - Line 133
    Version 2.5 for Windows    - Line 182
    Version 2.0 32-bit Edition - Line 129
    Version 3.0 32-bit Edition - Line 283
    Version 3.1 32-bit Edition - Line 315
    Version 4.0 32-bit Edition - Line 307

CAUSE

The second ASSERT (below) in WINCORE.CPP is the one being hit:

   BOOL CWnd::Attach(HWND hWndNew)
   {
      ASSERT(m_hWnd == NULL);   // Only attach once, detach on destroy
      ASSERT(FromHandlePermanent(hWndNew) == NULL);
         // Must not be already in permanent map
   ...

This implies that the control that we are attempting to subclass has already been subclassed.

With MFC dialog boxes, you can specify DDX/DDV member variables to associate with the controls of an MFC dialog class. These member variables can be real values (for example, int, CString) or control variables (for example, CEdit, CListBox). If the member variables are control variables, then the DDX/DDV handlers actually do subclass the dialog box controls using the control member variables. If you try to subclass those controls yourself, the above error can result.

RESOLUTION

If you want to subclass controls yourself, then do not associate DDX/DDV variables with those controls.

Alternatively, if you want to subclass controls but avoid the work, you can let the DDX/DDV routines work for you. To do this, use the following steps.

Create a dialog box template in App Studio.
Use ClassWizard to associate a C++ CDialog derived class with the template.
In ClassWizard, with the dialog class selected, choose Edit Variables and add a member variable of the appropriate control type, associating it with the control you want to subclass. With Visual C++ 1.5, choose the Member Variables tab and select Add Variable to accomplish this.
Edit the CDialog derived class definition to change the "// Dialog Data" section slightly. Change the class type for the member variable from the default (for example CEdit), to your own class derived from that control class (for example, CMyEdit), as follows:
```
      // Dialog Data
         //{{AFX_DATA(CTestDlg)
         enum { IDD = IDD_TEST_DLG };
         CMyEdit m_EditControl;
         //}}AFX_DATA
						
```
This will cause the control to be automatically subclassed with your own type of window.