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Microsoft KB Archive/92394

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Knowledge Base


How To Filter Keystrokes in Controls Derived from CEdit Class

Article ID: 92394

Article Last Modified on 11/21/2006



APPLIES TO

  • Microsoft Foundation Class Library 4.2, when used with:
    • Microsoft C/C++ Professional Development System 7.0
    • Microsoft Visual C++ 1.0 Professional Edition
    • Microsoft Visual C++ 1.5 Professional Edition
    • Microsoft Visual C++ 1.51
    • Microsoft Visual C++ 1.52 Professional Edition
    • Microsoft Visual C++ 1.0 Professional Edition
    • Microsoft Visual C++ 2.0 Professional Edition
    • Microsoft Visual C++ 2.1
    • Microsoft Visual C++ 2.2
    • Microsoft Visual C++ 4.0 Standard Edition



This article was previously published under Q92394


NOTE: Microsoft Visual C++ NET (2002) supported both the managed code model that is provided by the .NET Framework and the unmanaged native Windows code model. The information in this article applies to unmanaged Visual C++ code only.

SUMMARY

Using the Microsoft Foundation Classes (MFC), if an application derives an edit control of the CEdit class, it can capture messages for that control, process the messages, and then pass them to the Default member function for default processing, if desired.

MORE INFORMATION

One of the messages a control receives is the WM_CHAR message, which is processed by the OnChar member function. By default, OnChar calls the Default member function.

Attempting to change the character passed to an edit control using the following technique can create problems:

   afx_msg void CMyEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags)
   {
      if (nChar == 'a' || nChar == 'A')
         nChar = 'X';

      CEdit::OnChar(nChar, nRepCnt, nFlags);
   }
                

NOTE: This code attempts to change every "A" or "a" character into an "X" character by changing the wChar value before calling the base class CEdit::OnChar function. However, CEdit::OnChar calls the Default member function inherited from CWnd. Default uses the original wParam sent to the edit control with the WM_CHAR message and not the wChar value passed to the CEdit::OnChar member function.

To perform this type of processing, the application must directly call the DefWindowProc member function. The following code demonstrates this technique:

   /*
      Assume that CMyEdit is derived from the CEdit class.
   */ 

   afx_msg void CMyEdit::OnChar(UINT nChar, UINT nRepCnt, UINT nFlags)
   {
      if (nChar == 'a' || nChar == 'A')
         nChar = 'X';
      DefWindowProc(WM_CHAR, nChar, MAKELONG(nRepCnt, nFlags));
   }
                

Keywords: kbhowto kbkeyin kbuidesign kbarchitecture kbctrl KB92394