Why are my WCF Async with Rx unit tests unstable?

Question

I am using Rx and RxUI in a MVVM project and have a view model that queries its data from a WCF service asynchronously. In the unit tests I create a mock object that returns a Task with the expected value.

Here's a general idea of what my view model looks like

public class ViewModel : ReactiveObject
{
    private IContext _context;

    public ViewModel(IContext context)
    {
        _context = context;

        Observable.FromAsync(() => _context.WcfServiceCall())
              .Subscribe(result => {
                   Children.AddRange(results.Select(r => new ChildViewModel(r).ToList()));
               });
     }

     public ObservableCollection<ChildViewModel> { get; private set;}
}

My unit test looks like this

[TestFixture]
public class ViewModelTest : AssertionHelper
{
    [Test]
    public void ShouldSetChildren()
    {
        var c = new Mock<IContext>();
        c.Setup(q => q.WcfServiceCall())
            .Returns(Task.Run(() => new []{ 1,2,3,4,5,6 })):

        var vm = new ViewModel(c.Object);
        var p = vm.Children.First(); // this call sometimes fails

        ... 
     }
}

The issue I'm having is that I have over 400 tests that do this sort of thing and they all work most of the time but I randomly get failed tests, one or two at a time, that report the sequence has no values. This is unpredictable and random. I can run the tests again after a failure and all succeed. I have added the TestScheduler as described here but the problems persist.

Is there a better way to test methods that make asynchronous method calls?

Edit from Paul Bett's input: I see FromAsync does not take an IScheduler parameter but I do have SubscribeOn and ObserveOn available.

Alternatively, I could call the WCF async method directly and convert the returned Task to an observable. I'm not sure I understand when it is more appropriate to use Observable.FromAsync versus not using it.

score 1 · Accepted Answer · answered Jan 15 '13 at 23:45

1

Does Observable.FromAsync take an IScheduler parameter? Is it your Test Scheduler?

answered Jan 15 '13 at 23:45

Ana Betts

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I'm not passing IScheduler to FromAsync. If I do, should that be something like RxApp.TaskPoolScheduler? – Doug Mitchell Jan 16 '13 at 02:29
I see FromAsync does not accept an IScheduler parameter but there is ObserveOn and SubscribeOn extension methods. I've edited the original post concerning this. Could this be an issue with the way I'm calling the WCF methods? Do I really need FromAsync since the method is async already? – Doug Mitchell Jan 16 '13 at 16:06

score 0 · Answer 2 · answered Jan 16 '13 at 19:11

I realize that my unit tests were incorrect in that they were touching too many moving parts. I have tests that verify that the mocked web service calls are being made when expected and I really shouldn't include that complexity in tests that aren't focused on that test point.

I'm calling the wcf service when navigating to the view model. Here's a better representation of my view model with minor changes regarding specifying an IScheduler:

public class ViewModel : ReactiveObject, IRoutableViewModel
{
    private IContext _context;
    public ViewModel(IContext context)
    {
        _context = context;
        Weeks = new ReactiveCollection<WeekViewModel>();

        this.WhenNavigatedTo(() =>
        {
            _context.Service.GetWeeksAsync()
                .ToObservable()
                .ObserveOn(RxApp.DeferredScheduler)
                .Subscribe(result =>
                {
                    Weeks.AddRange(result.Select(w => WeekViewModel(w)));
                });
        });

        // ...
    }

    public ReactiveCollection<WeekViewModel> Weeks { get; private set; }
}

Instead of setting up the context to populate the Weeks collection then using the router to navigate to the ViewModel I am now adding objects to the Weeks collection in the unit tests and skipping navigation and the asynchronous web service calls. This has reduced the tests a bit, eliminated the instability as far as I can tell, and reduced the execution time of test suite.

So my unit tests look something like this

[TestFixture]
public class ViewModelTest : AssertionHelper
{
    [Test]
    public void ShouldSetChildren()
    {
        var c = new Mock<IContext>();
        var vm = new ViewModel(c.Object);
        vm.Children.AddRange((new []{1,2,3,4,5,6}).Select(i => new ChildViewModel(i)));

        var p = vm.Children.First(); // this is valid now - no timing issues

        ... 
    }
}

The code behaves correctly in the app but was problematic in the test runner so I believe that this solves my immediate issues.

Why are my WCF Async with Rx unit tests unstable?

2 Answers2