Question

For the problem I have these classes:

Public MustInherit Class BaseLeaf(Of T)
    Implements IBaseLeaf
    // etc

Public Class WebsiteLeaf
    Inherits BaseLeaf(Of Headline)
    // etc

Public Class WebsiteCollection
    Inherits BaseCollection(Of WebsiteLeaf)
    // etc

Public Class SubscriptionList
Private mCollection As BaseCollection(Of IBaseLeaf)

Public Sub LoadSubscriptions(ByVal collection As BaseCollection(Of IBaseLeaf))
    mCollection = collection
End Sub

In the Main class I am trying to call the following function:

Private Sub FetchSubscriptions(ByVal websites As WebsiteCollection)
        gUser.SubscriptionList.LoadSubscriptions(websites)
        // code
End Sub

This will access LoadSubscriptions with passing the “websites” variable.

As you can see LoadSubscriptions expects a BaseCollection(Of IBaseLeaf).

The “websites” variable is a WebsiteCollection, which is a BaseCollection(Of WebsiteLeaf(Of Headline)).

Now I am getting the error: Value of type ‘WebsiteCollection’ cannot be converted to ‘BaseCollection(Of IBaseLeaf)’.

What I am doing wrong here?

Answer 1

If class B is a descendent of class A this does not mean that Collection(Of B) is a descendant of Collection(Of A)!

If you have these definitions

Dim stringList As List(Of String)

Public Sub DoSomthingWithList(list As List(Of object))
     list.Add(Date.Now)
End Sub

and you could call the method like this

DoSomthingWithList(stringList)

then the method would try to add a Date to the list which is actually a list of strings! Since the parameter is typed as object the date would be boxed (i.e. converted to an object) but not converted to string.

Therefore the collections List(Of X) and List(Of Y) are never compatible, even if Y inherits X.

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Let’s look at another example

Public Class TestClass
    Implements IBaseLeaf      
    // etc

What happens if you call LoadSubscriptions with a WebsiteCollection?

Public Sub LoadSubscriptions(ByVal collection As BaseCollection(Of IBaseLeaf))      
    collection.Add(New TestClass())
End Sub