When is it appropriate to explicitly use Err.Clear?

Question

For example, the following function is used for checking whether a workbook is open:

Function BookOpen(Bk As String) As Boolean
    Dim T As Excel.Workbook 

    Err.Clear
    On Error Resume Next
    Set T = Application.Workbooks(Bk)
    BookOpen = Not T Is Nothing 
    Err.Clear 
    On Error GoTo 0 
End Function

Are these two Err.Clear statements necessary?

Answer 1

In this example

Function BookOpen(Bk As String) As Boolean
  Dim T As Excel.Workbook 
  Err.Clear
  On Error Resume Next
  Set T = Application.Workbooks(Bk)
  BookOpen = Not T Is Nothing 
  Err.Clear 
  On Error GoTo 0 
End Function

none of the uses is appropriate, because On Error resets the last error, so Err.Clear is redundant.

It's appropriate after actually handling a failed statement.

Function BookOpen(Bk As String) As Boolean
  Dim T As Excel.Workbook 

  On Error Resume Next
  Set T = Application.Workbooks(Bk)  ' this can fail...

  ' so handle a possible failure
  If Err.Number <> 0 Then
      MsgBox "The workbook named """ & Bk & """ does not exist."
      Err.Clear
  End If

  BookOpen = Not T Is Nothing 
End Function

If you have On Error Resume Next in effect, the program will continue after an error as if nothing had happened. There is no exception thrown, there is no warning, this is not structured error handling (i.e. it's nothing like try/catch blocks). Your program might end up in a very weird state if you don't do rigorous error checks.

This means you must check errors after. every. statement. that. can. fail. Be prepared to write a lot of If Err.Number <> 0 Then checks. Note that this is harder to get right than it seems.

Better is: Avoid long sections of code that have On Error Resume Next in effect like the plague. Break up operations into smaller functions/subs that do only one thing instead of writing a big function that does it all but can fail halfway through.

In short: Err.Clear makes your program behave predictably after a failed statement in an On Error Resume Next block. It marks the error as handled. That's its purpose.

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Of course in your sample it's easy to avoid error handling by using the commonly accepted way of checking whether a workbook (i.e. member of a collection) exists.

Function BookOpen(Bk As String) As Boolean
  Dim wb As Variant 

  BookOpen = False ' not really necessary, VB inits Booleans to False anyway

  For Each wb In Application.Workbooks
    If LCase(wb.Name) = LCase(Bk) Then
      BookOpen = True
      Exit For
    End If 
  Next
End Function