How to map different HTTP methods on the same url to different controllers?

Question

I have my API for a small part of my application split over two controllers, because of (external) requirements on the casing of JSON data in the API (some requests should use camelCasing, while others should use PascalCasing).

Now, I have a url that I want to map with PascalCasing for GET, but camelCasing for PUT, so I tried the following:

[PascalCasing] // custom attribute, part of our code
               // We configure all controllers that *don't* have this to use
               // camelCasing
public class PascalCasedController : ApiController
{
    [HttpGet]
    [Route("url/to/my/resource/{id}")]
    public IHttpActionResult(int id)
    {
        return Ok(GetResource(id));
    }
}

public class CamelCasedController : ApiController
{
    [HttpPut]
    [Route("url/to/my/resource/{id}")]
    public IHttpActionResult(int id, Resource resource)
    {
        SaveResource(id, resource);
        return Ok();
    }
}

The GET request works as expected, but if I try to PUT something there with Fiddler, I get the following error message:

Multiple controller types were found that match the URL. This can happen if attribute routes on multiple controllers match the requested URL.
The request has found the following matching controller types:
MyProject.PascalCaseController
MyProject.CamelCaseController

I realize this is probably because WebAPI maps routes to controllers first and actions next, but if HTTP methods are considered, there really isn't any ambiguity here. Is there any way that I can tell WebAPI how to do this, without having to have the methods in the same controller?

Answer 1

@Tomas - There's an interface "System.Web.Http.Dispatcher.IHttpControllerSelector" exposed in System.Web.Http assembly. You can use that interface and create your own HttpControllerSelector. You can then replace the DefaultControllerSelector with your custom controller selector in the HttpConfiguration during AreaRegistration.

httpConfig.Services.Replace(typeof(IHttpControllerSelector), new CustomControllerSelector(services.GetHttpControllerSelector()));

In this custom controller selector you can write your own implementation of SelectController() method of IHttpControllerSelector in which you can call GetControllerMapping() method of IHttpControllerSelector. This will give you the list of all the controllers registered. For every controller you can check for the DeclaredMethods and check for the CustomAttributes for each of the DeclaredMethods. In your case it will be either HttpGetAttribute or HttpPutAttribute.

Check the Method type of the incoming HttpRequestMessage (GET/PUT) and compare it against the value of the CustomAttributes. If you find a match of the combination of incoming request URL and the the respective Http Verb then you take that HttpControllerDiscriptor and return it from the SelectController() method..

This will allow you to have same URL with different methods in two different controllers.