How to mock subs with natural mocks

Imports MbUnit.Framework
Imports TypeMock
<TestFixture()> _
Public Class StringListTest
    <Test()> _
    Public Sub SomeTest()
        Dim mock As Mock = MockManager.Mock(GetType(StringList), Constructor.NotMocked)
        mock.ExpectAndReturn("Fill", New DynamicReturnValue(AddressOf Me.DynamicFill))

        'Execute code:
        Dim list As New StringList

        list.doSomething()

        'Verify results:
        Assert.AreEqual(2, list.Count)
        Assert.AreEqual("mockString1", list(0))
    End Sub

    Function DynamicFill(ByVal parameters() As Object, ByVal context As Object) As Object
        Dim list As StringList = DirectCast(context, StringList)
        list.Add("mockString1")
        list.Add("mockString2")
        Return Nothing
    End Function
End Class

Public Class StringList
    Inherits List(Of String)
    Public Sub Fill()
        Me.Add("t1")
        Me.Add("t2")
    End Sub

    Public Sub doSomething()
        Me.Fill()
        For Each s As String In Me
            Trace.WriteLine(s)
        Next
    End Sub
End Class