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 don't understand. I especially need to know how to correctly use it in functions. I use Bovey's rethrow version of the error handler (at bottom). When I started, I was using the basic boolean (non-rethrow) method and turned my subroutines into boolean functions. (P.S. I am switching back to the boolean method based on the answer.)

I need guidance on how to fit functions into this scheme. I want them to return their real values (a string or double, e.g., or -1 if they fail in some cases) so I can nest them in other functions and not just return an error handling boolean.

This is what a typical subroutine call to bDrawCellBorders(myWS) would look like within an entry point. Sub calls seem to be working well. (I.e. it is a subroutine that was turned into a function only so it can return a boolean to the error handling scheme.)

Sub UpdateMe()  ' Entry Point

    Const sSOURCE As String = "UpdateMe()"

    On Error GoTo ErrorHandler

    Set myWS = ActiveCell.Worksheet
    Set myRange = ActiveCell
    myWS.Unprotect

' lots of code

    If Not bDrawCellBorders(myWS) Then ERR.Raise glHANDLED_ERROR    ' Call subroutine

' lots of code

ErrorExit:
    On Error Resume Next
    Application.EnableEvents = True
    myWS.Protect AllowFormattingColumns:=True
    Exit Sub

ErrorHandler:
    If bCentralErrorHandler(msMODULE, sSOURCE,,True) Then  ' Call as Entry Point
        Stop
        Resume
    Else
        Resume ErrorExit
    End If
End Sub

However, I don't know how to extend this to real functions. This is based off an example in the book that was drawn up for a subroutine, and I just switched it to a function. Questions: * How do I call it? Is it simply like x = sngDoSomeMath(17) * Will its error handling function properly? * Where is the right place or places to call the error handling routine with bReThrow=true?

Public Function sngDoSomeMath(ByVal iNum As Integer) As Single

Dim sngResult As Single

Const sSOURCE As String = "sngDoSomeMath()"

On Error GoTo ErrorHandler

' example 1, input did not pass validation. don't want to 
' go up the error stack but just inform the
' calling program that they didn't get a good result from this 
' function call so they can do something else
If iNum <> 42 Then
    sngResult = -1    'function failed because I only like the number 42
    GoTo ExitHere
End If

' example 2, true error generated
sngResult = iNum / 0

sngDoSomeMath = lResult

ExitHere:
    Exit Function
ErrorHandler:

' Run cleanup code
'  ... here if any

' Then do error handling

If bCentralErrorHandler(msMODULE, sSOURCE, , , True) Then ' The true is for RETHROW
    Stop
    Resume
End If

End Function

The Error Handler Routine:

'
' Description:  This module contains the central error
'               handler and related constant declarations.
'
' Authors:      Rob Bovey, www.appspro.com
'               Stephen Bullen, www.oaltd.co.uk
'
' Chapter Change Overview
' Ch#   Comment
' --------------------------------------------------------------
' 15    Initial version
'
Option Explicit
Option Private Module

' **************************************************************
' Global Constant Declarations Follow
' **************************************************************
Public Const gbDEBUG_MODE As Boolean = False    ' True enables debug mode, False disables it.
Public Const glHANDLED_ERROR As Long = 9999     ' Run-time error number for our custom errors.
Public Const glUSER_CANCEL As Long = 18         ' The error number generated when the user cancels program execution.


' **************************************************************
' Module Constant Declarations Follow
' **************************************************************
Private Const msSILENT_ERROR As String = "UserCancel"   ' Used by the central error handler to bail out silently on user cancel.
Private Const msFILE_ERROR_LOG As String = "Error.log"  ' The name of the file where error messages will be logged to.


''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Comments: This is the central error handling procedure for the
'           program. It logs and displays any run-time errors
'           that occur during program execution.
'
' Arguments:    sModule         The module in which the error occured.
'               sProc           The procedure in which the error occured.
'               sFile           (Optional) For multiple-workbook
'                               projects this is the name of the
'                               workbook in which the error occured.
'               bEntryPoint     (Optional) True if this call is
'                               being made from an entry point
'                               procedure. If so, an error message
'                               will be displayed to the user.
'
' Returns:      Boolean         True if the program is in debug
'                               mode, False if it is not.
'
' Date          Developer       Chap    Action
' --------------------------------------------------------------
' 03/30/08      Rob Bovey       Ch15    Initial version
'
Public Function bCentralErrorHandler( _
       ByVal sModule As String, _
       ByVal sProc As String, _
       Optional ByVal sFile As String, _
       Optional ByVal bEntryPoint As Boolean, _
       Optional ByVal bReThrow As Boolean = True) As Boolean

    Static sErrMsg As String

    Dim iFile As Integer
    Dim lErrNum As Long
    Dim sFullSource As String
    Dim sPath As String
    Dim sLogText As String

    ' Grab the error info before it's cleared by
    ' On Error Resume Next below.
    lErrNum = ERR.Number
    ' If this is a user cancel, set the silent error flag
    ' message. This will cause the error to be ignored.
    If lErrNum = glUSER_CANCEL Then sErrMsg = msSILENT_ERROR
    ' If this is the originating error, the static error
    ' message variable will be empty. In that case, store
    ' the originating error message in the static variable.
    If Len(sErrMsg) = 0 Then sErrMsg = ERR.Description

    ' We cannot allow errors in the central error handler.
    On Error Resume Next

    ' Load the default filename if required.
    If Len(sFile) = 0 Then sFile = ThisWorkbook.Name

    ' Get the application directory.
    sPath = ThisWorkbook.Path
    If Right$(sPath, 1) <> "\" Then sPath = sPath & "\"

    ' Construct the fully-qualified error source name.
    sFullSource = "[" & sFile & "]" & sModule & "." & sProc

    ' Create the error text to be logged.
    sLogText = "  " & sFullSource & ", Error " & _
               CStr(lErrNum) & ": " & sErrMsg

    ' Open the log file, write out the error information and
    ' close the log file.
    iFile = FreeFile()
    Open sPath & msFILE_ERROR_LOG For Append As #iFile
    Print #iFile, Format$(Now(), "mm/dd/yy hh:mm:ss"); sLogText
    If bEntryPoint Or Not bReThrow Then Print #iFile,
    Close #iFile

    ' Do not display or debug silent errors.
    If sErrMsg <> msSILENT_ERROR Then

        ' Show the error message when we reach the entry point
        ' procedure or immediately if we are in debug mode.
        If bEntryPoint Or gbDEBUG_MODE Then
            Application.ScreenUpdating = True
            MsgBox sErrMsg, vbCritical, gsAPP_NAME
            ' Clear the static error message variable once
            ' we've reached the entry point so that we're ready
            ' to handle the next error.
            sErrMsg = vbNullString
        End If

        ' The return vale is the debug mode status.
        bCentralErrorHandler = gbDEBUG_MODE

    Else
        ' If this is a silent error, clear the static error
        ' message variable when we reach the entry point.
        If bEntryPoint Then sErrMsg = vbNullString
        bCentralErrorHandler = False
    End If

    'If we're using re-throw error handling,
    'this is not the entry point and we're not debugging,
    're-raise the error, to be caught in the next procedure
    'up the call stack.
    'Procedures that handle their own errors can call the
    'central error handler with bReThrow = False to log the
    'error, but not re-raise it.
    If bReThrow Then
        If Not bEntryPoint And Not gbDEBUG_MODE Then
            On Error GoTo 0
            ERR.Raise lErrNum, sFullSource, sErrMsg
        End If
    Else
        'Error is being logged and handled,
        'so clear the static error message variable
        sErrMsg = vbNullString
    End If

End Function

Answer 1

That is an amazing book by Rob.

My two cents of Error Handling (Either for a procedure or a Function) is based on KISS (Keep it simple Silly)

Understand what do you want from your error handler?

This is usually what I want/expect from my error handler...

1. Line on which the error happened
2. Error Number
3. Error Message
4. Reset Events if applicable

Lets break the above. As you are by now already aware how your error handler looks like, Consider this example.

Sub Sample()
    Dim i As Integer, j As Integer

    On Error GoTo Whoa

    Application.ScreenUpdating = False

    i = 1111111111

    For j = 1 To i
        Debug.Print ThisWorkbook.Sheets(1).Cells(i, 1).Value
    Next i

LetsContinue:
    Exit Sub
Whoa:
    MsgBox Err.Description
    Resume LetsContinue
End Sub

This is a very basic error handler but it's of very less help to me. So let's now tweak it to make it more useful. If you run the above code you get an error message like shown in the screenshot below and if you notice, it's not of much help.

Let's now tackle all the points that I mentioned in the Logic above

1. Line on which the error happened

There is a property called ERL which very few people are aware of. You can actually use it to get the line number of the code where the error happened. For that you have to ensure you number your code. See this example.

Sub Sample()
    Dim i As Integer, j As Integer

10      On Error GoTo Whoa

20      Application.ScreenUpdating = False

30      i = 1111111111

40      For j = 1 To i
50          Debug.Print ThisWorkbook.Sheets(1).Cells(i, 1).Value
60      Next j

LetsContinue:
70      Exit Sub
Whoa:
80      MsgBox Erl
90      Resume LetsContinue
End Sub

When you run the above code, you will get this

So now I know that the error happened on Line 30 which is i = 1111111111

Moving on to next

2. Error Number
3. Error Message

The error number and the error message can be retrieved from Err.Number and Err.Description respectively. So now let's combine Erl, Err.Number and Err.Description

Check this example

Sub Sample()
    Dim i As Integer, j As Integer

10      On Error GoTo Whoa

20      Application.ScreenUpdating = False

30      i = 1111111111

40      For j = 1 To i
50          Debug.Print ThisWorkbook.Sheets(1).Cells(i, 1).Value
60      Next j

LetsContinue:
70      Exit Sub
Whoa:
80      MsgBox "The Error Happened on Line : " & Erl & vbNewLine & _
           "Error Message : " & Err.Description & vbNewLine & _
           "Error Number : " & Err.Number
90      Resume LetsContinue
End Sub

When you run this code, you will get something like this.

You can choose to further customize the Error Message to make it more user friendly. For example

'~~> Message you want to deliver to the user in case the error happens
Const sMsg As String = "Please take a screenshot of this message and contact the developer for a resolution"
'~~> Title of your message box
Const sTitle As String = "Oopsie Daisies"

'~~> Change the above as applicable

Sub Sample()
    Dim i As Integer, j As Integer

10      On Error GoTo Whoa

20      Application.ScreenUpdating = False

30      i = 1111111111

40      For j = 1 To i
50          Debug.Print ThisWorkbook.Sheets(1).Cells(i, 1).Value
60      Next j

LetsContinue:
70      Exit Sub
Whoa:
80      MsgBox "The Error Happened on Line : " & Erl & vbNewLine & _
           "Error Message : " & Err.Description & vbNewLine & _
           "Error Number : " & Err.Number & vbNewLine & vbNewLine & _
           sMsg, vbCritical, sTitle
90      Resume LetsContinue
End Sub

On to the next one :)

Reset Events if applicable

When you are working with events and an error occurs, if there is no error handling, the code breaks. Unfortunately that doesn't reset the events. It is very important that you reset the events in the Error handler.

If you notice in the above code we are setting the Application.ScreenUpdating = False. When the code breaks, that event doesn't get reset. You will have to handle that in the Error handler LetsContinue in this case. See this example.

'~~> Message you want to deliver to the user in case the error happens
Const sMsg As String = "Please take a screenshot of this message and contact the developer for a resolution"
'~~> Title of your message box
Const sTitle As String = "Oopsie Daisies"

'~~> Change the above as applicable

Sub Sample()
    Dim i As Integer, j As Integer

10      On Error GoTo Whoa

20      Application.ScreenUpdating = False

30      i = 1111111111

40      For j = 1 To i
50          Debug.Print ThisWorkbook.Sheets(1).Cells(i, 1).Value
60      Next j

LetsContinue:
70      Application.ScreenUpdating = True
80      Exit Sub
Whoa:
90      MsgBox "The Error Happened on Line : " & Erl & vbNewLine & _
           "Error Message : " & Err.Description & vbNewLine & _
           "Error Number : " & Err.Number & vbNewLine & vbNewLine & _
           sMsg, vbCritical, sTitle
100     Resume LetsContinue
End Sub

Like Philippe, I also strongly suggest that you use MZ-Tools for VBA. I have been using it now for donkey years...

Hope this helps.