Excel VBA: Why "Sheets" collection is not a "Collection"?

Question

A simple question on Excel VBA: If "Sheets" object is a collection, why the following sentence returns false? (entered at the Inmediate panel)

debug.print TypeOf Sheets Is Collection
Falso

I have discovered it because I made a function that takes a Collection as parameter. The function works with variables that I have declared as Collection, but doesn't work with other collections (such as Sheets collection).

Perhaps VBA is failing in inheritance and/or polymorphism?

Edit: As JosieP and mehow have explained, the Sheets collection doesn't inherits from the Collection class/interface, so it is not a Collection, and can't be used polymorphically as a Collection.

So now the question is: Why Sheets isn't a subclass of Collection? Why they haven't inherited from Collection? It should be a piece of cake, given the methods/properties already implemented into Sheets. As I can see, the Collection class/interface only requires 4 methods:

Add
Count
Item
Remove

And Sheets already implements 4 similar methods/properties:

Add
Count
Item
Delete

So the mapping would be trivial. They could have converted Sheets collection into a Collection easily.

Edit 2: I thank mehow for the suggestion on asking MS, but I won't do it, since they will probably say "it is not a design bug, it is a feature" :). More food for thinking: Run the following code in a new workbook:

Sheets.Add.name = "sh2"
Sheets.Add.name = "sh3"

Dim col As Collection
Set col = New Collection
col.Add Sheets(1)
col.Add Sheets(2)
col.Add Sheets(3)

For Each ele In col
    Debug.Print "ele in col:  " & ele.name
Next ele

For Each ele In Sheets
    Debug.Print "ele in Sheets:  " & ele.name
Next ele

It is strange to me that both "col" and "Sheets" can be iterated the same way, with a "for each" loop, but they don't share a common interface. If I had to design the classes, the hierarchy would be:

    iIterable       (interface)
        |
        |
    iCollection     (interface)
       / \
      /   \
     /     \
Collection  cSheets    (classes)

and Sheets would be an object (instance) of cSheets class.

In this way, the function I made (that currently takes a Collection parameter), would take a iCollection parameter, so it could work both with a Collection instance and with Sheets.

Excel VBA: Why "Sheets" collection is not a "Collection"?

1 Answers