Question

Why does this extension:

<Extension()> _
    Public Function GetLastItem(ByVal l As List(Of Object)) As Object
        Return l(l.Count - 1)
    End Function

Work for instance of type List(of Object) but not for instance of type List(Of List(Of Object))?

Object is the most generic type in .net so List(of Object) is an Object and the extension should work?

Answer 1

Because a List(Of List(Of Object)) isn’t an Object.

Rewrite your extension like this:\

<Extension()>_
Public Function GetLastItem(ByVa l As List(Of T) As T
  Return l(l.Count - 1)
End Function

In fact, this is even more general:

<Extension()>_
Public Function GetLastItem(ByVa l As IEnumerable(Of T) As T
  Return l.ElementAt(l.Count() - 1)
End Function

Of course, you should just use Linq:

someListOfThings.Last();