Moq testing Delete method

I am trying to test my delete method for my service, to do this I am trying to add an item to the repository first. However, I don't think my mock repository add method is being called because _mockRepository.Object.GetAll() is always null. I have tried stepping in with the debugger and it just skips over it too. What am I doing wrong?

public class CommentServiceTest
{
    private Mock<IRepository<Comment>> _mockRepository;
    private ICommentService _service;
    private ModelStateDictionary _modelState;

    public CommentServiceTest()
    {
        _modelState = new ModelStateDictionary();
        _mockRepository = new Mock<IRepository<Comment>>();
        _service = new CommentService(new ModelStateWrapper(_modelState), _mockRepository.Object);
    }

    [Fact]
    public void Delete()
    {
        var comment = new Comment("BodyText")
        {
            Audio = new Audio(),
            Date = DateTime.Now
        };

        _mockRepository.Object.Add(comment);
        //Nothing in repository collection here still
        var commentToDelete = _mockRepository.Object.GetAll().First();
        _service.Delete(commentToDelete);

        Assert.DoesNotContain(commentToDelete, _mockRepository.Object.GetAll());
    }
}

public class Repository<T, TC> : IRepository<T> where T : class where TC : DbContext
{
    private bool _disposed;

    protected TC Context { get; }

    public Repository()
    {

    }

    public Repository(TC context)
    {
        Context = context;
    }

    public virtual IQueryable<T> GetAll()
    {
        return Context.Set<T>();
    }

    public virtual void Add(T entity)
    {
        Context.Set<T>().Add(entity);
        Save();
    }

    public virtual void Save()
    {
        Context.SaveChanges();
    }   
}

}