Example with Possible NHibernate Implementation

AuditAttribute.cs

public class AuditAttribute : ActionFilterAttribute
{
     public override void OnActionExecuting(ActionExecutingContext filterContext)
     {
          //Stores the Request in an Accessible object
          var request = filterContext.HttpContext.Request;
          //Generate an audit
          Audit audit = new Audit()
          {
               //Your Audit Identifier     
               AuditID = Guid.NewGuid(),
               //Our Username (if available)
               UserName = (request.IsAuthenticated) ? 
                          filterContext.HttpContext.User.Identity.Name : "Anonymous",
               //The IP Address of the Request
               IPAddress = request.ServerVariables["HTTP_X_FORWARDED_FOR"] ?? request.UserHostAddress,
               //The URL that was accessed
               AreaAccessed = request.RawUrl,
               //Creates our Timestamp
               TimeAccessed = DateTime.UtcNow
          };

          // Create your NHibernate session / repository here and 
          // save your audit object

          //Finishes executing the Action as normal 
          base.OnActionExecuting(filterContext);
     }
}

@rionmonster nevermind i solved that problem, but sadly im now stuck on the second part(advanced audit) part of your project.
Im stuck at
var sessionIdentifier = string.Join("", MD5.Create().ComputeHash(Encoding.ASCII.GetBytes(request.Cookies[FormsAuthentication.FormsCookieName].Value)).Select(s => s.ToString("x2")));
Where it always gives me the exception:
Object reference not set to an instance of an object

Any ideas please?