Excel VBA: On Error Goto statement not working ins

问题:

I'm trying to cycle through a table in excel. The first three columns of this table have text headings, the rest of them have dates as headings. I want to assign those dates, sequentially, to a Date-type variable, and then perform some operations based on the date

To do this I am using a foreach loop on myTable.ListColumns. Since the first three columns do not have date headers, I have tried to set the loop up so that, if there is an error assigning the header string to the date-type variable, the loop goes straight to the next column

This seems to work for the first column. However, when the second column's header is 'assigned' to the date-type variable, the macro encounters an error even though it is within an error-handling block

Dim myCol As ListColumn
For Each myCol In myTable.ListColumns
    On Error GoTo NextCol

    Dim myDate As Date
    myDate = CDate(myCol.Name)

    On Error GoTo 0

    'MORE CODE HERE

NextCol:
    On Error GoTo 0
Next myCol

To reiterate, the error is thrown on the second round of the loop, at the statement

myDate = CDate(myCol.Name)

Can anyone explain why the On Error statement stops working?

回答1:

With the code as shown, you're actually still considered to be within the error handling routine when you strike the next statement.

That means that subsequent error handlers are not allowed until you resume from the current one.

A better architecture would be:

    Dim myCol As ListColumn
    For Each myCol In myTable.ListColumns
        On Error GoTo ErrCol
        Dim myDate As Date
        myDate = CDate(myCol.Name)
        On Error GoTo 0
        ' MORE CODE HERE '
NextCol:
    Next myCol
    Exit Sub ' or something '

ErrCol:
    Resume NextCol

This clearly delineates error handling from regular code and ensures that the currently executing error handler finishes before you try to set up another handler.

This site has a good description of the problem:

---

Error Handling Blocks And On Error Goto

An error handling block, also called an error handler, is a section of code to which execution is tranferred via a On Error Goto <label>: statement. This code should be designed either to fix the problem and resume execution in the main code block or to terminate execution of the procedure. You can't use the On Error Goto <label>: statement merely skip over lines. For example, the following code will not work properly:

    On Error GoTo Err1:
    Debug.Print 1 / 0
    ' more code
Err1:
    On Error GoTo Err2:
    Debug.Print 1 / 0
    ' more code
Err2:

When the first error is raised, execution transfers to the line following Err1:. The error hander is still active when the second error occurs, and therefore the second error is not trapped by the On Error statement.

回答2:

You need to add resume of some sorts in your error handling code to indicate the error handling is over. Otherwise, the first error handler is still active and you are never "resolved."

See http://www.cpearson.com/excel/errorhandling.htm (specifically the heading "Error Handling Blocks And On Error Goto" and following section)

回答3:

Follow-up to paxdiablo's accepted answer. This is possible, allowing two error traps in the same sub, one after the other :

Public Sub test()
    On Error GoTo Err1:
    Debug.Print 1 / 0
    ' more code
Err1:
    On Error GoTo -1     ' clears the active error handler
    On Error GoTo Err2:  ' .. so we can set up another
    Debug.Print 1 / 0
    ' more code
Err2:
    MsgBox "Got here safely"
End Sub

Using On Error GoTo -1 cancels the active error handler and allows another to be set up (and err.clear doesn't do this!). Whether this is a good idea or not is left as an exercise for the reader, but it works!

回答4:

Clearing all property settings of the Err object is not the same as resetting the error handler.

Try this:

Sub TestErr()
Dim i As Integer
Dim x As Double
    On Error GoTo NextLoop
    For i = 1 To 2
10:     x = i / 0
NextLoop:
        If Err <> 0 Then
            Err.Clear
            Debug.Print "Cleared i=" & i
        End If
    Next
End Sub

You'll notice the just like the OP, it will catch the error properly when i =1 but it will fail on line 10 when i = 2, even though we used Err.Clear