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Class: ErrorHandlerVB6 in Category Error Handling : Visual Basic 6 from Total Visual SourceBook

Class that provides a standardized error handling mechanism to trap errors, track the location and type of the errors, and to allow the programmer to take appropriate response in VB6.

Procedure List

Procedure Name	Type	Description
(Declarations)	Declarations	Declarations and private variables for the CErrorHandlerVB6 class.
AppTitle	Property	Get the application title (defaults to the value of your application's app.title property).
CurrentOperation	Property	Get the current operation.
Destination	Property	Get the destination.
DisplayMsgOnError	Property	Get whether a message box is displayed when an error is handled.
ErrorDescription	Property	Get the description of the error that caused the error handler to be triggered.
ErrorLine	Property	Get the line number of the procedure that caused the error when the error handler was triggered.
ErrorNumber	Property	Get the error code that caused the error when the error handler is triggered.
IncludeExpandedInfo	Property	Get the value of IncludeExpandedInfo.
ProcName	Property	Get the name of the procedure containing the error which caused the error handler to be triggered.
TraceExecution	Property	Get the value of the TraceExecution property. If this property is true, a log file is maintained which tracks each procedure which is logged by the Push method. Even if no error occurs in the procedure, an entry is written. This information can be useful to trace the order in which procedures are called in your application. By default the information is stored in a file named "trace.log" in your application's directory.
Class_Initialize	Initialize	Set initial values to defaults which may be overridden with property settings.
Class_Terminate	Terminate	Close trace log if opened, and check to be sure that all Push calls are balanced by Pop calls. If you raise an assertion at this point, double-check your code to see that you are calling the error handler correctly.
ClearLog	Method	Clear the error log (or table depending on the setting of the Destination property). If Destination is a string property indicating a DOS text file, this function simply deletes the file. If Destination is a Recordset object, this method deletes all rows from the recordset.
HandleError	Method	Log errors and respond to run-time errors. This is the main routine which handles a run-time error. It is responsible for displaying an error message, writing log information to a file, and raising events back into the caller program. This method is called if a run-time error branches to a section of code which calls this method. This method should only be called AFTER an error has occurred. First the current state of the error object is saved (error message, line, description). The BeforeHandlerCalled event is raised in the program which instantiated the error handler. The Cancel argument to BeforeHandlerCalled is tested. If the user does not cancel the error handler, by setting Cancel to True, the error handler proceeds to save error state information to the log file. After the error information has been saved, the AfterHandlerCalled event is raised to give the caller a chance to provide a custom response to the error, and the "AppSpecificHandler" private subroutine is called. If your program is not receiving the BeforeHandlerCalled or AfterHandlerCalled events, you can use this procedure to provide custom error handling, for example to display a custom form, or to provide the user a chance to exit the program cleanly.
Pop	Method	Pops the current procedure name off the error handling stack. Normally this procedure is called when exiting a procedure normally when no error has occurred. It must be balanced by a call to the Push method when the procedure is first called.
Push	Method	Pushes the supplied procedure name onto the error handling stack. You must manually assign the name of the current procedure to the strProc argument. It must be balanced by a call to the Pop method which is called when the procedure exits normally.
AppSpecificErrHandler	Private	Custom error handler stub subroutine. The recommended way to use the CErrorHandlerVB6 class is to have it raise events into your program. You can receive the BeforeHandlerCalled and AfterHandlerCalled events. There you can provide specific functionality for your program, such as displaying a custom error message form, or closing the program cleanly. The advantage to using the events is that the CErrorHandlerVB6 object can be completely generic. All custom app-specific logic is contained in your program, not in the class itself. If you cannot use the events however, because you are declaring the error handler object variable in a standard module, which cannot receive events, you can customize this procedure to provide app-specific error-handling logic. All app-specific logic should be confined to this procedure. That way when you incorporate the CErrorHandlerVB6 class into your application, this is the only part of the code that will need to be modified.
GetLastErr	Private	This method is called to get the last error that occurred. We increment the pointer as one of the last things done in Push() in order to add the next procedure to the next item available in the stack array. However, if an error occurred before we could add the next procedure to the stack, we need to go back to the previous item in the array to get the error that occurred.
LogErrorToFile	Private	Logs the most recent error to a disk file. The name of the file is specified by setting the Destination property of the error handler object to a string containing the file name path.
LogErrorToTable	Private	Logs the most recent error to a table. The table is specified by setting the Destination property of the error handler object to a recordset object which you create in your application.

Example Code for Using Class: ErrorHandlerVB6

' Example of CErrorHandlerVB6
'
' To try this example, do the following:
' 1. Create a new form
' 2. Add a command button named 'cmdTest'
' 3. Add a check box named 'chkResponseMode'.
'      - When this box is unchecked the default error handling message will be displayed.
'      - When it is checked, a custom error handling action will be taken
' 4. Paste all the code from this example to the new form's module.

Private WithEvents mErrHandler As CErrorHandlerVB6

' This example assumes that the sample files are located in the folder named by the following constant.
Private Const mcstrSamplePath As String = "C:\TVSBSamp"

Private Sub Form_Load()
  On Error GoTo PROC_ERR

  ' Create the error handler object and set some of its optional properties
  Set mErrHandler = New CErrorHandlerVB6

  With mErrHandler
    .AppTitle = "My Application"
    .Destination = mcstrSamplePath & "\err.txt"
    .DisplayMsgOnError = True
    .IncludeExpandedInfo = True
    .CurrentOperation = "Initializing"
    .Push "Form_Load"
  End With

PROC_EXIT:
  mErrHandler.Pop
  Exit Sub

PROC_ERR:
  mErrHandler.HandleError
  GoTo PROC_EXIT
End Sub

' Standard error handling style used for all non-trivial procs
Private Sub cmdTest_Click()
  On Error GoTo PROC_ERR
  mErrHandler.Push "cmdTest_Click"

  Call Proc1

PROC_EXIT:
  mErrHandler.Pop
  Exit Sub

PROC_ERR:
  mErrHandler.HandleError
  GoTo PROC_EXIT
End Sub

Private Sub Proc1()
  On Error GoTo PROC_ERR
  mErrHandler.Push "Proc1"

  Call Proc2

PROC_EXIT:
  mErrHandler.Pop
  Exit Sub

PROC_ERR:
  mErrHandler.HandleError
  GoTo PROC_EXIT
End Sub

Private Sub Proc2()
  On Error GoTo PROC_ERR
  mErrHandler.Push "Proc2"

  ' Specify a checkpoint to narrow down the location of the error if you are not using line numbers
  mErrHandler.CurrentOperation = "I'm about to die"

  ' Simulate an error
  MsgBox (1 / 0)

  ' This code is bypassed by the error handler
  mErrHandler.CurrentOperation = "I'll never make it"

PROC_EXIT:
  mErrHandler.Pop
  Exit Sub

PROC_ERR:
  mErrHandler.HandleError
  GoTo PROC_EXIT
End Sub

Private Sub chkResponseMode_Click()
  On Error GoTo PROC_ERR
  mErrHandler.Push "chkResponseMode_Click"

  ' When unchecked, let the error handler display a standard error message. When checked, no message
  ' will be displayed, so display a custom message in the BeforeHandlerCalled event of the error object

  mErrHandler.DisplayMsgOnError = Not (chkResponseMode.Value = vbChecked)

PROC_EXIT:
  mErrHandler.Pop
  Exit Sub

PROC_ERR:
  mErrHandler.HandleError
  GoTo PROC_EXIT

End Sub

Private Sub mErrHandler_AfterHandlerCalled()
  ' This event is raised after the .HandleError method is called.

  If mErrHandler.DisplayMsgOnError = False Then
    MsgBox "Closing application"
    ' If you are writing the error to a text file this will open the file for viewing
    Shell "notepad.exe " & mErrHandler.Destination, vbNormalFocus
    Unload Me
  End If

End Sub

Private Sub mErrHandler_BeforeHandlerCalled(Cancel As Boolean)
  ' This event is raised when the .HandleError method is called on the errorhandler object.
  ' If the Cancel argument to this event is set to True, then the AfterHandlerCalled event will
  ' not be raised

  Dim strCustomMessage As String

  If mErrHandler.DisplayMsgOnError = False Then
    With mErrHandler
      strCustomMessage = "Custom Error Handler" & vbCrLf & _
          "Something bad happened while doing: " & .CurrentOperation & vbCrLf & _
          "The current procedure is: " & .ProcName & vbCrLf & _
          "The error message is: " & .ErrorDescription & vbCrLf & _
          "The error number is: " & .ErrorNumber & vbCrLf & _
          "The error log will now be opened."

      MsgBox strCustomMessage, , .AppTitle

      If MsgBox("Close the application?", vbQuestion + vbYesNo, .AppTitle) = vbNo Then
        Cancel = True
      End If

    End With
  End If

End Sub

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