CSRF session linked protection with ASP.NET MVC

AntiForgerySessionDataProvider.cs

using System;
using System.Web;
using System.Web.Helpers;

namespace CSRF.Infrastructure
{
    public class AntiForgerySessionDataProvider : IAntiForgeryAdditionalDataProvider
    {
        private readonly Func<string, string> _sessionKeyValuePair = sessionId => string.Format("Session: {0}", sessionId);
        public string GetAdditionalData(HttpContextBase context)
        {
            return _sessionKeyValuePair(context.Session.SessionID);
        }

        public bool ValidateAdditionalData(HttpContextBase context, string additionalData)
        {
            return additionalData == _sessionKeyValuePair(context.Session.SessionID);
        }
    }
}

CsrfController.cs

using System.Collections.Generic;
using System.Web.Mvc;

namespace CSRF.Controllers
{
    public class CsrfTestController : Controller
    {
        public ActionResult Index()
        {
            return View(Memory.Mem);
        }

        [ValidateAntiForgeryToken]
        public ActionResult Create(string someVal)
        {
            Memory.Mem.Add(someVal);
            return RedirectToAction("Index");
        }
    }

    public static class Memory
    {
        public static HashSet<string> Mem = new HashSet<string>();
    }
}

global.asax.cs

using System.Web.Helpers;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;
using CSRF.Infrastructure;

namespace CSRF
{
    public class WebApiApplication : 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);

            AntiForgeryConfig.AdditionalDataProvider = new AntiForgerySessionDataProvider();
            
            
        }
    }
}

Index.cshtml

@model HashSet<string>

@{
    ViewBag.Title = "title";
}

<h2>title</h2>
<ul>
    @foreach (var m in Model)
    {
        <li>@m</li>
    }
</ul>

@using (Html.BeginForm("Create", "CsrfTest", FormMethod.Post))
{
    <input name="someVal" type="text"/>
    @Html.AntiForgeryToken()
}