public class UserBuilder
    {
        private User _user;

        public UserBuilder()
        {
            _user = new User
            {
                FirstName = Guid.NewGuid().ToString(),
                LastName = Guid.NewGuid().ToString(),
                Password = Guid.NewGuid().ToString(),
                CardNumber = Guid.NewGuid().ToString()
            };
        }

        public User Build()
        {
            return _user;
        }
    }

 [TestFixture]
    public class MainPageViewModelTest
    {
+
        public MainPageViewModelTest()
        private Mock<INavigationService> _navigationService;

        [SetUp]
        public void Setup()
        {
            _navigationService = new Mock<INavigationService>();
        }

        [Test]
        public void ShouldGiveTheActiveUserACardnummer_AsUserName()
        {
            var user = new UserBuilder().Build();
            UserRepository.SetActiveUser(user);

            User retrievedUser = UserRepository.GetActiveUser();
            var tempSut = new MainPageViewModel(_navigationService.Object);

            Assert.AreEqual(tempSut.UserName, retrievedUser.CardNumber);
        }
    }