Why would VBA TypeOf operator fail

Question

I have been fighting with an Excel 2007 problem for several days now. Below is a listing of all facts I can think of that might be relevant:

1. IDetailShee

Answer 1

I found this question after posting my own similar issue as TypeOf fails to work with Excel workbook's ActiveSheet that implements interface

I don't have a definitive explanation, but I think I do have a workaround.

I suspect it is because [the code] is implementing an interface on Sheet1 or Chart, and is extending Sheet1/Chart1, but Sheet1 is already extending Worksheet (and Chart1 is already extending Chart).

In my testing, I am able to force VBA to return the real value of TypeOf by firstly accessing a property of the sheet. That means, doing something ugly like:

'Explicitly access ThisWorkbook.ActiveSheet.Name before using TypeOf
If TypeOf ThisWorkbook.Sheets(ThisWorkbook.ActiveSheet.Name) Is PublicInterface Then

Answer 2

There are a few ways you could cheat using CallByName. You're going to have to work around this bug one way or another.

A quick dirty example

Every sheet that starts with an implementing line should have a public GetType function. I attached the "TestSheet" sub to a button on my ribbon. It puts the returned type name in cell A1 to demonstrate the function.

Module1

'--- Start Module1 ---
Option Explicit

Public Sub TestSheet()
  Dim obj As Object
  Set obj = ActiveSheet
  ActiveSheet.[A1] = GetType(obj)
End Sub

Public Function GetType(obj As Object) As String
  Dim returnValue As String
  returnValue = TypeName(obj)
  On Error Resume Next
  returnValue = CallByName(obj, "GetType", VbMethod)
  Err.Clear
  On Error GoTo 0
  GetType = returnValue
End Function
'--- End Module1 ---

Sheet1

'--- Start Sheet1 ---
Implements Class1
Option Explicit

Public Function Class1_TestFunction()
End Function

Public Function GetType() As String
    GetType = "Class1"
End Function
'--- End Sheet1 ---

Answer 3

If you're not trusting TypeOf, plough on and ignore errors:

Dim sht as EXCEL.WorkSheet
For Each sht in ActiveWorkbook.Worksheets
  'If TypeOf sht is IDetailSheet Then
  Dim DetailSheet As IDetailSheet
  On Error Resume Next
  Set DetailSheet = sht
  On Error GoTo 0
  If Not DetailSheet Is Nothing Then
    DetailSheets.Add DetailSheet, DetailSheet.Name
  End If
Next sht

If this doesn't work, the worksheets really aren't IDetailSheet at that time at least.