VBA Excel Error Handling - especially in functions - Professional Excel Development Style

Question

I got the book \"Professional Excel Development\" by Rob Bovey and it is opening up my eyes.

I am refitting my code with error handling. However, there is a lot I do

Answer 1

I needed a bit more help on this specific technique so I went right to the source and Mr. Bovey was gracious enough to reply. He gave me permission to post his response to the StackOverflow community.

The instructions below refer to his preferred method of error handling for functions the "boolean error handling" technique and not to the alternate "rethrow method", both described in his book "Professional Excel Development" 2nd edition.

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Hi Shari,

In answer to your questions about error handling in functions, there are three error handling scenarios you can have with a function in VBA:

1) The function is so trivial that is doesn't need an error handler. In the unlikely event an error occurs in a function like this it will spill over into the error handler of the calling procedure.

2) A non-trivial function needs an error handler and uses the Boolean return value system described in the book. Any other values the function needs to return are returned through ByRef arguments. This case covers the vast majority of functions I write. There are some things you can't do with functions like this, feeding them directly into the argument of another function is one example, but I consider this a good tradeoff in order to achieve bullet proof error handling.

3) A non-trivial function needs an error handler and must return a value not related to its error status. This is a rare situation because I can convert 99% plus of these into case 2 by restructuring my code. If you can't do this, your only choice is to select an arbitrary return value that is out of the range of normal return values and use this to indicate that an error has occurred. If the caller of this function sees this arbitrary error flag value it knows it can't continue.

Rob Bovey Application Professionals http://www.appspro.com/

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Code Example (Shari W)

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' Show how to call a function using this error handling method.
Const giBAD_RESULT As Integer = -1

Function TestMath()   ' An Entry Point

    Dim sngResult As Single
    Dim iNum As Integer

    ' Call the function, actual result goes in sngResult but it returns the error handling boolean.
    ' A true error like Div 0 will go to error handler.

    ' Set Up Error Handling for Entry Point
    Application.EnableCancelKey = xlErrorHandler
    Dim bUserCancel As Boolean
    Const sSOURCE As String = "TestMath()"
    On Error GoTo ErrorHandler
    ' End Error Set Up

    iNum = 0   ' Try 0 to create error
    If Not bDoSomeMath(iNum, sngResult) Then ERR.Raise glHANDLED_ERROR
    ' If function does parameter checking and wants to return a bad input code, check for that.
    If sngResult = giBAD_RESULT Then
        MsgBox ("Bad input to bDoSomeMath " & iNum)
    Else
        MsgBox ("I believe the answer is " & sngResult)
    End If

ErrorExit:
    On Error Resume Next
    Exit Function

ErrorHandler:
    If bCentralErrorHandler(msMODULE, sSOURCE, , True) Then
        Stop
        Resume
    Else
        Resume ErrorExit
    End If
End Function
Function bDoSomeMath(ByVal iNum As Integer, ByRef sngResult As Single) As Boolean

    ' Error handling Set Up
    Dim bReturn As Boolean
    Const sSOURCE As String = "bDoSomeMath()"
    On Error GoTo ErrorHandler
    bReturn = True
    ' End Error Set Up

    If iNum < 0 Or iNum > 1000 Then
        sngResult = giBAD_RESULT   'function failed because I only like the numbers 0 to 1000
        GoTo ErrorExit
    Else
        sngResult = 100 / iNum   ' generate a true error by iNum = 0
    End If

ErrorExit:
    On Error Resume Next
    bDoSomeMath = bReturn
    Exit Function

ErrorHandler:
    bReturn = False
    If bCentralErrorHandler(msMODULE, sSOURCE, , , True) Then
        Stop
        Resume
    Else
        Resume ErrorExit
    End If

End Function