Global.asax.cs

using MvcApplication3.Controllers;
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 MvcApplication3
{
    // メモ: IIS6 または IIS7 のクラシック モードの詳細については、
    // http://go.microsoft.com/?LinkId=9394801 を参照してください

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

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

            ValueProviderFactories.Factories.Remove(ValueProviderFactories.Factories.OfType<JsonValueProviderFactory>().FirstOrDefault());
            ValueProviderFactories.Factories.Add(new JsonDotNetValueProviderFactory());
        }
    }
}

MyController.cs

using Newtonsoft.Json;
using Newtonsoft.Json.Converters;
using Newtonsoft.Json.Serialization;
using System;
using System.Collections.Generic;
using System.Dynamic;
using System.Globalization;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace MvcApplication3.Controllers
{
    public class MyController : Controller
    {
        protected override JsonResult Json(object data, string contentType, System.Text.Encoding contentEncoding, JsonRequestBehavior behavior)
        {
            return new JsonDotNetResult
            {
                Data = data,
                ContentType = contentType,
                ContentEncoding = contentEncoding,
                JsonRequestBehavior = JsonRequestBehavior.AllowGet
            };
        }
    }

    // see http://stackoverflow.com/a/17245644
    public class JsonDotNetResult : JsonResult
    {
        private static readonly JsonSerializerSettings Settings = new JsonSerializerSettings
        {
            ContractResolver = new CamelCasePropertyNamesContractResolver(),
            Converters = new List<JsonConverter> { new StringEnumConverter() }
        };

        public override void ExecuteResult(ControllerContext context)
        {
            if (this.JsonRequestBehavior == JsonRequestBehavior.DenyGet &&
                string.Equals(context.HttpContext.Request.HttpMethod, "GET", StringComparison.OrdinalIgnoreCase))
            {
                throw new InvalidOperationException("GET request not allowed");
            }

            var response = context.HttpContext.Response;

            response.ContentType = !string.IsNullOrEmpty(this.ContentType) ? this.ContentType : "application/json";

            if (this.ContentEncoding != null)
            {
                response.ContentEncoding = this.ContentEncoding;
            }

            if (this.Data == null)
            {
                return;
            }

            response.Write(JsonConvert.SerializeObject(this.Data, Settings));
        }
    }

    // see http://www.dalsoft.co.uk/blog/index.php/2012/01/10/asp-net-mvc-3-improved-jsonvalueproviderfactory-using-json-net/
    public sealed class JsonDotNetValueProviderFactory : ValueProviderFactory
    {
        public override System.Web.Mvc.IValueProvider GetValueProvider(ControllerContext controllerContext)
        {
            if (controllerContext == null)
                throw new ArgumentNullException("controllerContext");

            if (!controllerContext.HttpContext.Request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
                return null;

            string bodyText = null;
            using (var reader = new System.IO.StreamReader(controllerContext.HttpContext.Request.InputStream))
            {
                bodyText = reader.ReadToEnd();
            }
            return String.IsNullOrEmpty(bodyText) ? null : new DictionaryValueProvider<object>(JsonConvert.DeserializeObject<ExpandoObject>(bodyText, new ExpandoObjectConverter()), CultureInfo.CurrentCulture);
        }
    }
}