Web API 2 Cors Request Error

Question

I am trying to get CORS set up for a project I am working on with WebAPI 2. I started having issues, so I created a demo app directly from asp.net forums here. Everything was working correctly until I needed to use json as the content type. Then I started getting:

Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

I understand with this content type sends preflight requests, but I am dumbfounded how I can get this to pass. Am I missing something? As soon as I remove the "contentType: 'application/json'" attribute from AJAX request, it works.

TestController.cs

[Authorize]
[EnableCors(origins: "*", headers: "*", methods: "*")]
public class TestController : ApiController
{
    // GET api/<controller>
    public HttpResponseMessage Get()
    {
        return new HttpResponseMessage()
        {
            Content = new StringContent("GET: Test message")
        };
    }

    public HttpResponseMessage Post([FromBody]string name)
    {
        return new HttpResponseMessage()
        {
            Content = new StringContent("POST: Test message")
        };
    }

    public HttpResponseMessage Put()
    {
        return new HttpResponseMessage()
        {
            Content = new StringContent("PUT: Test message")
        };
    }
}

WebApiConfig.cs

public static void Register(HttpConfiguration config)
    {
        // Web API configuration and services
        config.EnableCors();

        // Web API routes
        config.MapHttpAttributeRoutes();

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

Ajax Request

$.ajax({
            type: "POST",
            url: 'http://localhost:17515/',
            data: JSON.stringify("Test"),
            xhrFields: {
                withCredentials: true
            },
            contentType: "application/json"
        });

Possible duplicate of [OPTIONS 405 (Method Not Allowed) web api 2](http://stackoverflow.com/questions/26649361/options-405-method-not-allowed-web-api-2) — Gusman, Feb 26 '16 at 21:52

score 0 · Answer 1 · answered Feb 26 '16 at 21:55

It client will first send an OPTIONS request to the server. To this request, the server should add a header:

Access-Control-Allow-Origin: http://localhost:17822

This indicates that the API running on port 17515 accepts requests from the client served by port 17822.

You could try changing your attribute to:

[EnableCors(origins: "http://localhost:17822", headers: "*", methods: "*")]

We haven't had good experiences using EnableCors, so we handle OPTIONS requests using OWIN, simply returning 200 OK and manually adding the appropriate headers to all OPTIONS request sent by approved origins.

There is a good article on CORS on MSDN (likely you have already seen it): https://msdn.microsoft.com/en-us/magazine/dn532203.aspx

score 0 · Answer 2 · answered Feb 26 '16 at 22:49

In your WebApiConfig.cs, you could try enabling the cors attributes there instead of on the controller.

        var cors = new EnableCorsAttribute("*", "*", "*");
        config.EnableCors(cors);

rather than just

        config.EnableCors();

This is working on a test project I have running at the moment.

Web API 2 Cors Request Error

2 Answers2