How To Filter Keystrokes in Controls Derived from CEdit Class (92394)

The Microsoft Foundation Classes (MFC), when used with:

Microsoft C/C++ for MS-DOS 7.0

Microsoft Visual C++ for Windows, 16-bit edition 1.0

Microsoft Visual C++ for Windows, 16-bit edition 1.5

Microsoft Visual C++ for Windows, 16-bit edition 1.51

Microsoft Visual C++ for Windows, 16-bit edition 1.52

Microsoft Visual C++, 32-bit Editions 1.0

Microsoft Visual C++, 32-bit Editions 2.0

Microsoft Visual C++, 32-bit Editions 2.1

Microsoft Visual C++, 32-bit Editions 2.2

Microsoft Visual C++, 32-bit Editions 4.0

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));
   }

Modification Type:

Minor

Last Reviewed:

7/1/2004

Keywords:

kbArchitecture kbCtrl kbhowto kbKeyIn KbUIDesign KB92394