async/await broken in ASP.NET projects

[b-szypelow]

I'm using Mediator.Net with Autofac in an ASP.NET WebApi2 project. I've found out the IRequestHandler is not working properly when it's an async method with awaits.

I was able to reproduce the issue on a blank new WebApi project with the following code:

    public class DefaultController : ApiController
    {
        private readonly IMediator m;
        public DefaultController(IMediator m) => this.m = m;

        [Route("ping"), HttpGet]
        public async Task<IHttpActionResult> Ping() 
            => Ok(await m.RequestAsync<Ping, Pong>(new Ping()));

        [Route("hang"), HttpGet]
        public async Task<IHttpActionResult> Ping2()
            => Ok(await m.RequestAsync<Ping2, Pong2>(new Ping2()));
    }

    public class Ping : IRequest { }
    public class Ping2 : IRequest { }
    public class Pong : IResponse { }
    public class Pong2 : IResponse { }

    public class WorkingHandler : IRequestHandler<Ping, Pong>
    {
        public Task<Pong> Handle(ReceiveContext<Ping> context)
        {
            return Task.FromResult(new Pong());
        }
    }

    public class BrokenHandler : IRequestHandler<Ping2, Pong2>
    {
        public async Task<Pong2> Handle(ReceiveContext<Ping2> context)
        {
            await Task.Yield(); // Or whatever awaitable
            return new Pong2(); // Never reached
        }
    }

The Configuration is as folllows:

    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            config.MapHttpAttributeRoutes();

            var b = new ContainerBuilder();
            b.RegisterApiControllers(Assembly.GetExecutingAssembly());
            b.RegisterMediator(new MediatorBuilder().RegisterHandlers(Assembly.GetExecutingAssembly()));
            var c = b.Build();

            config.DependencyResolver = new AutofacWebApiDependencyResolver(c);
        }
    }

The /ping endpoint works as expected. The /hang endpoint never terminates.

I was not able to reproduce the issue in unit tests. It only happens in ASP.NET.

My first guess is that it's a deadlock caused by a blocking call of Result in RequestPipe.cs which interfered with the way IIS is handling tasks.

[mayuanyang]

Hi @b-szypelow,

Thanks for keeping in touch, will dig in a bit more and review your PR soon.

Regards
Eddy

[mayuanyang]

Have merged the PR and push a new version to nuget, please update and validate. thanks

[b-szypelow]

I've updated my project and it works now. Thank you.