Why are my WCF Async with Rx unit tests unstable? - system.reactive

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.

Answer 1

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

Answer 2

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.