Creating Custom Assertions

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In some circumstances, you might not want to break into your code each time an assertion fails. For example, you might want to log assertion information to a file and use an error handler to handle any errors that result from the bad data. With a little bit of planning, you can create code to use while debugging that will let you handle assertions according to a flag you pass to a general routine. For example, the following procedure accepts arguments representing an assertion expression to test, the text of the assertion expression, the calling procedure's name, and a flag indicating how to display or log a failed assertion:

Function CustomAssertError(varExpression As Variant, _
                           strExpression As String, _
                           strCallingProc As String, _
                           Optional intOutputType As Integer = 0) As Boolean

   Dim intFileNum        As Integer
   Dim strErrorMessage   As String
   Const DEBUG_LOGFILE   As String = "c:\CustomAssertLog.txt"
   
   #If FLAG_DEBUG = True Then
      If varExpression = False Then
         strErrorMessage = "ASSERTION FAILURE! " & Now() & vbCrLf _
            & "The expression: " & vbCrLf & "'" & strExpression & "'" _
            & vbCrLf & "Called from: " & vbCrLf & "'" & strCallingProc _
            & "'" & vbCrLf & "failed!"

         Select Case intOutputType
            Case 0
               ' Display in message box.
               MsgBox strErrorMessage
            Case 1
               ' Write to Debug window.
               Debug.Print strErrorMessage
            Case 2
               ' Write to text file on disk.
               intFileNum = FreeFile
               Open DEBUG_LOGFILE For Append As #intFileNum
               Write #intFileNum, strErrorMessage
               Close #intFileNum
            Case Else
               Stop
         End Select
      End If
   #End If
End Function

See Also

Writing Error-Free Code | Design-Time Tools | Run-Time Tools | Script Debugging Tools | Understanding Conditional Compilation | Using Assertions | Handling Errors