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Q142481: FCSVAL32 Control-by-Control Validation in MFC

Article: Q142481
Product(s): Microsoft C Compiler
Version(s): 4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbcode kbfile kbSample kbDlg kbMFC KbUIDesign kbVC kbVC400 kbVC500 kbVC600 kbGrpDSMFCAT
Last Modified: 02-JUL-2002

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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, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SUMMARY
=======

The FCSVAL32 sample application was created to show how an application can do
control-by-control validation in a dialog box.

The following files are available for download from the Microsoft Download
Center:

Visual C++ 6.0:

  DownloadDownload Fcsval32.exe now
  (http://download.microsoft.com/download/vc40pro/Sample4/1/NT4/EN-US/Fcsval32.exe)

For additional information about how to download Microsoft Support files, click
the following article number to view the article in the Microsoft Knowledge
Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft scanned this file for viruses. Microsoft used the most current
virus-detection software that was available on the date that the file was
posted. The file is stored on secure servers that prevent any unauthorized
changes to the file.

Visual C++ .NET:

  DownloadDownload Fcsval32.exe now
  (http://download.microsoft.com/download/VisualStudioNET/sample/1.20/WIN98MeXP/EN-US/Fcsval32.exe)

Release Date: June 27, 2002

For additional information about how to download Microsoft Support files, click
the following article number to view the article in the Microsoft Knowledge
Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft scanned this file for viruses. Microsoft used the most current
virus-detection software that was available on the date that the file was
posted. The file is stored on secure servers that prevent any unauthorized
changes to the file.


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

The application itself is just a modal dialog box displayed by the
CWinApp::InitInstance(). After displaying the dialog box, InitInstance() simply
quits the application.

The important part of the sample takes place in the dialog box class
implementation: There are two edit controls. The first takes input of an integer
between 1 and 20. The second takes a character string as input with length less
than or equal to 5. When you tab or mouse-click from control to control within
the displayed dialog box, the contents of the control that is losing focus are
validated.

The CFocusDlg Class
-------------------

The application's functionality centers around the CFocusDlg class and its
implementation of four message handlers (discussed below). Normal data exchange
(DDX) and validation (DDV) using the routines provided by MFC take place in
OnInitialUpdate(), when the dialog box is first displayed, and when the user
chooses the OK button to accept the input. This is default behavior provided by
ClassWizard when member variables are connected to dialog box controls and can
be examined in the dialog class DoDataExchange() function.

  

  Special Case 1
  --------------

  The first special case, for which this sample is written, involves validating
  control contents when switching focus from one control to the next. This is
  done by handling the EN_KILLFOCUS notification sent by the edit control that
  is losing focus. The idea here is to check the contents and, if they are not
  valid, to display the message box, inform the user, and then set the focus
  back to the control from which it came.

  Unfortunately, some difficulties arise when trying to set the focus (or
  display the message boxes) within a Killfocus message handler. At this point,
  Windows is in an indeterminate state as it is moving focus from one control
  to the other. This is a bad place to do the validation and SetFocus() call.

  The solution here is to post a user-defined message to the dialog box (parent)
  and do the validation and SetFocus() there, thus waiting for a safer time to
  do the work. (See "CFocusDlg::OnEditLostFocus()" in the file FOCUSDLG.CPP and
  "WM_EDITLOSTFOCUS user-defined message" in the file FOCUSDLG.H.)

  Another thing you will notice about this function is that it uses TRY/CATCH to
  do the validation. The provided DDX/DDV routines throw CUserExceptions when
  failing to validate or load a control's data. You should catch these and do
  the SetFocus() in the CATCH block.

  Special Case 2
  --------------

  The second special case is when the user clicks the Cancel button while there
  is still invalid data in one of the controls. In this situation, the user
  (normally) does not want to know whether his or her input is correct because
  he or she is quitting the application. You might consider prompting the user
  to save and validate the input, but normally Cancel means what it says. Focus
  is sent to the Cancel button before the IDCANCEL notification is sent to the
  dialog box, causing EN_KILLFOCUS to be generated by the control.

  Because we want to do control validation, we maintain a flag called
  "m_bValidate". When the user clicks Cancel, the dialog box is notified of the
  mouse-click by the child button through a WM_PARENTNOTIFY message. If we
  handle this message, we can set the m_bValidate flag to FALSE, which prevents
  the validation from taking place in the OnEditLostFocus() function. Notice
  the "if (m_bValidate)" right at the beginning.

  IMPORTANT NOTE:

  To ensure that the dialog receives the WM_PARENTNOTIFY message we have to
  explicitly remove the WS_EX_NOPARENTNOTIFY style from the Cancel button. For
  example:

      BOOL CFocusDlg::OnInitDialog()
      {
         CDialog::OnInitDialog();

         // Remove this style so we get the WM_PARENTNOTIFY when the
         // user clicks on the Cancel button
         GetDlgItem(IDCANCEL)->ModifyStyleEx(WS_EX_NOPARENTNOTIFY,0);

         // Rest of OnInitDialog
         ...
         ...
      }

  Special Case 3
  --------------

  The third special case is when the user inputs bad data and then decides to
  bring another application to the foreground, possibly to do some other work
  or check some other information before completing this dialog box' form. The
  edit control generates a EN_KILLFOCUS when the application loses activation
  but, again, you might not necessarily want to validate. So, handle the
  WM_ACTIVATEAPP message and based on the bActive flag that is passed as a
  parameter, set the "m_bValidate" flag to disable validation. Upon
  reactivation, the same message gets handled and this time the bActivate flag
  is TRUE, causing us to re-enable validation.

  Special Case 4
  --------------

  The fourth special case is when the user ends the Windows session either
  through the Program Manager or the ExitWindows() function. In this case,
  handle the WM_QUERYENDSESSION message. If the dialog box controls pass
  validation (coded like the OnOK() function), then return TRUE from this
  handler to allow Windows to terminate. Otherwise, return FALSE so that the
  user can input the correct data.

Note for Combobox
-----------------

For a combobox that is not a drop list combobox (i.e. if there is an edit control
associated with the combobox) the code in the sample can cause infinite
recursion.

The following code (in focusdlg.cpp) is used in the sample to set the focus to
the control if the validation fails.

      CATCH(CUserException, e)
      {
          // Validation failed - user already alerted,
          // setfocus() back to the original control.

          ASSERT(bOK == FALSE);
          GetDlgItem(id)->SetFocus();// Reset the focus.
      }

This should be modified so that the focus is set to the edit control of the
combobox if the validation fails.

      CATCH(CUserException, e)
      {
          // Validation failed - user already alerted,
          // setfocus() back to the original control.

          ASSERT(bOK == FALSE);
          GetDlgItem(id)->GetWindow( GW_CHILD )->SetFocus();
          // Reset the focus to the edit control in
          // the combobox
      }

Additional query words: 4.00 killfocus queryendsession parentnotify combo box

======================================================================
Keywords          : kbcode kbfile kbSample kbDlg kbMFC KbUIDesign kbVC kbVC400 kbVC500 kbVC600 kbGrpDSMFCATL kbDialog 
Technology        : kbAudDeveloper kbMFC
Version           : :4.0,4.1,4.2,5.0,6.0

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