DevTut

# Error Handling

# Avoiding error conditions

When a runtime error occurs, good code should handle it. The best error handling strategy is to write code that checks for error conditions and simply avoids executing code that results in a runtime error.

One key element in reducing runtime errors, is writing small procedures that do one thing. The fewer reasons procedures have to fail, the easier the code as a whole is to debug.

Avoiding runtime error 91 - Object or With block variable not set:

This error will be raised when an object is used before its reference is assigned. One might have a procedure that receives an object parameter:

Private Sub DoSomething(ByVal target As Worksheet)
    Debug.Print target.Name
End Sub

If target isn't assigned a reference, the above code will raise an error that is easily avoided by checking if the object contains an actual object reference:

Private Sub DoSomething(ByVal target As Worksheet)
    If target Is Nothing Then Exit Sub
    Debug.Print target.Name
End Sub

If target isn't assigned a reference, then the unassigned reference is never used, and no error occurs.

This way of early-exiting a procedure when one or more parameter isn't valid, is called a guard clause.

Avoiding runtime error 9 - Subscript out of range:

This error is raised when an array is accessed outside of its boundaries.

Private Sub DoSomething(ByVal index As Integer)
    Debug.Print ActiveWorkbook.Worksheets(index)
End Sub

Given an index greater than the number of worksheets in the ActiveWorkbook, the above code will raise a runtime error. A simple guard clause can avoid that:

Private Sub DoSomething(ByVal index As Integer)
    If index > ActiveWorkbook.Worksheets.Count Or index <= 0 Then Exit Sub
    Debug.Print ActiveWorkbook.Worksheets(index)
End Sub

Most runtime errors can be avoided by carefully verifying the values we're using before we use them, and branching on another execution path accordingly using a simple If statement - in guard clauses that makes no assumptions and validates a procedure's parameters, or even in the body of larger procedures.

# Custom Errors

Often when writing a specialized class, you'll want it to raise its own specific errors, and you'll want a clean way for user/calling code to handle these custom errors. A neat way to achieve this is by defining a dedicated Enum type:

Option Explicit
Public Enum FoobarError
    Err_FooWasNotBarred = vbObjectError + 1024
    Err_BarNotInitialized
    Err_SomethingElseHappened
End Enum

Using the vbObjectError built-in constant ensures the custom error codes don't overlap with reserved/existing error codes. Only the first enum value needs to be explicitly specified, for the underlying value of each Enum member is 1 greater than the previous member, so the underlying value of Err_BarNotInitialized is implicitly vbObjectError + 1025.

# Raising your own runtime errors

A runtime error can be raised using the Err.Raise statement, so the custom Err_FooWasNotBarred error can be raised as follows:

Err.Raise Err_FooWasNotBarred

The Err.Raise method can also take custom Description and Source parameters - for this reason it's a good idea to also define constants to hold each custom error's description:

Private Const Msg_FooWasNotBarred As String = "The foo was not barred."
Private Const Msg_BarNotInitialized As String = "The bar was not initialized."

And then create a dedicated private method to raise each error:

Private Sub OnFooWasNotBarredError(ByVal source As String)
    Err.Raise Err_FooWasNotBarred, source, Msg_FooWasNotBarred
End Sub

Private Sub OnBarNotInitializedError(ByVal source As String)
    Err.Raise Err_BarNotInitialized, source, Msg_BarNotInitialized
End Sub

The class' implementation can then simply call these specialized procedures to raise the error:

Public Sub DoSomething()
    'raises the custom 'BarNotInitialized' error with "DoSomething" as the source:
    If Me.Bar Is Nothing Then OnBarNotInitializedError "DoSomething"
    '...
End Sub

The client code can then handle Err_BarNotInitialized as it would any other error, inside its own error-handling subroutine.

Note: the legacy Error keyword can also be used in place of Err.Raise, but it's obsolete/deprecated.

# Resume keyword

An error-handling subroutine will either:

  • run to the end of the procedure, in which case execution resumes in the calling procedure.
  • or, use the Resume keyword to resume execution inside the same procedure.

The Resume keyword should only ever be used inside an error handling subroutine, because if VBA encounters Resume without being in an error state, runtime error 20 "Resume without error" is raised.

There are several ways an error-handling subroutine may use the Resume keyword:

  • Resume used alone, execution continues on the statement that caused the error. If the error isn't actually handled before doing that, then the same error will be raised again, and execution might enter an infinite loop.
  • Resume Next continues execution on the statement immediately following the statement that caused the error. If the error isn't actually handled before doing that, then execution is permitted to continue with potentially invalid data, which may result in logical errors and unexpected behavior.
  • Resume [line label] continues execution at the specified line label (or line number, if you're using legacy-style line numbers). This would typically allow executing some cleanup code before cleanly exiting the procedure, such as ensuring a database connection is closed before returning to the caller.

# On Error Resume Next

The On Error statement itself can use the Resume keyword to instruct the VBA runtime to effectively ignore all errors.

If the error isn't actually handled before doing that, then execution is permitted to continue with potentially invalid data, which may result in logical errors and unexpected behavior.

The emphasis above cannot be emphasized enough. On Error Resume Next effectively ignores all errors and shoves them under the carpet. A program that blows up with a runtime error given invalid input is a better program than one that keeps running with unknown/unintended data - be it only because the bug is much more easily identifiable. On Error Resume Next can easily hide bugs.

The On Error statement is procedure-scoped - that's why there should normally be only one, single such On Error statement in a given procedure.

However sometimes an error condition can't quite be avoided, and jumping to an error-handling subroutine only to Resume Next just doesn't feel right. In this specific case, the known-to-possibly-fail statement can be wrapped between two On Error statements:

On Error Resume Next
[possibly-failing statement]
Err.Clear 'resets current error
On Error GoTo 0

The On Error GoTo 0 instruction resets error handling in the current procedure, such that any further instruction causing a runtime error would be unhandled within that procedure and instead passed up the call stack until it is caught by an active error handler. If there is no active error handler in the call stack, it will be treated as an unhandled exception.

Public Sub Caller()
    On Error GoTo Handler
    
    Callee
    
    Exit Sub
Handler:
    Debug.Print "Error " & Err.Number & " in Caller."
End Sub

Public Sub Callee()
    On Error GoTo Handler
    
    Err.Raise 1     'This will be handled by the Callee handler.
    On Error GoTo 0 'After this statement, errors are passed up the stack.
    Err.Raise 2     'This will be handled by the Caller handler.    
    
    Exit Sub
Handler:
    Debug.Print "Error " & Err.Number & " in Callee."
    Resume Next
End Sub

# On Error statement

Even with guard clauses, one cannot realistically always account for all possible error conditions that could be raised in the body of a procedure. The On Error GoTo statement instructs VBA to jump to a line label and enter "error handling mode" whenever an unexpected error occurs at runtime. After handling an error, code can resume back into "normal" execution using the Resume keyword.

Line labels denote subroutines: because subroutines originate from legacy BASIC code and uses GoTo and GoSub jumps and Return statements to jump back to the "main" routine, it's fairly easy to write hard-to-follow spaghetti code if things aren't rigorously structured. For this reason, it's best that:

  • a procedure has one and only one error-handling subroutine
  • the error-handling subroutine only ever runs in an error state

This means a procedure that handles its errors, should be structured like this:

Private Sub DoSomething()
    On Error GoTo CleanFail

    'procedure code here

CleanExit:
    'cleanup code here
    Exit Sub

CleanFail:
    'error-handling code here
    Resume CleanExit
End Sub

# Error Handling Strategies

Sometimes you want to handle different errors with different actions. In that case you will inspect the global Err object, which will contain information about the error that was raised - and act accordingly:

CleanExit:
    Exit Sub

CleanFail:
    Select Case Err.Number
        Case 9
            MsgBox "Specified number doesn't exist. Please try again.", vbExclamation
            Resume
        Case 91
            'woah there, this shouldn't be happening.
            Stop 'execution will break here
            Resume 'hit F8 to jump to the line that raised the error
        Case Else
            MsgBox "An unexpected error has occurred:" & vbNewLine & Err.Description, vbCritical
            Resume CleanExit
    End Select
End Sub

As a general guideline, consider turning on the error handling for entire subroutine or function, and handle all the errors that may occur within its scope. If you need to only handle errors in the small section section of the code -- turn error handling on and off a the same level:

Private Sub DoSomething(CheckValue as Long)

    If CheckValue = 0 Then
        On Error GoTo ErrorHandler   ' turn error handling on
        ' code that may result in error
        On Error GoTo 0              ' turn error handling off - same level
    End If

CleanExit:
    Exit Sub

ErrorHandler:
    ' error handling code here
    ' do not turn off error handling here
    Resume

End Sub

# Line numbers

VBA supports legacy-style (e.g. QBASIC) line numbers. The Erl hidden property can be used to identify the line number that raised the last error. If you're not using line numbers, Erl will only ever return 0.

Sub DoSomething()
10 On Error GoTo 50
20 Debug.Print 42 / 0
30 Exit Sub
40
50 Debug.Print "Error raised on line " & Erl ' returns 20
End Sub

If you are using line numbers, but not consistently, then Erl will return the last line number before the instruction that raised the error.

Sub DoSomething()
10 On Error GoTo 50
   Debug.Print 42 / 0
30 Exit Sub

50 Debug.Print "Error raised on line " & Erl 'returns 10
End Sub

Keep in mind that Erl also only has Integer precision, and will silently overflow. This means that line numbers outside of the integer range (opens new window) will give incorrect results:

Sub DoSomething()
99997 On Error GoTo 99999
99998 Debug.Print 42 / 0
99999
      Debug.Print Erl   'Prints 34462
End Sub

The line number isn't quite as relevant as the statement that caused the error, and numbering lines quickly becomes tedious and not quite maintenance-friendly.

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