You can enhance this collection to provide additional features. To make the collection strongly typed, modify the Add method so its Item parameter has a specific data type. You can also add data validation. The following code allows the program to add only OrderItem objects to the collection. It also verifies that each OrderItem has a non-blank Name value and a Quantity of at least 1.

When it adds a new item, it raises the ItemAdded event so the main program can take action if necessary. You could add siilar events to other methods. For example, you could raise a BeforeRemove event in the Remove subroutine. If you pass a Cancel parameter to the event ByRef, the main program could set Cancel to True to tell the Remove subroutine to cancel the removal.

Public Event ItemAdded()

' Add an item to the collection.
Public Sub Add(ByVal Item As OrderItem, Optional ByVal key _
    As Variant, Optional ByVal before As Variant, Optional _
    ByVal after As Variant)
    ' Make sure the new item contains valid data.
    If Len(Item.Name) = 0 Then
        Err.Raise 1, "OrderItemCollection.Add", _
            "OrderItem must have a non-blank Name."
    End If
    If Item.Quantity < 1 Then
        Err.Raise 1, "OrderItemCollection.Add", _
            "OrderItem must have Quantity > 0."
    End If

    ' Add the item.
    m_Items.Add Item, key, before, after

    ' Raise the ItemAdded event.
    RaiseEvent ItemAdded
End Sub