ASP.net MVC returning JSONP

Question

I am looking to return some JSON across domains and I understand that the way to do this is through JSONP rather than pure JSON. I am using ASP.net MVC so I was thinking about just extending the JSONResult type and then extendig Controller so that it also implemented a Jsonp method. Is this the best way to go about it or is there a built in ActionResult which might be better?

Edit: I went ahead and did that. Just for reference sake I added a new result:

public class JsonpResult : System.Web.Mvc.JsonResult
    {
        public override void ExecuteResult(ControllerContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException("context");
            }

            HttpResponseBase response = context.HttpContext.Response;

            if (!String.IsNullOrEmpty(ContentType))
            {
                response.ContentType = ContentType;
            }
            else
            {
                response.ContentType = "application/json";
            }
            if (ContentEncoding != null)
            {
                response.ContentEncoding = ContentEncoding;
            }
            if (Data != null)
            {
                // The JavaScriptSerializer type was marked as obsolete prior to .NET Framework 3.5 SP1
#pragma warning disable 0618
                HttpRequestBase request = context.HttpContext.Request;

                JavaScriptSerializer serializer = new JavaScriptSerializer();
                response.Write(request.Params["jsoncallback"] + "(" + serializer.Serialize(Data) + ")");
#pragma warning restore 0618
            }
        }
    }

and also a couple of methods to a superclass of all my controllers:

protected internal JsonpResult Jsonp(object data)
        {
            return Jsonp(data, null /* contentType */);
        }

        protected internal JsonpResult Jsonp(object data, string contentType)
        {
            return Jsonp(data, contentType, null);
        }

        protected internal virtual JsonpResult Jsonp(object data, string contentType, Encoding contentEncoding)
        {
            return new JsonpResult
            {
                Data = data,
                ContentType = contentType,
                ContentEncoding = contentEncoding
            };
        }

Works like a charm.

Answer 1

I just made a blog post about this exact thing and used essentially the same approach as you have outlined above except for adding a little action filter on top to make enabling JSONP on existing controller implementations a little less painful. You can read all about it here:

http://blogorama.nerdworks.in/entry-EnablingJSONPcallsonASPNETMVC.aspx