A JSONP media-type formatter for ASP .NET WebApi (latest stable version). Copy the class and add this to your Global.asax.s var config = GlobalConfiguration.Configuration; config.Formatters.Add(new JsonpFormatter(null,null)); Original code from: https://gist.github.com/jonathanhunt/2967547

JSONPMediaTypeFormatter

using System;
using System.IO;
using System.Net;
using System.Net.Http;
using System.Net.Http.Formatting;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using Newtonsoft.Json;

namespace Test
{
    
    public class JsonpFormatter : MediaTypeFormatter
    {
        private readonly string _callbackQueryParameter;
        private readonly JsonSerializerSettings _jsonSerializerSettings;

        
        public JsonpFormatter(JsonSerializerSettings jsonSerializerSettings, string callbackQueryParameter)
        {
            _jsonSerializerSettings = jsonSerializerSettings ?? new JsonSerializerSettings();
            _callbackQueryParameter = callbackQueryParameter ?? "callback";
            SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/json"));
            SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/javascript"));
        }

        private string CallbackFunction { get; set; }

        public override bool CanReadType(Type type)
        {
            return true;
        }

        public override bool CanWriteType(Type type)
        {
            return true;
        }

        public override Task<object> ReadFromStreamAsync(Type type, Stream readStream, HttpContent content, IFormatterLogger formatterLogger)
        {
            var serializer = JsonSerializer.Create(_jsonSerializerSettings);

            return Task.Factory.StartNew(() =>
            {
                using (var streamReader = new StreamReader(readStream, Encoding.UTF8))
                {
                    using (var jsonTextReader = new JsonTextReader(streamReader))
                    {
                        return serializer.Deserialize(jsonTextReader, type);
                    }
                }
            });
        }


        public override Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content, TransportContext transportContext)
        {
                var serializer = JsonSerializer.Create(_jsonSerializerSettings);
                var isJsonp = !string.IsNullOrEmpty(CallbackFunction);

                return Task.Factory.StartNew(() =>
                {
                    using (var jsonTextWriter = new JsonTextWriter(new StreamWriter(writeStream, Encoding.UTF8)) { CloseOutput = false })
                    {
                        if (isJsonp)
                        {
                            jsonTextWriter.WriteRaw(CallbackFunction + "(");
                            jsonTextWriter.Flush();
                        }

                        serializer.Serialize(jsonTextWriter, value);
                        jsonTextWriter.Flush();

                        if (isJsonp)
                        {
                            jsonTextWriter.WriteRaw(")");
                            jsonTextWriter.Flush();
                        }
                    }
                });
        }

       

        public override MediaTypeFormatter GetPerRequestFormatterInstance(Type type, HttpRequestMessage request, MediaTypeHeaderValue mediaType)
        {
            var formatter = new JsonpFormatter(_jsonSerializerSettings, _callbackQueryParameter)
            {
                CallbackFunction = GetJsonCallbackFunction(request)
            };

            return formatter;
        }

        private string GetJsonCallbackFunction(HttpRequestMessage request)
        {
            if (request.Method != HttpMethod.Get) return null;

            var query = request.RequestUri.ParseQueryString();
            var queryVal = query[_callbackQueryParameter];

            if (string.IsNullOrEmpty(queryVal)) return null;

            return queryVal;
        }
    }
}

I'm new to .Net WebAPI and am having the CORS problem so I am trying the JSONP formatter. It appears that I have successfully added this to my webapi project as described above. What is a simple way to verify that the JSONP formatter is functioning?