public class JsonpMediaTypeFormatter : JsonMediaTypeFormatter

{

  private string _callbackQueryParameter;

  public JsonpMediaTypeFormatter()

  {

    SupportedMediaTypes.Add(DefaultMediaType);

    SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/javascript"));

    // need a lambda here so that it'll always get the 'live' value of CallbackQueryParameter.

    MediaTypeMappings.Add(new Mapping(() => CallbackQueryParameter, "application/javascript"));

  }

  public string CallbackQueryParameter

  {

    get { return _callbackQueryParameter ?? "callback"; }

    set { _callbackQueryParameter = value; }

  }

  public override Task WriteToStreamAsync(Type type, object value, Stream writeStream, HttpContent content,

                                          TransportContext transportContext)

  {

    var callback = GetCallbackName();

    if (!String.IsNullOrEmpty(callback))

    {

      // select the correct encoding to use.

      Encoding encoding = SelectCharacterEncoding(content.Headers);

      // write the callback and opening paren.

      return Task.Factory.StartNew(() =>

        {

          var bytes = encoding.GetBytes(callback + "(");

          writeStream.Write(bytes, , bytes.Length);

        })

      // then we do the actual JSON serialization...

      .ContinueWith(t => base.WriteToStreamAsync(type, value, writeStream, content, transportContext))

      // finally, we close the parens.

      .ContinueWith(t =>

        {

          var bytes = encoding.GetBytes(")");

          writeStream.Write(bytes, , bytes.Length);

        });

    }

    return base.WriteToStreamAsync(type, value, writeStream, content, transportContext);

  }

  private string GetCallbackName()

  {

    if (HttpContext.Current.Request.HttpMethod != "GET")

      return null;

    return HttpContext.Current.Request.QueryString[CallbackQueryParameter];

  }

  #region Nested type: Mapping

  private class Mapping : MediaTypeMapping

  {

    private readonly Func<string> _param; 

    public Mapping(Func<string> discriminator, string mediaType)

      : base(mediaType)

    {

      _param = discriminator;

    }

    public override double TryMatchMediaType(HttpRequestMessage request)

    {

      if (request.RequestUri.Query.Contains(_param() + "="))

        return 1.0;

      return 0.0;

    }

  }

  #endregion

}

public class JsonCallbackAttribute

    : ActionFilterAttribute

{

    private const string CallbackQueryParameter = "callback";

    public override void OnActionExecuted(HttpActionExecutedContext context)

    {

        var callback = context.Request.GetQueryNameValuePairs().Where(item => item.Key == CallbackQueryParameter).Select(item => item.Value).SingleOrDefault();

        if (!string.IsNullOrEmpty(callback))

        {

            var jsonBuilder = new StringBuilder(callback);

            jsonBuilder.AppendFormat("({0})", context.Response.Content.ReadAsStringAsync().Result);

            context.Response.Content = new StringContent(jsonBuilder.ToString());

        }

        base.OnActionExecuted(context);

    }

}