public async void AddingStringQuestion()
        {
            var questionnaire    = _dbMock.Object.Questionnaires.First(q => q.Id == 1);
            var expectedQuestion = _dbMock.Object.Questions.OfType <StringQuestion>().First();

            var question = await _questionnaireService.AddQuestion(questionnaire.Id, expectedQuestion);

            Assert.NotNull(_questionnaireService.GetQuestionFromQuestionnaire(questionnaire.Id, question.Id));
            Assert.Equal(expectedQuestion.Contents, question.Contents);

            _dbMock.Verify(x => x.SaveChangesAsync(), Times.Once);
        }