Global.asax.cs

using System.Web;
using System.Web.Helpers;
using System.Web.Hosting;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;

namespace DevExtreme.MVC.Demos {
    public class MvcApplication : System.Web.HttpApplication {
        protected void Application_Start() {
            AreaRegistration.RegisterAllAreas();
            FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
            GlobalConfiguration.Configure(WebApiConfig.Register);
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            BundleConfig.RegisterBundles(BundleTable.Bundles);

            AntiForgeryConfig.SuppressXFrameOptionsHeader = true;
        }

        public override void Init() {
            PostAuthenticateRequest += MvcApplication_PostAuthenticateRequest;
            base.Init();
        }

        private void MvcApplication_PostAuthenticateRequest(object sender, System.EventArgs e) {
            if(HttpContext.Current.Request.AppRelativeCurrentExecutionFilePath.Contains("~/api")) {
                HttpContext.Current.SetSessionStateBehavior(System.Web.SessionState.SessionStateBehavior.Required);
            }
        }
    }
}