vreksam/WebApi/App_Start/WebApiConfig.cs

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

namespace WebApi
{
    public class JsonContentNegotiator : IContentNegotiator
    {
        private readonly JsonMediaTypeFormatter _jsonFormatter;

        public JsonContentNegotiator(JsonMediaTypeFormatter formatter)
        {
            _jsonFormatter = formatter;
        }

        public ContentNegotiationResult Negotiate(Type type, HttpRequestMessage request, IEnumerable<MediaTypeFormatter> formatters)
        {
            var result = new ContentNegotiationResult(_jsonFormatter, new MediaTypeHeaderValue("application/json"));
            return result;
        }
    }

    public static class WebApiConfig
    {
        public static void Register(HttpConfiguration config)
        {
            // Web API configuration and services

            //DEBUG
            config.EnableSystemDiagnosticsTracing();

            // Web API routes
            config.MapHttpAttributeRoutes();

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

            // Force always json response
            var jsonFormatter = new JsonMediaTypeFormatter();
            //optional: set serializer settings here
            //config.Services.Replace(typeof(IContentNegotiator), new JsonContentNegotiator(jsonFormatter));
        }
    }
}