how can I test method from service when I use unit of work and repository pattern

Question

How can I test ClassifyComment() method from my service. I have that test code:

[TestClass]
public class SpamServiceTest
{
    [TestMethod]
    public void ClassifyCommentTest()
    {
        var spamComments = Builder<Comments>.CreateListOfSize(10).All().With(x => x.Content = "spam spam spam")
            .Build().AsQueryable();

        var mocker = new AutoMoqer();

        mocker.GetMock<IUnitOfWork>()
                .Setup(x => x.CommentsRepository.GetComments(It.Is<bool>(y => y == true)))
                .Returns(spamComments);

        //.......
    }
}

But it gives me error: Can not instantiate proxy of class: CommentsRepository. Could not find a parameterless constructor.

Below is my code which I want test:

public class SpamService : ISpamService
{
    private readonly IUnitOfWork _unitOfWork;

    public SpamService(IUnitOfWork unitOfWork)
    {
        _unitOfWork = unitOfWork;
    }

    public bool ClassifyComment(Comments comment)
    {
        var spam = _unitOfWork.CommentsRepository.GetComments(true).ToList();
        //.............
    }
}

public class UnitOfWork : IUnitOfWork
{
    private DatabaseContext context = new DatabaseContext();
    private CommentsRepository commentsRepository;

    public CommentsRepository CommentsRepository
    {
        get
        {
            if (this.commentsRepository == null)
            {
                this.commentsRepository = new CommentsRepository(context);
            }
            return commentsRepository;
        }
    }          
}

public class CommentsRepository : ICommentsRepository
{
    private DatabaseContext context;

    public CommentsRepository(DatabaseContext context)
    {
        this.context = context;
    }

    public virtual IQueryable<Comments> GetComments(bool isSpam)
    {
        //.......
    }
}

Answer 1

IUnityOfWork should return a ICommentsRepository , ie an interface, not an implementation. The mock of IUnityOfWork should return a mock of ICommentsRepository.

Let the abstraction work with other abstractions, not with implementations.