Async method does not return asp.net mvc 4

Question

I'm having a problem with an async method that I implemented. The method basically makes a HttpRequest to a resource and deserializes the string if the request is successful. I wrote a test for the method, and it works. But the method does never return when I call it from a controller?

    public async Task<IEnumerable<T>> Get()
    {
        try
        {
            var resourceSegmentUri = new Uri(_uri, UriKind.Relative);

            var response = await _client.GetAsync(resourceSegmentUri);

            if (response.IsSuccessStatusCode)
            {
                var submission = await response.Content.ReadAsStringAsync();
                return JsonConvert.DeserializeObject<IEnumerable<T>>(submission);
            }

            if (response.Content != null)
            {
                var message = response.Content.ReadAsStringAsync();
                throw new WebException(message.Result, (WebExceptionStatus)response.StatusCode);
            }

        }
        catch (WebException e)
        {
            Logger.Error("GET Request failed with status: {0}", e.Status);
            throw;
        }

        throw new Exception();
    }

Code that never returns:

public ActionResult Index()
{
   var api = new Api();
   var test = api.Get().Result; //Never returns
   return View();
}

Test that works:

[Test]
public void GetShouldReturnIfSuccessfulRequest()
{
    var api = new Api();
    var submission = api.Get();

    Console.WriteLine(JsonConvert.SerializeObject(submission));
    Assert.NotNull(submission);
}

Does anyone know the problem?

I'm not sure I understand your question. There is no return ...; statement in your Index() method, therefore it can't return anything. — Dennis Traub
– Dennis Traub, Commented Apr 16, 2013 at 10:56
Sorry missed the return statement, but it's inrelevant really. — Bj Blazkowicz
– Bj Blazkowicz, Commented Apr 16, 2013 at 11:01
Change your test so that is gets the result of the async method otherwise you're not testing what you intended. api.Get() should probably be api.Get().Result — Mårten Wikström
– Mårten Wikström, Commented Apr 16, 2013 at 11:23

Nick Butler · Accepted Answer · 2013-04-16 11:03:22Z

12

You've got a deadlock because you're calling .Result in your controller action.

If you use async/await then you have to use asynchronous actions too.

So something like this should fix it:

public async Task<ActionResult> Index()
{
  var api = new Api();
  var test = await api.Get(); // Should return
}

There's a comprehensive article about this here: Using Asynchronous Methods in ASP.NET MVC 4

answered Apr 16, 2013 at 11:03

Nick Butler

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7 Comments

Bj Blazkowicz Over a year ago

Does not work. I have tried it. Basically in my real solution I wrap the Get() method in a service method that returns a model to the controller.

Nick Butler Over a year ago

@BjBlazkowicz Then the best option would be to make the service method async too. If you start using async/await you have to use it all the way up the call stack.

Bj Blazkowicz Over a year ago

I have made the service method async, still does not work. The thing is that I wrote a test for it in NUnit and it works fine. But the method never returns when calling it from a controller.

Nick Butler Over a year ago

@BjBlazkowicz Just saw your edit - in your test, submission will be the Task not the result, so it's not really testing your method.

Stephen Cleary Over a year ago

+1. @BjBlazkowicz: I explain the cause of the deadlock on my blog and in my recent MSDN article.

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