Question

The codes below are exactly the same, except that one is C# and the other one is VB.Net.
C# compiles just fine, but VB.Net throws the warning:

Interface ‘System.IObserver(Of Foo)’ is ambiguous with another
implemented interface ‘System.IObserver(Of Bar)’ due to the ‘In’ and
‘Out’ parameters in ‘Interface IObserver(Of In T)’

Why does VB.Net show the warning and not C#? And most important, how can I resolve this problem?

Obs: I’m using .Net Framework 4 with Visual Studio 2010 Ultimate.

VB.Net Code:

Module Module1

    Sub Main()

    End Sub

    Public Class Foo
    End Class
    Public Class Bar
    End Class
    Public Class Beholder
        Implements IObserver(Of Foo)
        Implements IObserver(Of Bar)

#Region "Impl"
        Public Sub OnCompleted() Implements System.IObserver(Of Bar).OnCompleted

        End Sub

        Public Sub OnError([error] As System.Exception) Implements System.IObserver(Of Bar).OnError

        End Sub

        Public Sub OnNext(value As Bar) Implements System.IObserver(Of Bar).OnNext

        End Sub

        Public Sub OnCompleted1() Implements System.IObserver(Of Foo).OnCompleted

        End Sub

        Public Sub OnError1([error] As System.Exception) Implements System.IObserver(Of Foo).OnError

        End Sub

        Public Sub OnNext1(value As Foo) Implements System.IObserver(Of Foo).OnNext

        End Sub
#End Region

    End Class

End Module

C# Code:

 class Program {
        static void Main(string[] args) {
        }
    }

    public class Foo { }
    public class Bar { }
    public class Beholder : IObserver<Foo>, IObserver<Bar> {
        #region IObserver<Foo> Members

        public void OnCompleted() {
            throw new NotImplementedException();
        }

        public void OnError(Exception error) {
            throw new NotImplementedException();
        }

        public void OnNext(Foo value) {
            throw new NotImplementedException();
        }

        #endregion

        #region IObserver<Bar> Members


        public void OnNext(Bar value) {
            throw new NotImplementedException();
        }

        #endregion
    }

Answer 1

Summing up:

• VB appears to be giving the warning unnecessarily here. I’ll mention it to the VB testers when they’re back from Christmas vacation.
• This is a suspicious programming practice regardless of whether it is safe or not; it’s a bit strange to implement two versions of the same interface.
• If instead you chose a covariant interface like IEnumerable<T> then the warning would be justified. If you have an object that is both a sequence of Turtles and a sequence of Giraffes, then what happens when you implicitly convert it to sequence of Animal? Do you get Turtles or Giraffes? The runtime just picks one, which is not necessarily the behaviour you want.

For some interesting discussion of the last point see the comments to my 2007 article on the subject:

http://blogs.msdn.com/b/ericlippert/archive/2007/11/09/covariance-and-contravariance-in-c-part-ten-dealing-with-ambiguity.aspx