Question

Does anyone know why the behaviour of throwing exceptions within a PUT action method in a controller in ASP.NET WebAPI would be different from any other actions and verb types?

Essentially I have the code below (I normally have an exception filter globally registered but I’ve disabled that to ensure it isn’t the cause).

public class JourneysController : ApiController
{
    private IJourneyRepository repository = null;

    public JourneysController(IJourneyRepository repository)
    {
        this.repository = repository;
    }

    public async Task<Journey> GetById(string id)
    {
        throw new BusinessException("test1");

        var item = await repository.Get(id);
        if (item == null) throw new HttpResponseException(HttpStatusCode.NotFound);
        return item;
    }

    public async Task<HttpResponseMessage> Post(HttpRequestMessage request, [FromBody]Journey value)
    {
        throw new BusinessException("test2");

        value = await repository.Add(value);

        var response = request.CreateResponse<Journey>(HttpStatusCode.Created, value);

        string uri = Url.Link("DefaultApi", new { id = value.Id });
        response.Headers.Location = new Uri(uri);
        return response;
    }

    public async void Put(string id, [FromBody]Journey value)
    {
        throw new BusinessException("test3");

        value.Id = id;
        if (!await repository.Update(value)) throw new HttpResponseException(HttpStatusCode.NotFound);
    }

    public HttpResponseMessage Delete(string id)
    {
        throw new BusinessException("test4");

        repository.Remove(id);
        return new HttpResponseMessage(HttpStatusCode.NoContent);
    }
}

When I execute (using Chrome Advanced REST client) the GET, POST and DELETE verbs I get a payload like the following (no easy way to copy/paste out of the client):

500 Internal Server Error 

Date: Tue, 25 Sep 2012 15:37:56 GMT 
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Content-Length: 813 
Pragma: no-cache 

{
    "Message": "An error has occurred.",
    "ExceptionMessage": "test4",
    "ExceptionType": "KieranBenton.LeaveNow.Services.Model.BusinessException",
    "StackTrace": " <snip>"
}

However the PUT action returns me a yellow screen of death payload like:

<span><H1>Server Error in '/' Application.<hr width=100% size=1 color=silver></H1>

        <h2> <i>test3</i> </h2></span>

        <font face="Arial, Helvetica, Geneva, SunSans-Regular, sans-serif ">

        <b> Description: </b>An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

        <b> Exception Details:    </b>KieranBenton.LeaveNow.Services.Model.BusinessException: test3

Has anyone got any idea why this might be happening or if I’ve actually stumbled across a bug?

Thanks.

Answer 1

One guideline for async is to avoid async void.

The async equivalent of a synchronous method returning void is an async method returning Task. async void only exists so that event handlers may be async.

As you noticed, the exception handling is different. Any exceptions raised from an async void are sent directly to the SynchronizationContext that was current when the method started. This is natural for event handlers, but is confusing in other scenarios.

ASP.NET establishes a SynchronizationContext that represents the current HTTP request, so any exceptions from an async void method are not passed up to the WebAPI code that’s running the controller; they get passed around the WebAPI framework straight to the ASP.NET request context.