ASP.NET/MVC4 HandleError Filter to keep the ViewData set by Controller for Error View & It's layouts

HandleErrorWithViewDataAttribute.cs

using System;
using System.Linq;
using System.Web.Mvc;

namespace ObjectCraftworks
{
    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
    public class HandleErrorWithViewDataAttribute:HandleErrorAttribute
    {
        public override void OnException(ExceptionContext filterContext)
        {
            //skip if already handled
            if (filterContext.ExceptionHandled == true)
                return;
            base.OnException(filterContext);
            //skip if not handled by base
            if (filterContext.ExceptionHandled == false)
                return;
            
            var result = filterContext.Result as ViewResult;
            //Skip if not ViewResult
            if (result == null)
                return;
            //Skip the keys that is already in ViewResult's ViewData
            var keysToBeAdded = filterContext.Controller
                                             .ViewData
                                             .Where(item => !result.ViewData.ContainsKey(item.Key));
            foreach (var item in keysToBeAdded)
            {
                result.ViewData.Add(item);
            }
        }
    }
}