How to implement HttpMessageHandler in Web API?

Question

In an ASP.NET 4.5 MVC 4 Web API project, I want to add a custom HttpMessageHandler. I've changed WebApiConfig class (in \App_Satrt\WebApiConfig.cs), as follows:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        config.Routes.MapHttpRoute(
            name: "DefaultApi",
            routeTemplate: "api/{controller}/{id}",
            defaults: new { id = RouteParameter.Optional },
            constraints: null,
            handler: new MyCustomizedHttpMessageHandler()
        );
    }
}

Then I developed MyCustomizedHttpMessageHandler:

public class MyCustomizedHttpMessageHandler : HttpMessageHandler
{
    protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        IPrincipal principal = new GenericPrincipal(
            new GenericIdentity("myuser"), new string[] { "myrole" });
        Thread.CurrentPrincipal = principal;
        HttpContext.Current.User = principal;

        return Task<HttpResponseMessage>.Factory.StartNew(() => request.CreateResponse());
    }
}

However, the request to the API (let's say http://mylocalhost.com/api/values), always returns status code 200, without any data. I mean it never gets to ValuesController.cs's 'GET()' method.

What have I missed? How can I implement HttpMessageHandler properly?

PS: Have already read this one: https://stackoverflow.com/a/12030785/538387 , doesn't help me.

Answer 1

Here you are creating a HttpMessageHandler which short circuits the request and doesn't let the request pass through the rest of the pipeline. Instead, you should create a DelegatingHandler.

Also there are 2 kinds of message handler pipelines in Web API. One is a regular pipeline in which all requests for all routes pass through and other where one could have message handlers specific to certain routes only.

1. Try to create a DelegatingHandler and add it to your HttpConfiguration's list of message handlers:

   config.MessageHandlers.Add(new HandlerA())
   

2. If you want to add a route specific message handler, then you could do the following:

   config.Routes.MapHttpRoute(
               name: "DefaultApi",
               routeTemplate: "api/{controller}/{id}",
               defaults: new { id = RouteParameter.Optional },
               constraints: null,
               handler: 
                      HttpClientFactory.CreatePipeline(
                             new HttpControllerDispatcher(config), 
                             new DelegatingHandler[]{new HandlerA()})
               );
   

This Web Api Poster shows the pipeline flow.

Answer 2

To write a custom message handler, you should derive from System.Net.Http.DelegatingHandler

class CustomMessageHandler : DelegatingHandler
{
    protected override Task<HttpResponseMessage> 
      SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
    {
        IPrincipal principal = new GenericPrincipal(
            new GenericIdentity("myuser"), new string[] { "myrole" });
        Thread.CurrentPrincipal = principal;
        HttpContext.Current.User = principal;

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

And call base.SendAsync to send the request to the inner handler.