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.

Thymine · Accepted Answer · 2016-02-18 20:52:49Z

24

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.

Try to create a DelegatingHandler and add it to your HttpConfiguration's list of message handlers:
```
config.MessageHandlers.Add(new HandlerA())
```

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.

edited Feb 18, 2016 at 20:52

Thymine

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answered Mar 4, 2013 at 17:25

Kiran

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4 Comments

Tohid Over a year ago

Thank you. Solution (1) works well. But number 2, new HttpControllerDispatcher() says: System.Web.Http.Dispatcher.HttpControllerDispatcher does not contain a constructor that takes 0 arguments - Update: I fixed your code. Thank you.

Tohid Over a year ago

Please also add this part of Cuong Le's answer to your answere: To write a custom message handler, you should derive from System.Net.Http.DelegatingHandler.

0xcaff Over a year ago

The link for the poster is broken.

William Over a year ago

in which file do we need to add the code: config.MessageHandlers.Add(new HandlerA())

cuongle · Accepted Answer · 2013-03-04 15:02:19Z

17

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.

answered Mar 4, 2013 at 15:02

cuongle

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3 Comments

Tohid Over a year ago

@Cuonge Le - Thank you for the answer. After I replaced CustomMessageHandler with your code, it raises error on return base.SendAsync(request, cancellationToken); saying: The inner handler has not been assigned. . What I missed again?

Tohid Over a year ago

@Counge Le - The combination of your answer and Kirran Chall's, has fixed the issue. Thank you again.

jaxxbo Over a year ago

@Cuong Le what is the equivalent for .NET 4.0? I could not find DelegatingHandler for .NET .4... how can do messagehandling in 4.0. Thanks for your help.

Bahtiyar Özdere · Accepted Answer · 2020-12-20 12:59:25Z

I used @cuongle answer to solve my problem. Just with one addition. So I did not get "The inner handler has not been assigned.". Thank you @coungle.

public class CustomMessageHandler : DelegatingHandler
{
        public CustomMessageHandler ()
        {
              //add this to solve "The inner handler has not been assigned"
               InnerHandler = new HttpClientHandler();
        }
    
        protected override async Task<HttpResponseMessage> 
          SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            // Before request 
            IPrincipal principal = new GenericPrincipal(
                new GenericIdentity("myuser"), new string[] { "myrole" });
            Thread.CurrentPrincipal = principal;
            HttpContext.Current.User = principal;
    
            var result = await base.SendAsync(request, cancellationToken);
    
            // After request when response arrives
            
            return result;
        }
}

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