Properly Handling Errors in VBA (Excel)
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.
Two main purposes for error handling:
- 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)
- 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):
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.
BTW, if you ever need me to do your company logo, look me up at http://www.MySuperCrappyLogoLabels99.com
I definitely wouldn't use Block1. It doesn't seem right having the Error block in an IF statement unrelated to Errors.
Blocks 2,3 & 4 I guess are variations of a theme. I prefer the use of Blocks 3 & 4 over 2 only because of a dislike of the GOTO statement; I generally use the Block4 method. This is one example of code I use to check if the Microsoft ActiveX Data Objects 2.8 Library is added and if not add or use an earlier version if 2.8 is not available.
Option Explicit
Public booRefAdded As Boolean 'one time check for references
Public Sub Add_References()
Dim lngDLLmsadoFIND As Long
If Not booRefAdded Then
lngDLLmsadoFIND = 28 ' load msado28.tlb, if cannot find step down versions until found
On Error GoTo RefErr:
'Add Microsoft ActiveX Data Objects 2.8
Application.VBE.ActiveVBProject.references.AddFromFile _
Environ("CommonProgramFiles") + "\System\ado\msado" & lngDLLmsadoFIND & ".tlb"
On Error GoTo 0
Exit Sub
RefErr:
Select Case Err.Number
Case 0
'no error
Case 1004
'Enable Trust Centre Settings
MsgBox ("Certain VBA References are not available, to allow access follow these steps" & Chr(10) & _
"Goto Excel Options/Trust Centre/Trust Centre Security/Macro Settings" & Chr(10) & _
"1. Tick - 'Disable all macros with notification'" & Chr(10) & _
"2. Tick - 'Trust access to the VBA project objects model'")
End
Case 32813
'Err.Number 32813 means reference already added
Case 48
'Reference doesn't exist
If lngDLLmsadoFIND = 0 Then
MsgBox ("Cannot Find Required Reference")
End
Else
For lngDLLmsadoFIND = lngDLLmsadoFIND - 1 To 0 Step -1
Resume
Next lngDLLmsadoFIND
End If
Case Else
MsgBox Err.Number & vbCrLf & Err.Description, vbCritical, "Error!"
End
End Select
On Error GoTo 0
End If
booRefAdded = TRUE
End Sub
I keep things simple:
At the module level I define two variables and set one to the name of the module itself.
Private Const ThisModuleName As String = "mod_Custom_Functions"
Public sLocalErrorMsg As String
Within each Sub/Function of the module I define a local variable
Dim ThisRoutineName As String
I set ThisRoutineName to the name of the sub or function
' Housekeeping
On Error Goto ERR_RTN
ThisRoutineName = "CopyWorksheet"
I then send all errors to an ERR_RTN: when they occur, but I first set the sLocalErrorMsg to define what the error actually is and provide some debugging info.
If Len(Trim(FromWorksheetName)) < 1 Then
sLocalErrorMsg = "Parameter 'FromWorksheetName' Is Missing."
GoTo ERR_RTN
End If
At the bottom of each sub/function, I direct the logic flow as follows
'
' The "normal" logic goes here for what the routine does
'
GoTo EXIT_RTN
ERR_RTN:
On Error Resume Next
' Call error handler if we went this far.
ErrorHandler ThisModuleName, ThisRoutineName, sLocalErrorMsg, Err.Description, Err.Number, False
EXIT_RTN:
On Error Resume Next
'
' Some closing logic
'
End If
I then have a seperate module I put in all projects called "mod_Error_Handler".
'
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Subroutine Name: ErrorHandler '
' '
' Description: '
' This module will handle the common error alerts. '
' '
' Inputs: '
' ModuleName String 'The name of the module error is in. '
' RoutineName String 'The name of the routine error in in. '
' LocalErrorMsg String 'A local message to assist with troubleshooting.'
' ERRDescription String 'The Windows Error Description. '
' ERRCode Long 'The Windows Error Code. '
' Terminate Boolean 'End program if error encountered? '
' '
' Revision History: '
' Date (YYYYMMDD) Author Change '
' =============== ===================== =============================================== '
' 20140529 XXXXX X. XXXXX Original '
' '
'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
'
Public Sub ErrorHandler(ModuleName As String, RoutineName As String, LocalErrorMsg As String, ERRDescription As String, ERRCode As Long, Terminate As Boolean)
Dim sBuildErrorMsg As String
' Build Error Message To Display
sBuildErrorMsg = "Error Information:" & vbCrLf & vbCrLf
If Len(Trim(ModuleName)) < 1 Then
ModuleName = "Unknown"
End If
If Len(Trim(RoutineName)) < 1 Then
RoutineName = "Unknown"
End If
sBuildErrorMsg = sBuildErrorMsg & "Module Name: " & ModuleName & vbCrLf & vbCrLf
sBuildErrorMsg = sBuildErrorMsg & "Routine Name: " & RoutineName & vbCrLf & vbCrLf
If Len(Trim(LocalErrorMsg)) > 0 Then
sBuildErrorMsg = sBuildErrorMsg & "Local Error Msg: " & LocalErrorMsg & vbCrLf & vbCrLf
End If
If Len(Trim(ERRDescription)) > 0 Then
sBuildErrorMsg = sBuildErrorMsg & "Program Error Msg: " & ERRDescription & vbCrLf & vbCrLf
If IsNumeric(ERRCode) Then
sBuildErrorMsg = sBuildErrorMsg & "Program Error Code: " & Trim(Str(ERRCode)) & vbCrLf & vbCrLf
End If
End If
MsgBox sBuildErrorMsg, vbOKOnly + vbExclamation, "Error Detected!"
If Terminate Then
End
End If
End Sub
The end result is a pop-up error message teling me in what module, what soubroutine, and what the error message specifically was. In addition, it also will insert the Windows error message and code.
Block 2 doesn't work because it doesn't reset the Error Handler potentially causing an endless loop. For Error Handling to work properly in VBA, you need a Resume statement to clear the Error Handler. The Resume also reactivates the previous Error Handler. Block 2 fails because a new error would go back to the previous Error Handler causing an infinite loop.
Block 3 fails because there is no Resume statement so any attempt at error handling after that will fail.
Every error handler must be ended by exiting the procedure or a Resume statement. Routing normal execution around an error handler is confusing. This is why error handlers are usually at the bottom.
But here is another way to handle an error in VBA. It handles the error inline like Try/Catch in VB.net There are a few pitfalls, but properly managed it works quite nicely.
Sub InLineErrorHandling()
'code without error handling
BeginTry1:
'activate inline error handler
On Error GoTo ErrHandler1
'code block that may result in an error
Dim a As String: a = "Abc"
Dim c As Integer: c = a 'type mismatch
ErrHandler1:
'handle the error
If Err.Number <> 0 Then
'the error handler has deactivated the previous error handler
MsgBox (Err.Description)
'Resume (or exit procedure) is the only way to get out of an error handling block
'otherwise the following On Error statements will have no effect
'CAUTION: it also reactivates the previous error handler
Resume EndTry1
End If
EndTry1:
'CAUTION: since the Resume statement reactivates the previous error handler
'you must ALWAYS use an On Error GoTo statement here
'because another error here would cause an endless loop
'use On Error GoTo 0 or On Error GoTo <Label>
On Error GoTo 0
'more code with or without error handling
End Sub
Sources:
- http://www.cpearson.com/excel/errorhandling.htm
- http://msdn.microsoft.com/en-us/library/bb258159.aspx
The key to making this work is to use a Resume statement immediately followed by another On Error statement. The Resume is within the error handler and diverts code to the EndTry1 label. You must immediately set another On Error statement to avoid problems as the previous error handler will "resume". That is, it will be active and ready to handle another error. That could cause the error to repeat and enter an infinite loop.
To avoid using the previous error handler again you need to set On Error to a new error handler or simply use On Error Goto 0 to cancel all error handling.