Rhino Mocks Error: Previous method 'IEnumerator.MoveNext();' requires a return value or an exception to throw

Question

I have the following test code:

parentViewModel = MockRepository.GenerateMock<IParentViewModel>();
parentViewModel.Expect(x => x.GetPropertyValue<IEnumerable<Milestone>>("JobMilestones")).Return(new Milestone[0]);

viewModel = new JobPenaltiesViewModel(j, new Penalty[0], _opContext, parentViewModel);

Assert.That(viewModel.Milestones.Count(), Is.EqualTo(0));
parentViewModel.VerifyAllExpectations();

List<string> propsChanged = new List<string>();
viewModel.PropertyChanged += (s, e) => propsChanged.Add(e.PropertyName);

parentViewModel.Raise(x => x.PropertyChanged += null, parentViewModel, new PropertyChangedEventArgs("JobMilestones"));

AssertPropertiesChangedAsExepected(propsChanged, 1, "Milestones");

Milestone m1 = GenerateMilestone(j);
List<Milestone> milestones1 = new List<Milestone> { m1 };
parentViewModel.Expect(x => x.GetPropertyValue<IEnumerable<Milestone>>("JobMilestones")).Return(milestones1).Repeat.Any();

IEnumerable<Milestone> milestones = viewModel.Milestones;
Assert.That(milestones.Count(), Is.EqualTo(1));
parentViewModel.VerifyAllExpectations();

All the tests and assertions succeed up until the:

Assert.That(milestones.Count(), Is.EqualTo(1));

That's where I get the exception:

Previous method 'IEnumerator.MoveNext();' requires a return value or an exception to throw.

I've tried everything I can think of, and my testing seems to indicate that the parentViewModel Mock is returning null, or an empty enumeration (i.e. when I use the debugger to inspect the returned value the 'Results View' says the enumeration returned no results).

What am I missing here?

Answer 1

milestones.Count() is executing like that (as this is an IEnumerable object):

1. Set counter to 0.
2. Get the first element.
3. Add 1 to the counter.
4. Move to the next element.
5. Step 3 until next element is null
6. Return the counter

So I suggest you to get some rewriting.

Option 1:

1. Create not IEnumerable collection, but some stronger object, like List or Array:

   var milestones = viewModel.Milestones.ToArray();
//var milestones = viewModel.Milestones.ToList();

   After that you can use respectively Count and Length property for the Assert check:

   Assert.That(milestones.Count, Is.EqualTo(1));
//Assert.That(milestones.Length, Is.EqualTo(1));

2. Create a local variable to store count parameter:

   var count = viewModel.Milestones.Count(); // .Count() method executes here. Assert.That(count, Is.EqualTo(1));