Extended JsonMediaTypeFormatter to Support JSONP

Global.asax.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace API {

    public class WebApiApplication : System.Web.HttpApplication {
        protected void Application_Start() {
            AreaRegistration.RegisterAllAreas();

            var config = GlobalConfiguration.Configuration;
            config.Formatters.Insert(0, new JsonpMediaTypeFormatter());

            WebApiConfig.Register(GlobalConfiguration.Configuration);
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);
        }
    }
}

JsonpMediaTypeFormatter.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Http.Formatting;
using System.Net.Http.Headers;
using System.Threading.Tasks;
using System.Web;

namespace API {
    public class JsonpMediaTypeFormatter : JsonMediaTypeFormatter{

        private string _callbackQueryParamter;

        public JsonpMediaTypeFormatter(){
            SupportedMediaTypes.Add(DefaultMediaType);
            SupportedMediaTypes.Add(new MediaTypeWithQualityHeaderValue("text/javascript"));

            MediaTypeMappings.Add(new UriPathExtensionMapping("jsonp", DefaultMediaType));
        }

        public string CallbackQueryParameter{
            get { return _callbackQueryParamter ?? "callback"; }
            set { _callbackQueryParamter = value; }
        }

        public override System.Threading.Tasks.Task WriteToStreamAsync(Type type, object value, System.IO.Stream writeStream, System.Net.Http.HttpContent content, System.Net.TransportContext transportContext){
            string callback;
            if(IsJsonpRequest(out callback)){
                return Task.Factory.StartNew(() => {
                    var writer = new StreamWriter(writeStream);
                    writer.Write(callback + "(");
                    writer.Flush();
                    base.WriteToStreamAsync(type, value, writeStream, content, transportContext).Wait();
                    writer.Write(")");
                    writer.Flush();
                });
            }
            return base.WriteToStreamAsync(type, value, writeStream, content, transportContext);
        }

        private bool IsJsonpRequest(out string callback){
            callback = null;
            switch(HttpContext.Current.Request.HttpMethod){
                case "POST":
                    callback = HttpContext.Current.Request.Form[CallbackQueryParameter];
                    break;
                default:
                    callback = HttpContext.Current.Request.QueryString[CallbackQueryParameter];
                    break;
            }
            return !string.IsNullOrEmpty(callback);
        }
    }
}

I get an error on the global.asax

Method not found: 'System.Web.Http.Services.DefaultServices System.Web.Http.HttpConfiguration.get_Services()'

Any idea what I am doing wrong?

HttpContext.Current returns null in my case (.NET 4.5). I fixed this in the following code:

public class JsonpMediaTypeFormatter : JsonMediaTypeFormatter
    {
        private string _callbackQueryParamter;
        private HttpRequestMessage HttpRequest;
        public JsonpMediaTypeFormatter()
        {
            SupportedMediaTypes.Add(DefaultMediaType);
            SupportedMediaTypes.Add(new MediaTypeWithQualityHeaderValue("text/javascript"));

            MediaTypeMappings.Add(new UriPathExtensionMapping("jsonp", DefaultMediaType));
        }
        public string CallbackQueryParameter
        {
            get { return _callbackQueryParamter ?? "callback"; }
            set { _callbackQueryParamter = value; }
        }
        public override MediaTypeFormatter GetPerRequestFormatterInstance(Type type,
                                            HttpRequestMessage request,  MediaTypeHeaderValue mediaType)
        {
            HttpRequest = request;
            return base.GetPerRequestFormatterInstance(type, request, mediaType);
        }
        public override System.Threading.Tasks.Task WriteToStreamAsync(Type type, object value, System.IO.Stream writeStream, System.Net.Http.HttpContent content, System.Net.TransportContext transportContext)
        {
            string callback;

            if (IsJsonpRequest(out callback))
            {
                return Task.Factory.StartNew(() =>
                {
                    var writer = new StreamWriter(writeStream);
                    writer.Write(callback + "(");
                    writer.Flush();
                    base.WriteToStreamAsync(type, value, writeStream, content, transportContext).Wait();
                    writer.Write(")");
                    writer.Flush();
                });
            }
            return base.WriteToStreamAsync(type, value, writeStream, content, transportContext);
        }
        private bool IsJsonpRequest(out string callback)
        {
            var query = HttpUtility.ParseQueryString(HttpRequest.RequestUri.Query);
            callback = query[CallbackQueryParameter];
            return !string.IsNullOrEmpty(callback);
        }
    }