WebApiContrib.Formatting.Jsonp provides a JSONP MediaTypeFormatter implementation for ASP.NET Web API.

In order to add it to your Web API solution, run Install-Package WebApiContrib.Formatting.Jsonp from your NuGet Package Manager console in Visual Studio.

To use the JsonpMediaTypeFormatter, add the following code to your configuration in Global.asax.cs:

FormatterConfig.RegisterFormatters(GlobalConfiguration.Configuration.Formatters);

The FormatterConfig class (typically in /App_Start/FormatterConfig.cs) looks this:

public class FormatterConfig
{
    public static void RegisterFormatters(MediaTypeFormatterCollection formatters)
    {
        var jsonFormatter = formatters.JsonFormatter;
        jsonFormatter.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();

        var jsonpFormatter = new JsonpMediaTypeFormatter(jsonFormatter, /* default */ callbackQueryParameter = "callback");
        formatters.Add(jsonpFormatter);
    }
}

By default, you must specify text/javascript as the media type to accept. If you leave this out, you will likely send back application/json and find errors in your browser console. In addition to the standard Accept header, the JsonpMediaTypeFormatter allows query string and URI path extension mapping. To use the query string mapping, add ?format=jsonp&callback=? to your request URI. (NOTE: you need to specify the callback=?, or whatever you name your callback, in the query string anyway.)

** NOTE: URI path extension is currently not working. ** URI path extension mapping uses the routing mechanisms. If you are using Attribute Routing, you should add "/{format}" after each route if you plan to use the URI mapping for jsonp, e.g. [Route("api/value/{id:int}/{format?}")]. If you will require the Content-Type header to specify text/javascript, then you can leave your routes alone. (See the sample applications for examples.)

If you are using traditional routing, update your Default ASP.NET Web API route in /App_Start/WebApiConfig.cs:

config.Routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}/{format}",
    defaults: new { id = RouteParameter.Optional, format = RouteParameter.Optional }
);

To see the JsonpMediaTypeFormatter in action, just clone this project, run the WebContribContrib.Formatting.Jsonp.SampleWebHost project web application, and then start the WebApiContrib.Formatting.Jsonp.SampleJQueryClient web application and hit the "Get JSONP" button.