Properly Handling Errors in VBA (Excel)

Two main purposes for error handling:

  1. Trap errors you can predict but can't control the user from doing (e.g. saving a file to a thumb drive when the thumb drives has been removed)
  2. For unexpected errors, present user with a form that informs them what the problem is. That way, they can relay that message to you and you might be able to give them a work-around while you work on a fix.

So, how would you do this?

First of all, create an error form to display when an unexpected error occurs.

It could look something like this (FYI: Mine is called frmErrors): Company Error Form

Notice the following labels:

  • lblHeadline
  • lblSource
  • lblProblem
  • lblResponse

Also, the standard command buttons:

  • Ignore
  • Retry
  • Cancel

There's nothing spectacular in the code for this form:

Option Explicit

Private Sub cmdCancel_Click()
  Me.Tag = CMD_CANCEL
  Me.Hide
End Sub

Private Sub cmdIgnore_Click()
  Me.Tag = CMD_IGNORE
  Me.Hide
End Sub

Private Sub cmdRetry_Click()
  Me.Tag = CMD_RETRY
  Me.Hide
End Sub

Private Sub UserForm_Initialize()
  Me.lblErrorTitle.Caption = "Custom Error Title Caption String"
End Sub

Private Sub UserForm_QueryClose(Cancel As Integer, CloseMode As Integer)
  'Prevent user from closing with the Close box in the title bar.
    If CloseMode <> 1 Then
      cmdCancel_Click
    End If
End Sub

Basically, you want to know which button the user pressed when the form closes.

Next, create an Error Handler Module that will be used throughout your VBA app:

'****************************************************************
'    MODULE: ErrorHandler
'
'   PURPOSE: A VBA Error Handling routine to handle
'             any unexpected errors
'
'     Date:    Name:           Description:
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'03/22/2010    Ray      Initial Creation
'****************************************************************
Option Explicit

Global Const CMD_RETRY = 0
Global Const CMD_IGNORE = 1
Global Const CMD_CANCEL = 2
Global Const CMD_CONTINUE = 3

Type ErrorType
    iErrNum As Long
    sHeadline As String
    sProblemMsg As String
    sResponseMsg As String
    sErrorSource As String
    sErrorDescription As String
    iBtnCap(3) As Integer
    iBitmap As Integer
End Type

Global gEStruc As ErrorType
Sub EmptyErrStruc_S(utEStruc As ErrorType)
  Dim i As Integer
    
  utEStruc.iErrNum = 0
  utEStruc.sHeadline = ""
  utEStruc.sProblemMsg = ""
  utEStruc.sResponseMsg = ""
  utEStruc.sErrorSource = ""
  For i = 0 To 2
    utEStruc.iBtnCap(i) = -1
  Next
  utEStruc.iBitmap = 1

End Sub
Function FillErrorStruct_F(EStruc As ErrorType) As Boolean
  'Must save error text before starting new error handler
  'in case we need it later
  EStruc.sProblemMsg = Error(EStruc.iErrNum)
  On Error GoTo vbDefaultFill

  EStruc.sHeadline = "Error " & Format$(EStruc.iErrNum)
  EStruc.sProblemMsg = EStruc.sErrorDescription
  EStruc.sErrorSource = EStruc.sErrorSource
  EStruc.sResponseMsg = "Contact the Company and tell them you received Error # " & Str$(EStruc.iErrNum) & ". You should write down the program function you were using, the record you were working with, and what you were doing."
  
   Select Case EStruc.iErrNum
       'Case Error number here
       'not sure what numeric errors user will ecounter, but can be implemented here
       'e.g.
       'EStruc.sHeadline = "Error 3265"
       'EStruc.sResponseMsg = "Contact tech support. Tell them what you were doing in the program."

     Case Else
       
       EStruc.sHeadline = "Error " & Format$(EStruc.iErrNum) & ": " & EStruc.sErrorDescription
       EStruc.sProblemMsg = EStruc.sErrorDescription
      
   End Select
   
   GoTo FillStrucEnd
   
vbDefaultFill:

  'Error Not on file
  EStruc.sHeadline = "Error " & Format$(EStruc.iErrNum) & ": Contact Tech Support"
  EStruc.sResponseMsg = "Contact the Company and tell them you received Error # " & Str$(EStruc.iErrNum)
FillStrucEnd:

  Exit Function

End Function
Function iErrorHandler_F(utEStruc As ErrorType) As Integer
  Static sCaption(3) As String
  Dim i As Integer
  Dim iMCursor As Integer
  
  Beep

  'Setup static array
  If Len(sCaption(0)) < 1 Then
    sCaption(CMD_IGNORE) = "&Ignore"
    sCaption(CMD_RETRY) = "&Retry"
    sCaption(CMD_CANCEL) = "&Cancel"
    sCaption(CMD_CONTINUE) = "Continue"
  End If

  Load frmErrors
  
  'Did caller pass error info?  If not fill struc with the needed info
  If Len(utEStruc.sHeadline) < 1 Then
    i = FillErrorStruct_F(utEStruc)
  End If

  frmErrors!lblHeadline.Caption = utEStruc.sHeadline
  frmErrors!lblProblem.Caption = utEStruc.sProblemMsg
  frmErrors!lblSource.Caption = utEStruc.sErrorSource
  frmErrors!lblResponse.Caption = utEStruc.sResponseMsg
  
  frmErrors.Show
  iErrorHandler_F = frmErrors.Tag   ' Save user response
  Unload frmErrors                  ' Unload and release form

  EmptyErrStruc_S utEStruc          ' Release memory

End Function

You may have errors that will be custom only to your application. This would typically be a short list of errors specifically only to your application. If you don't already have a constants module, create one that will contain an ENUM of your custom errors. (NOTE: Office '97 does NOT support ENUMS.). The ENUM should look something like this:

Public Enum CustomErrorName
  MaskedFilterNotSupported
  InvalidMonthNumber
End Enum

Create a module that will throw your custom errors.

'********************************************************************************************************************************
'    MODULE: CustomErrorList
'
'   PURPOSE: For trapping custom errors applicable to this application
'
'INSTRUCTIONS:  To use this module to create your own custom error:
'               1.  Add the Name of the Error to the CustomErrorName Enum
'               2.  Add a Case Statement to the raiseCustomError Sub
'               3.  Call the raiseCustomError Sub in the routine you may see the custom error
'               4.  Make sure the routine you call the raiseCustomError has error handling in it
'
'
'     Date:    Name:           Description:
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'03/26/2010    Ray       Initial Creation
'********************************************************************************************************************************
Option Explicit
Const MICROSOFT_OFFSET = 512 'Microsoft reserves error values between vbObjectError and vbObjectError + 512
'************************************************************************************************
'  FUNCTION:  raiseCustomError
'
'   PURPOSE:  Raises a custom error based on the information passed
'
'PARAMETERS:  customError - An integer of type CustomErrorName Enum that defines the custom error
'             errorSource - The place the error came from
'
'   Returns:  The ASCII vaule that should be used for the Keypress
'
'     Date:    Name:           Description:
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'03/26/2010    Ray       Initial Creation
'************************************************************************************************
Public Sub raiseCustomError(customError As Integer, Optional errorSource As String = "")
  Dim errorLong As Long
  Dim errorDescription As String
  
  errorLong = vbObjectError + MICROSOFT_OFFSET + customError
  
  Select Case customError
  
    Case CustomErrorName.MaskedFilterNotSupported
      errorDescription = "The mask filter passed is not supported"
      
    Case CustomErrorName.InvalidMonthNumber
      errorDescription = "Invalid Month Number Passed"
      
    Case Else
      errorDescription = "The custom error raised is unknown."
      
  End Select
  
  Err.Raise errorLong, errorSource, errorDescription
  
End Sub

You are now well equipped to trap errors in your program. You sub (or function), should look something like this:

Public Sub MySub(monthNumber as Integer)
  On Error GoTo eh  
  
  Dim sheetWorkSheet As Worksheet
  
  'Run Some code here
  
  '************************************************
  '*   OPTIONAL BLOCK 1:  Look for a specific error
  '************************************************
  'Temporarily Turn off Error Handling so that you can check for specific error
  On Error Resume Next
  'Do some code where you might expect an error.  Example below:
  Const ERR_SHEET_NOT_FOUND = 9 'This error number is actually subscript out of range, but for this example means the worksheet was not found
  
  Set sheetWorkSheet = Sheets("January")
  
  'Now see if the expected error exists
  
  If Err.Number = ERR_SHEET_NOT_FOUND Then
    MsgBox "Hey!  The January worksheet is missing.  You need to recreate it."
    Exit Sub
  ElseIf Err.Number <> 0 Then
    'Uh oh...there was an error we did not expect so just run basic error handling 
    GoTo eh
  End If
  
  'Finished with predictable errors, turn basic error handling back on:
  On Error GoTo eh
  
  '**********************************************************************************
  '*   End of OPTIONAL BLOCK 1
  '**********************************************************************************
  
  '**********************************************************************************
  '*   OPTIONAL BLOCK 2:  Raise (a.k.a. "Throw") a Custom Error if applicable
  '**********************************************************************************
  If not (monthNumber >=1 and monthnumber <=12) then
    raiseCustomError CustomErrorName.InvalidMonthNumber, "My Sub"
  end if
  '**********************************************************************************
  '*   End of OPTIONAL BLOCK 2
  '**********************************************************************************
  
  'Rest of code in your sub
  
  goto sub_exit

eh:
  gEStruc.iErrNum = Err.Number
  gEStruc.sErrorDescription = Err.Description
  gEStruc.sErrorSource = Err.Source
  m_rc = iErrorHandler_F(gEStruc)
  
  If m_rc = CMD_RETRY Then
    Resume
  End If

sub_exit:
  'Any final processing you want to do.
  'Be careful with what you put here because if it errors out, the error rolls up.  This can be difficult to debug; especially if calling routine has no error handling.
  
  Exit Sub 'I was told a long time ago (10+ years) that exit sub was better than end sub...I can't tell you why, so you may not want to put in this line of code.  It's habit I can't break :P
End Sub

A copy/paste of the code above may not work right out of the gate, but should definitely give you the gist.


You've got one truly marvelous answer from ray023, but your comment that it's probably overkill is apt. For a "lighter" version....

Block 1 is, IMHO, bad practice. As already pointed out by osknows, mixing error-handling with normal-path code is Not Good. For one thing, if a new error is thrown while there's an Error condition in effect you will not get an opportunity to handle it (unless you're calling from a routine that also has an error handler, where the execution will "bubble up").

Block 2 looks like an imitation of a Try/Catch block. It should be okay, but it's not The VBA Way. Block 3 is a variation on Block 2.

Block 4 is a bare-bones version of The VBA Way. I would strongly advise using it, or something like it, because it's what any other VBA programmer inherting the code will expect. Let me present a small expansion, though:

Private Sub DoSomething()
On Error GoTo ErrHandler

'Dim as required

'functional code that might throw errors

ExitSub:
    'any always-execute (cleanup?) code goes here -- analagous to a Finally block.
    'don't forget to do this -- you don't want to fall into error handling when there's no error
    Exit Sub

ErrHandler:
    'can Select Case on Err.Number if there are any you want to handle specially

    'display to user
    MsgBox "Something's wrong: " & vbCrLf & Err.Description

    'or use a central DisplayErr routine, written Public in a Module
    DisplayErr Err.Number, Err.Description

    Resume ExitSub
    Resume
End Sub

Note that second Resume. This is a trick I learned recently: It will never execute in normal processing, since the Resume <label> statement will send the execution elsewhere. It can be a godsend for debugging, though. When you get an error notification, choose Debug (or press Ctl-Break, then choose Debug when you get the "Execution was interrupted" message). The next (highlighted) statement will be either the MsgBox or the following statement. Use "Set Next Statement" (Ctl-F9) to highlight the bare Resume, then press F8. This will show you exactly where the error was thrown.

As to your objection to this format "jumping around", A) it's what VBA programmers expect, as stated previously, & B) your routines should be short enough that it's not far to jump.

Tags:

Excel

Vba

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