MVC ConvertEmptyStringToNull

     [HttpPost]
        public JsonResult CreateOrder(O_CreateOrder obj)
        {
            IOrderBiz GOBiz = Ioc.IoCFactory.Instance.CurrentContainer.Resolve<IOrderBiz>("GroupOrder");

            ICreateOrder createobject = GOBiz.CreateOrder<O_CreateOrder>(obj);

            return Json(createobject,null,null);
        }

usingSystem.ComponentModel.DataAnnotations;
 

     [DisplayFormat(ConvertEmptyStringToNull = false)] 
        public string GuaranteeType
        {
            get;
            set;
        }