Passing Generic List as Type Parameter

Question

Is there any way to call to get the call to Me.DoSomething2 inside of MainApplication.CallDoSomething to work in the code below.

Public Interface ICovariance(Of Out T As Grandparent) 
End Interface

Public MustInherit Class Grandparent
End Class

Public Class Parent : Inherits Grandparent
End Class

Public Class Child : Inherits Parent
End Class

Public Class CustomList(Of T As Grandparent) : Implements ICovariance(Of T)
End Class

Public Class CustomList2(Of T As Parent)
    Inherits CustomList(Of T)

    Public Sub MyCustomList2Method()
    End Sub
End Class

Public Class MainApplication
    Public Sub CallDoSomething()
        'This Works
        Me.DoSomething(Of CustomList2(Of Child))()

        'This does not work
         Me.DoSomething2(Of CustomList2(Of Child))()
    End Sub

    ' This method works because of the interface and covariance 
    Public Function DoSomething(Of T As {ICovariance(Of Grandparent), New})() As T
        Dim instance As New T

        'This method can't be called because I'm working with an interface
        instance.MyCustomList2Method()
        Return instance
    End Function

    ' This method does not work. 
    ' I want T to be a CustomList2 that contains Parent 
    ' (or anything that inherits Parent)
    Public Function DoSomething2(Of T As {CustomList2(Of Parent), New})() As T
        Dim instance As New T

        'This method would work if I could call this method
        instance.MyCustomList2Method()

        Return instance
    End Function
End Class

I understand that that type parameters on DoSomething2 are expecting a CustomList2(Of Parent), but I'd like to pass a Customlist2(Of Child) to it. I can get this to work with an interface that uses covariance. Using an interface does not allow me toe call methods, such as CustomList2.MyCustomerList2Method.

This is a simplified example Could anyone provide some insight?

Answer 1

Try adding second generic type parameter to your second method:

Public Function DoSomething2(Of T1 As {Parent}, T2 As {CustomList2(Of T1), New})() As T2
    Dim instance As New T2

    'This method would work if I could call this method
    instance.MyCustomList2Method()

    Return instance
End Function

When it looks like that, you can call it like this:

Me.DoSomething2(Of Child, CustomList2(Of Child))()