json - Setting Access-Control-Allow-Origin in ASP.Net MVC - simplest possible method

Question

I have a simple actionmethod, that returns some json. It runs on ajax.example.com. I need to access this from another site someothersite.com.

If I try to call it, I get the expected...:

Origin http://someothersite.com is not allowed by Access-Control-Allow-Origin.

I know of two ways to get around this: JSONP and creating a custom HttpHandler to set the header.

Is there no simpler way?

Is it not possible for a simple action to either define a list of allowed origins - or simple allow everyone? Maybe an action filter?

Optimal would be...:

return json(mydata, JsonBehaviour.IDontCareWhoAccessesMe);

Answer 1

For plain ASP.NET MVC Controllers

Create a new attribute

public class AllowCrossSiteJsonAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        filterContext.RequestContext.HttpContext.Response.AddHeader("Access-Control-Allow-Origin", "*");
        base.OnActionExecuting(filterContext);
    }
}

Tag your action:

[AllowCrossSiteJson]
public ActionResult YourMethod()
{
    return Json("Works better?");
}

For ASP.NET Web API

using System;
using System.Web.Http.Filters;

public class AllowCrossSiteJsonAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
    {
        if (actionExecutedContext.Response != null)
            actionExecutedContext.Response.Headers.Add("Access-Control-Allow-Origin", "*");

        base.OnActionExecuted(actionExecutedContext);
    }
}

Tag a whole API controller:

[AllowCrossSiteJson]
public class ValuesController : ApiController
{

Or individual API calls:

[AllowCrossSiteJson]
public IEnumerable<PartViewModel> Get()
{
    ...
}

For Internet Explorer <= v9

IE <= 9 doesn't support CORS. I've written a javascript that will automatically route those requests through a proxy. It's all 100% transparent (you just have to include my proxy and the script).

Download it using nuget corsproxy and follow the included instructions.

Blog post | Source code