//[ExpectedException]
        public void UpdateTagNames_With_Empty_Tags()
        {
            var tagRepository = Substitute.For<ITopicTagRepository>();
            var topicRepository = Substitute.For<ITopicRepository>();
            var topicTagService = new TopicTagService(tagRepository, topicRepository);
            const string oldName = "";
            const string newName = "";
            tagRepository.GetTagName(oldName).Returns(x => null);

            topicTagService.UpdateTagNames(newName, oldName);

            tagRepository.DidNotReceiveWithAnyArgs().Update(Arg.Is<TopicTag>(x => x.Tag == null));
        }
        public void UpdateTagNames_With_Non_Existing_Tag()
        {
            var tagRepository = Substitute.For<ITopicTagRepository>();
            var topicRepository = Substitute.For<ITopicRepository>();
            var topicTagService = new TopicTagService(tagRepository, topicRepository);
            var oldName = "bilbo";
            var newName = "baggins";
            tagRepository.GetTagName(oldName).Returns(x => null);

            topicTagService.UpdateTagNames(newName, oldName);

            tagRepository.DidNotReceiveWithAnyArgs().Update(Arg.Is<TopicTag>(x => x.Tag == null));
        }