DelegatingHandler not executing ASP.Net Web Api

Question

today i encountered a strange behavior in my Web Api application

protected void Application_Start() {

    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    GlobalConfiguration.Configuration
        .MessageHandlers.Add(new DummyMessageHandler());
}

And my DelegatingHandler looks like this.

public class DummyMessageHandler : DelegatingHandler {

    protected override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken) {

*       if (request.Headers.Authorization.Scheme == "Basic")
            Thread.CurrentPrincipal = new GenericPrincipal(
                new GenericIdentity("Authenticated"), new string[0]);

        return base.SendAsync(request, cancellationToken);
    }
}

The problem I encountered was that the delegating handlers are not being executed. I have a breakpoint in the line marked with a * and the execution of my code never stops there.

My nuget packages.config is the following:

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Microsoft.AspNet.Mvc" version="4.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.Razor" version="2.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.Web.Optimization" version="1.0.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebApi" version="4.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebApi.Client" version="4.1.0-alpha-120809" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebApi.Core" version="4.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebApi.WebHost" version="4.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.AspNet.WebPages" version="2.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.Net.Http" version="2.0.20710.0" targetFramework="net40" />
  <package id="Microsoft.Web.Infrastructure" version="1.0.0.0" targetFramework="net40" />
  <package id="Newtonsoft.Json" version="4.5.8" targetFramework="net40" />
  <package id="WebGrease" version="1.1.0" targetFramework="net40" />
</packages>

I'm looking at this for a long time, can you point me to something I am missing ? Thank you

I'm not able to reproduce your issue. In a newly created MVC4 Web.API project I've copied your DummyMessageHandler and registered it in the Application_Start and if I put a breakpoint there it is being hit. — nemesv
– nemesv, Commented Aug 17, 2012 at 0:19
Maybe the problem is happening because i have been updating this project since beta. I will install the last mvc4 and see if it solves it. — Filipe Pinheiro
– Filipe Pinheiro, Commented Aug 17, 2012 at 0:43
After installing mvc i created a project and all worked fine until I started adding packages with nuget. The result was this github.com/mccalltd/AttributeRouting/issues/112. — Filipe Pinheiro
– Filipe Pinheiro, Commented Aug 17, 2012 at 2:16

Shaun Luttin · Accepted Answer · 2014-05-06 19:51:21Z

14

What you have done is correct. The problem might be happening because the DelegatingHandler only runs when you call a Web API Controller action. For example:

This will invoke your Message Handler, because it's an ApiController.

public class ValuesController : ApiController
{
    // GET api/values
    public IEnumerable<string> Get()
    {
        return new string[] { "value1", "value2" };
    }
}

This will NOT, because it's just a Controller.

public class HomeController : Controller
{
    public ActionResult Index()
    {
        ViewBag.Title = "Home Page";

        return View();
    }
}

Make sure you are calling a Web API Controller action, otherwise the debugger will not hit your break point.

edited May 6, 2014 at 19:51

answered May 6, 2014 at 19:38

Shaun Luttin

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2 Comments

MacX Over a year ago

Is it also possible to invoke the handler on a specific path, allthough there is no controller action mapped to it?

fernacolo Over a year ago

Had same issue and this solution worked for me. Looks like an implict/hidden requirement from ASP.NET, requiring a specific type of controller.

Toan Nguyen · Accepted Answer · 2014-05-06 20:49:22Z

7

you should register your handler in the WebApiConfig file, not in the global.asax

 public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {

            config.MapHttpAttributeRoutes();
            config.MessageHandlers.Add( new DummyMessageHandler());
            config.Routes.MapHttpRoute(
                name: "DefaultApi",
                routeTemplate: "api/{controller}/{id}",
                defaults: new { id = RouteParameter.Optional }
            );
        }


    }

edited May 6, 2014 at 20:49

answered Feb 12, 2014 at 22:54

Toan Nguyen

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6 Comments

Shaun Luttin Over a year ago

-1 GlobalConfiguration.Configuration.MessageHandlers.Add() does work for adding Message Handlers to the pipeline. Further, if we do choose to add a Message Handler in the WebApiConfig, we use config.MessageHandlers.Add() not config.Filters.Add(). The latter is for adding Action Filters not Message Handlers.

Toan Nguyen Over a year ago

@ShaunLuttin That was the valid point. Probably I mistyped Filters instead of MessageHandlers. Thanks

Shaun Luttin Over a year ago

Thanks for updating the answer. Now, why do you say to register the handler in WebApiConfig instead of in the Global.asax. Both work and is a matter of style. Your answer is misleading; it implies that registering in Global.asax won't work. That's why the -1 stands.

Toan Nguyen Over a year ago

Well, I said "You should" not "You must" to make registering message handlers more centralized. You are able to register a handler event even in your data access layer by accessing the global variable, but apparently that wouldn't be a recommended way.

Shaun Luttin Over a year ago

Good point, Toan. It is better to register the DelegateHandler within WebApiConfig, as you suggest. The question, though, is about the handler not executing; it is not about the recommended place to register it. You're answer, while relevant and important, does not directly answer the question. That's why I haven't reversed the down vote.

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