title: Cannot access to User.Identity under OnActionExecuting(ActionExecutingContext filterContext)
body: <p>I am not sure what do I am missing but I really cannot refer to 
   <code>User.Identity</code> under <code>OnActionExecuting(ActionExecutingContext filterContext)</code></p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.Web.Mvc;
  using System.Web.Security;
  using System.Web.Routing;
  
  .....
  
      public class RealUserAttribute : ActionFilterAttribute
      {
              public override void OnActionExecuting(ActionExecutingContext filterContext)
              {
                   /// if (User.Identity.IsAuthenticated)
  </code></pre>
  
  <p>Any clue how to fix it?</p>