How to test async exceptions in Akka.NET actor with a ReceiveAsync

Question

I have a try/catch block in both my application and unit test. I'm trying to create a test that catches an exception being thrown in my actor. When debugging through the code I get the exception being thrown but in my test I never receive the exception.

Application:

public class FooActor : ReceiveActor {
    private readonly IFooService fooService;
    private readonly IChildActorFactory childCreatorFactory;
    private IActorRef barActor;

    public FooActor(IFooService fooService, IChildActorFactory childCreatorFactory) {   
        this.fooService = fooService;
        this.childCreatorFactory = childCreatorFactory;

        ReceiveAsync<bool>(async (x) => await StartAsync(x).ConfigureAwait(false));
    }

    protected override void PreStart() {
        barActor = childCreatorFactory.Create<BarActor>(Context, "BarActor");
    }

    public async Task StartAsync(bool start) {
        try {
            if (start) {
                var fooInformation = await fooService.GetInformationAsync().ConfigureAwait(false);

                if (fooInformation != null) {
                    barActor.Tell(fooInformation);
                }
            }
        } catch (Exception exception) {
            throw new Exception($"Unhandled exception. Actor {Self.Path.Name};", exception);
        }
    }
}

Test:

[Fact]
public void StartAsync_ThrowsException_ExceptionThrown() {
    using (var mock = AutoMock.GetLoose()) {
        //Arrange
        Sys.UseAutofac(mock.Container);
        var mockChildActorFactory = mock.Mock<IChildActorFactory>();
        var mockBarService = mock.Mock<IBarService>();

        mockBarService.Setup(x => x.GetInformationAsync()).Throws(new Exception());

        var props = Props.Create(() => new FooActor(mockBarService.Object, mockChildActorFactory.Object));

        var fooActorName = "FooActor";
        var fooActor = new TestActorRef<FooActor>(Sys, props, null, fooActorName);

        try {
            // Act
            fooActor.Receive(true);
        } catch (Exception exception) {
            // Assert
            Assert.Equal($"Unhandled Exception. Actor { fooActorName }.", exception.Message);
        }
    }
}

score 0 · Answer 1 · answered Nov 27 '19 at 20:16

The problem is in the async/await operators in ReceiveAsync method:

ReceiveAsync<bool>(async (x) => await StartAsync(x).ConfigureAwait(false));

when execution context reached the await operation it just start Task at thread pool(simplified) and returns to the caller. I.e. when

fooActor.Receive(true);

completed, the actual task with StartAsync(x) may not be started yet. So when actual exception is thrown your test is already finished without any exception.

How to test async exceptions in Akka.NET actor with a ReceiveAsync

1 Answers1