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

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.