using System;

using Moq;

public class Program

{

    public static void Test<T>()

    {

        var mock = new Mock<IObserver<T>>();

        mock.Setup(s => s.OnCompleted())

            .Callback(() => Console.WriteLine("actually called"));

​

        var observable = mock.Object;

        observable.OnCompleted();

​

        mock.Verify(x => x.OnCompleted(), Times.AtLeastOnce);

    }

    public static void Main()

    {

        Test<int>();

    }

}