public void ThrowsArgumentNullExceptionForNullViewModel()
            {
                var model = new Person();
                var vmManager = new ViewModelManager();

                ExceptionTester.CallMethodAndExpectException<ArgumentNullException>(() => vmManager.UnregisterModel(null, model));
            }

            public void RegistersModelForViewModel()
            {
                var model = new Person();
                var vm = new TestViewModel(model);
                var vmManager = new ViewModelManager();

                vmManager.RegisterViewModelInstance(vm);
                vmManager.RegisterModel(vm, model);

                var foundVm = vmManager.GetViewModelsOfModel(model).First();

                Assert.AreEqual(vm, foundVm);
            }

 public void UnregisterViewModelInstance_NotExistingViewModel()
 {
     var manager = new ViewModelManager();
     manager.UnregisterViewModelInstance(new InterestingViewModel());
 }

        public void UnregisterViewModelInstance_ExistingViewModel()
        {
            var manager = new ViewModelManager();
            var viewModel = new InterestingViewModel();

            manager.RegisterViewModelInstance(viewModel);
            manager.UnregisterViewModelInstance(viewModel);
        }

        public void UnregisterViewModelInstance_Null()
        {
            var manager = new ViewModelManager();

            ExceptionTester.CallMethodAndExpectException<ArgumentNullException>(() => manager.UnregisterViewModelInstance(null));
        }

 public void RegisterViewModelInstance_ViewModel()
 {
     var manager = new ViewModelManager();
     manager.RegisterViewModelInstance(new InterestingViewModel());
 }

            public void ReturnsNullForUnregisteredModel()
            {
                var model = new Person();
                var vmManager = new ViewModelManager();

                var foundVm = vmManager.GetViewModelsOfModel(model).FirstOrDefault();

                Assert.IsNull(foundVm);
            }

        public void RemoveInterestedViewModelInstance_SecondArgumentNull()
        {
            var manager = new ViewModelManager();

            ExceptionTester.CallMethodAndExpectException<ArgumentNullException>(() => manager.RemoveInterestedViewModelInstance(typeof(InterestingViewModel), null));
        }

            public void ReturnsViewModelForMultiRegisteredViewModel()
            {
                var firstvm = new TestViewModel(){FirstName = "Rajiv", LastName = "Mounguengue"};
                var secondvm = new TestViewModel();
                var vmManager = new ViewModelManager();

                vmManager.RegisterViewModelInstance(firstvm);
                vmManager.RegisterViewModelInstance(secondvm);
                var foundvm = vmManager.GetFirstOrDefaultInstance<TestViewModel>();

                Assert.AreEqual(firstvm, foundvm);
            }

            public void ReturnsViewModelForRegisteredViewModel()
            {
                var vm = new TestViewModel();
                var vmManager = new ViewModelManager();

                vmManager.RegisterViewModelInstance(vm);
                var foundvm = vmManager.GetFirstOrDefaultInstance<TestViewModel>();

                Assert.AreEqual(vm, foundvm);
            }

            public void ReturnsNullForUnregisteredViewModel()
            {
                var vmManager = new ViewModelManager();

                var foundvm = vmManager.GetFirstOrDefaultInstance<TestViewModel>();

                Assert.IsNull(foundvm);
            }

            public void ReturnsChildViewModelsUsingParentInstance()
            {
                var parentViewModel = new TestViewModel() as IRelationalViewModel;
                var childViewModel = new TestViewModel() as IRelationalViewModel;
                var viewModelManager = new ViewModelManager();

                parentViewModel.RegisterChildViewModel(childViewModel as IViewModel);
                childViewModel.SetParentViewModel(parentViewModel as IViewModel);

                viewModelManager.RegisterViewModelInstance(parentViewModel as IViewModel);
                viewModelManager.RegisterViewModelInstance(childViewModel as IViewModel);

                var foundViewModels = viewModelManager.GetChildViewModels(parentViewModel as IViewModel);

                Assert.IsNotNull(foundViewModels);
                Assert.IsTrue(foundViewModels.Contains(childViewModel));
            }

            public void ReturnsNullForUnregisteredChildViewModels()
            {
                var viewModelManager = new ViewModelManager();

                var foundViewModels = viewModelManager.GetChildViewModels(42);

                Assert.AreEqual(0, foundViewModels.Count());
            }

        public void AddInterestedViewModelInstance_ViewModelForNotExistingInterestedInViewModel()
        {
            var manager = new ViewModelManager();

            var interestedViewModel = new InterestedViewModel();

            manager.AddInterestedViewModelInstance(typeof(InterestingViewModel), interestedViewModel);
        }

        public void RemoveInterestedViewModelInstance_FirstArgumentNull()
        {
            var manager = new ViewModelManager();

            ExceptionTester.CallMethodAndExpectException<ArgumentNullException>(() => manager.RemoveInterestedViewModelInstance(null, null));
        }

            public void ShouldFailsDueToANonIViewModelType()
            {
                var viewModelManager = new ViewModelManager();

                ExceptionTester.CallMethodAndExpectException<ArgumentException>(() => viewModelManager.GetFirstOrDefaultInstance(typeof(Type)));
            }

        public void RemoveInterestedViewModelInstance_ViewModelForNotRegisteredInterestedInViewModel()
        {
            var manager = new ViewModelManager();

            var interestedViewModel = new InterestedViewModel();

            manager.RemoveInterestedViewModelInstance(typeof(InterestingViewModel), interestedViewModel);
        }

            public void MustBeNotNullAfterConstructed()
            {
                var vmManager = new ViewModelManager();

                Assert.IsNotNull(vmManager.ActiveViewModels);
            }

            public void MustExistsForRegisteredViewModels()
            {
                var firstvm = new TestViewModel();
                var secondvm = new TestViewModel();
                var vmManager = new ViewModelManager();

                vmManager.RegisterViewModelInstance(firstvm);
                vmManager.RegisterViewModelInstance(secondvm);

                var vmList = vmManager.ActiveViewModels.ToList();

                Assert.IsTrue(vmList.Any(vm => TagHelper.AreTagsEqual(vm.UniqueIdentifier, firstvm.UniqueIdentifier)));
                Assert.IsTrue(vmList.Any(vm => TagHelper.AreTagsEqual(vm.UniqueIdentifier, secondvm.UniqueIdentifier)));
            }

            public void UnregistersAllModelForViewModel()
            {
                var model = new Person();
                var vm = new TestViewModel(model);
                var vmManager = new ViewModelManager();

                vmManager.RegisterViewModelInstance(vm);
                vmManager.RegisterModel(vm, model);
                
                var foundVm = vmManager.GetViewModelsOfModel(model).First();

                Assert.AreEqual(vm, foundVm);

                vmManager.UnregisterAllModels(vm);

                foundVm = vmManager.GetViewModelsOfModel(model).FirstOrDefault();

                Assert.IsNull(foundVm);
            }