public async Task ShouldNotGetAuthor()
        {
            var expected = new FailedOperationResult <Author>
            {
                Code = ConstOperationCodes.AUTHOR_NOT_FOUND
            };

            var result = await mockCourseLibraryService.Object.GetAuthor(Guid.Empty);

            var comparer = new Comparer();

            Assert.True(comparer.Compare(result, expected));
        }
        public async Task ShouldNotAddAuthor()
        {
            var authorToAdd = new Author
            {
                FirstName = "exsits"
            };
            var expected = new FailedOperationResult <Author>
            {
                Code = ConstOperationCodes.AUTHOR_NAME_ALREADY_EXISTS
            };
            var result = await mockCourseLibraryService.Object.AddAuthor(authorToAdd);

            var comparer = new Comparer();

            Assert.True(comparer.Compare(result, expected));
        }