public class WwwRequirementAttribute : FilterAttribute, IAuthorizationFilter
    {
        public void OnAuthorization(AuthorizationContext filterContext)
        {
            var url = filterContext.HttpContext.Request.Url.Host;
            bool startsWith3W = url.StartsWith("www.", StringComparison.OrdinalIgnoreCase);
            if (!startsWith3W)
            {
                filterContext.Result = new RedirectResult("www."+url, true);
            }

        }
    }

[WwwRequirement]
        public ActionResult Index()
        {
            return View();
        }