I have in interface called iproduct, that needs to define that a collection needs to be implemented how is this done.

Or would it just be done in the class that implements the interface, i'm trying to build a fully OO shooping cart for some sites that I gotta build.

Hi,

How about: 

Public Interface iProduct

    Property someCollection() As ICollection

    Property ProductID() As Integer

    Property Quantity() As Integer

    Property Price() As Decimal

End Interface  

Public Class Product
    Implements iProduct

    Public Property someCollection() As System.Collections.ICollection Implements iProduct.someCollection
        Get

        End Get
        Set(ByVal value As System.Collections.ICollection)

        End Set
    End Property

Having the shopping cart collection inside of the product seems to be a case of the cart before the horse...no pun intended. If you have two products, do they each contain their own shiopping cart collection?

I do not believe you would want the item interface to contain the shopping cart interface.

Products are products - not collections
The shopping cart is a collection of Products.
In .Net 2.0, you could express that collection with Generics.
In .Net 1.1, you could create a ShoppingCart class that inherits from CollectionBase.
Then you add type specific access methods so the cart only accepts Products.

I was thinking that a product would contain a collection of additional definable properties, eg
If the product as a t-shirt you could define size and color.
Hence a collection inside of the product.