I have recently discovered that VB.NET already^* supports duck typing and without resorting to generics too boot. The solution is purely dynamic, just like with Ruby. Here's the analogous code sample I've been using in my other post about duck typing:

Class Customer
    Dim _first_name, _last_name As String
    Public Sub New(ByVal first_name As String, ByVal last_name As String)
      _first_name = first_name
      _last_name = last_name
    End Sub
    Public Sub append_name_to_file(ByVal file)
      file &= _first_name & _last_name
    End Sub
  End Class

I used different operator here (&) but the point is that file parameter has no type declared, is not implicitly treated as Object and is dynamically checked for existence of said operator (&). Quite cool.

^*Tested with Visual Studio 2005 which will be available in 7 weeks. I think this won't work in 2003 (at least not in this manner).