Asp.Net WebApi2 Enable CORS not working with AspNet.WebApi.Cors 5.2.3

Question

I tried to follow the steps at http://enable-cors.org/server_aspnet.html to have my RESTful API (implemented with ASP.NET WebAPI2) work with cross origin requests (CORS Enabled). It's not working unless I modify the web.config.

I installed WebApi Cors dependency:

install-package Microsoft.AspNet.WebApi.Cors -ProjectName MyProject.Web.Api

Then in my App_Start I've got the class WebApiConfig as follows:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        var corsAttr = new EnableCorsAttribute("*", "*", "*");
        config.EnableCors(corsAttr);

        var constraintsResolver = new DefaultInlineConstraintResolver();

        constraintsResolver.ConstraintMap.Add("apiVersionConstraint", typeof(ApiVersionConstraint));
        config.MapHttpAttributeRoutes(constraintsResolver); 
        config.Services.Replace(typeof(IHttpControllerSelector), new NamespaceHttpControllerSelector(config));
        //config.EnableSystemDiagnosticsTracing(); 
        config.Services.Replace(typeof(ITraceWriter), new SimpleTraceWriter(WebContainerManager.Get<ILogManager>())); 
        config.Services.Add(typeof(IExceptionLogger), new SimpleExceptionLogger(WebContainerManager.Get<ILogManager>()));
        config.Services.Replace(typeof(IExceptionHandler), new GlobalExceptionHandler()); 
    }
}

but after that I run the application, I request a resource with Fiddler like: http://localhost:51589/api/v1/persons and in the response I cannot see the HTTP headers that I should see such as:

• Access-Control-Allow-Methods: POST, PUT, DELETE, GET, OPTIONS
• Access-Control-Allow-Origin: *

Am I missing some step? I have tried with the following annotation on the controller:

[EnableCors(origins: "http://example.com", headers: "*", methods: "*")]

Same result, no CORS enabled.

However, if I add the following in my web.config (without even installing the AspNet.WebApi.Cors dependency) it works:

<system.webServer>

<httpProtocol>
  <!-- THESE HEADERS ARE IMPORTANT TO WORK WITH CORS -->
  <!--
  <customHeaders>
    <add name="Access-Control-Allow-Origin" value="*" />
    <add name="Access-Control-Allow-Methods" value="POST, PUT, DELETE, GET, OPTIONS" />
    <add name="Access-Control-Allow-Headers" value="content-Type, accept, origin, X-Requested-With, Authorization, name" />
    <add name="Access-Control-Allow-Credentials" value="true" />
  </customHeaders>
  -->
</httpProtocol>
<handlers>
  <!-- THESE HANDLERS ARE IMPORTANT FOR WEB API TO WORK WITH  GET,HEAD,POST,PUT,DELETE and CORS-->
  <!--

  <remove name="WebDAV" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="GET,HEAD,POST,PUT,DELETE" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
  <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
  <remove name="OPTIONSVerbHandler" />
  <remove name="TRACEVerbHandler" />
  <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="*" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
-->
</handlers>

Any help would be much appreciated!

Thank you.

Answer 1

I've created a pared-down demo project for you.

• Source: https://github.com/bigfont/webapi-cors
• Api Link: https://cors-webapi.azurewebsites.net/api/values

You can try the above API Link from your local Fiddler to see the headers. Here is an explanation.

Global.ascx

All this does is call the WebApiConfig. It's nothing but code organization.

public class WebApiApplication : System.Web.HttpApplication
{
    protected void Application_Start()
    {
        WebApiConfig.Register(GlobalConfiguration.Configuration);
    }
}

WebApiConfig.cs

The key method for your here is the EnableCrossSiteRequests method. This is all that you need to do. The EnableCorsAttribute is a globally scoped CORS attribute.

public static class WebApiConfig
{
    public static void Register(HttpConfiguration config)
    {
        EnableCrossSiteRequests(config);
        AddRoutes(config);
    }

    private static void AddRoutes(HttpConfiguration config)
    {
        config.Routes.MapHttpRoute(
            name: "Default",
            routeTemplate: "api/{controller}/"
        );
    }

    private static void EnableCrossSiteRequests(HttpConfiguration config)
    {
        var cors = new EnableCorsAttribute(
            origins: "*", 
            headers: "*", 
            methods: "*");
        config.EnableCors(cors);
    }
}

Values Controller

The Get method receives the EnableCors attribute that we applied globally. The Another method overrides the global EnableCors.

public class ValuesController : ApiController
{
    // GET api/values
    public IEnumerable<string> Get()
    {
        return new string[] { 
            "This is a CORS response.", 
            "It works from any origin." 
        };
    }

    // GET api/values/another
    [HttpGet]
    [EnableCors(origins:"http://www.bigfont.ca", headers:"*", methods: "*")]
    public IEnumerable<string> Another()
    {
        return new string[] { 
            "This is a CORS response. ", 
            "It works only from two origins: ",
            "1. www.bigfont.ca ",
            "2. the same origin." 
        };
    }
}

Web.config

You do not need to add anything special into web.config. In fact, this is what the demo's web.config looks like - it's empty.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
</configuration>

Demo

var url = "https://cors-webapi.azurewebsites.net/api/values"

$.get(url, function(data) {
  console.log("We expect this to succeed.");
  console.log(data);
});

var url = "https://cors-webapi.azurewebsites.net/api/values/another"

$.get(url, function(data) {
  console.log(data);
}).fail(function(xhr, status, text) {
  console.log("We expect this to fail.");
  console.log(status);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>