Microsoft KB Archive/114962

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Control-by-Control Validation in MFC

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Q114962

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The information in this article applies to:


 * The Microsoft Foundation Classes (MFC), used with:
 * Microsoft Visual C++, versions 1.0, 1.5

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

MORE INFORMATION
Fcsval.exe

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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 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.

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.

Additional query words: kbinf killfocus queryendsession parentnotify

Keywords : kbfile kbMFC KbUIDesign kbVC

Issue type :

Technology : kbAudDeveloper kbMFC