Mocking extension methods

NSubIssueTests.cs

//I saw this and thought there was a simple way around subbing extension methods
//http://blogs.clariusconsulting.net/kzu/how-to-mock-extension-methods/

//My choice of solution was to just refactor the IRepo interface

namespace NSubIssueTests
{
    [TestFixture]
    public class ExtensionOnInterfaceTests
    {
        [Test]
        public void NSubstitute_ReturnsForAnyArgs_ExpectSubstitutedValue()
        {
            //arrange
            var userRepo = Substitute.For<IRepo<User>>();

            userRepo.AddOrAttach(new User()).ReturnsForAnyArgs(new User { Id = Guid.NewGuid() });

            var mu = new ManageUsers(userRepo);

            //act
            var result = mu.MethodNotBehavingAsExpectedWithSubstitution(new User());

            //assert
            Assert.AreNotEqual(Guid.Empty, result);
        }
    }

    public static class Extensions
    {
        //Here: I thought because it extends an interface 'IRepo<>' there might have been some way to stub here.
        public static TResult AddOrAttach<TSrc, TResult>(this IRepo<TResult> repo, TSrc model) where TSrc : Domain
        {
            throw new NotSupportedException("Because we have ReturnsForAnyArgs() on this extension method do not expect to enter here");

            //because I use the real IRepo here, with void return methods, stubbing it doesn't help
        }
    }

    //-------------------------------------
    //The rest is just to support the above:

    public interface IRepo<TEntity>
    {
        void Add(TEntity e);
        void Attach(TEntity e);
        void AddOrAttach<TSrc, TResult>(TSrc model);
    }

    public class User : Domain { public String UserName { get; set; } }

    public class Domain { public virtual Guid Id { get; set; } }

    public class ManageUsers
    {
        private readonly IRepo<User> _userRepo;

        public ManageUsers(IRepo<User> userRepo)
        {
            _userRepo = userRepo;
        }

        public Guid MethodNotBehavingAsExpectedWithSubstitution(User u)
        {
            var result = _userRepo.AddOrAttach(u);
            
            return result.Id;
        }
    }

    public class BasicRepo<T> : IRepo<T>
    {
        public void Add(T e)
        {
            throw new NotImplementedException();
        }

        public void Attach(T e)
        {
            throw new NotImplementedException();
        }

        public void AddOrAttach<TSrc, TResult>(TSrc model)
        {
            throw new NotImplementedException();
        }
    }
}