Task ToObservable Error Handling

Question

I have the following method which is responsible for calling my service class and passing the results off to another method for saving them in my db:

public IObservable<bool> SyncSessions()
{
    var subject = new ReplaySubject<bool>();

    try
    {
        var query = new ByFilterQuery { SearchPeriodStartTime = DateTime.Now };
        var sessions = _sessionService.GetSessions(query).Result;
        var saved = SaveSessions(sessions);
        subject.OnNext(saved);
        subject.OnCompleted();
    }
    catch (Exception ex)
    {
        subject.OnError(ex);
    }

    return subject;
 }

_sessionService.GetSessions will throw a HttpRequestException if the server returns a 500 or something similar. I have a unit test which mocks this behaviour and want to test my method handles the error gracefully.

Is there any better ways of propagating the error in the Rx fashion? I tried doing:

_sessionService.GetSessions(query).ToObservable().Select(SaveSessions);

But this threw my error instead of it being passed to the error handling action in the calling method. I also plan to merge this method with several others and handle the errors in a combined manor.

EDIT: Here is how I was subscribing to the observable

Exception error = null;
_sessionManager
    .SyncSessions()
    .Subscribe(null, e => error = e);

Assert.That(error, Is.Not.Null.After(500));

I'm passing null into the first parameter as I don't really care about that in the context of this test

Answer 1

First, for the original code you posted, I recommend using an AsyncSubject instead of a ReplaySubject. Think of it as being optimized for the single-result case.

Now on to your question...

Whenever an observable source generates an exception, that exception is propagated through the OnError handler. If, when you subscribe to the observable, you do not supply an OnError handler, then the exception will be thrown.

The small code you posted:

_sessionService.GetSessions(query).ToObservable().Select(SaveSessions);

is not enough. Without also posting the code that shows how you subscribe to the observable I can only guess that you did not supply an OnError handler.

Edit based on OP's test code

Is your mock service returning a Task that throws an exception, or is the mock service just throwing the exception? Unless you are using async/await, a function that is supposed to return a Task, but throws an exception while creating that Task will raise the exception immediately instead of returning a failed Task. ToObservable never even gets called because GetSessions throws. If you change your mock service to return a failed Task then your test will probably work correctly.

If you want to catch immediate exceptions as well as failed tasks, then you can use Defer to defer execution of your method until an observer subscribes. It will also collect any immediately thrown exceptions in addition to failed tasks:

return Observable
    .Defer(() => _sessionService.GetSessions(query))
    .Select(SaveSessions);