How to properly mock extension methods with generics in xUnit?

Question

So I'm trying to mock a service of mine, here's the real code:

public class PhaseService : IPhaseService
    {
        private readonly IRepository<Phase> _phaseRepository;
        private readonly IMapper _mapper;
        private readonly HrbContext _context;

        public PhaseService(IRepository<Phase> phaseRepository, IMapper mapper, HrbContext context)
        {
            _phaseRepository = phaseRepository;
            _mapper = mapper;
            _context = context;
        }

        public async Task<PhaseDto> GetAsync(Guid id)
        {
            var result = await _phaseRepository.GetActiveAsync(id);
            return _mapper.Map<PhaseDto>(result);
        }
}

It uses an Extension Method, this is here:

namespace HRB_Server.Application.Extensions
{
    public static class RepositoryExtensions
    {
        /// <summary>
        /// Returns the entity to which the given id is a match (no navigation properties loaded). Throws exceptions if the entity is not found or if is not active.
        /// </summary>
        public static async Task<T> GetActiveAsync<T>(this IRepository<T> repo, Guid id)
            where T : BaseEntity
        {
            T entity = await repo.GetAsync(id);

            if (entity == null)
            {
                throw new EntityNotFoundException(typeof(T), id);
            }

            if (!entity.IsActive)
            {
                throw new EntityNotActiveException(typeof(T), id);
            }

            return entity;
        }
}

Here's my xUnit test:

namespace HRB_Server.Tests.Services
{
    public class PhaseServiceTest
    {
        private readonly Mock<IRepository<Phase>> _repository;
        private readonly Mock<IMapper> _mapper;
        private readonly Mock<HrbContext> _context;

        public PhaseServiceTest()
        {
            _repository = new Mock<IRepository<Phase>>();
            //_mapper = new Mock<IMapper>();
            _mapper = null;
            //_context = new Mock<HrbContext>(new DbContextOptions<HrbContext>(), new HttpContextAccessor());
            _context = null;
        }

        [Fact]
        public void GetPhase_ActivePhaseObject_PhaseShouldExist()
        {
            // Arrange
            var newGuid = Guid.NewGuid();
            var phase = GetSamplePhase(newGuid);

            _repository.Setup(x => RepositoryExtensions.GetActiveAsync<Phase>(_repository, It.IsAny<Guid>()))
                .Returns(GetSamplePhase(newGuid));

            var phaseService = new PhaseService(_repository.Object, _mapper.Object, _context.Object);

            // Act
            var result = phaseService.GetAsync(newGuid);

            // Assert (expected, actual)
            Assert.Equal(phase.Result.Id, newGuid);
        }
}

The error I'm getting is in the Setup of the _repository:

repository.Setup(x => RepositoryExtensions.GetActiveAsync<Phase>(_repository, It.IsAny<Guid>()))
                    .Returns(GetSamplePhase(newGuid));  

It says it cannot convert the Mocked repository to a real one, but shouldn't I use the mocked repository here?

What I'm trying to achieve is testing my REAL service and mocking the repository, right? Am I doing it correctly here?

Answer 1

Assuming you are using MOQ, do not try to mock the extension method.

Since you control the code of the extension method then mock a safe path through the extension method.

The extension uses GetAsync in this case and that is what needs to be mocked assuming that is not an extension as well.

//...

 _repository
    .Setup(x => x.GetAsync(It.IsAny<Guid>()))
    .ReturnsAsync(GetSamplePhase(newGuid));

//...

It will allow the test when exercised to go through GetActiveAsync code and if it fails, also throw the Exceptions etc as described in the code.