public void ChangeFirstnameTest()
        {
            //Arrange
            int index    = 0;
            var testUser = GetUserCollection().ElementAt(index);

            //Act
            userMockRepository.Setup(x => x.Get(index.ToString())).Returns(GetUserCollection().ElementAt(index));
            mapper.Setup(x => x.Map <User>(It.IsAny <User>())).Returns(GetUserCollection().ElementAt(index));
            userService.ChangeFirstname(testUser, "NewFirstname");

            //Assert
            Assert.Equal(testUser.FirstName, "NewFirstname");
        }