1 
Index
 2 @*一级根节点*@ 3 @Html.DropDownList("YiJi", ViewBag.user as SelectList, new { onchange = "City()" }) 4  5  6     二级根节点 7  8  9 10 11     三级根节点12 13 14 15 

CityBll bll = new CityBll();        /// 
        /// 主页面        /// 
        /// 
              public ActionResult Index()        {            var list = bll.getList().Where(m => m.PId == 0).ToList();            SelectList list2 = new SelectList(list, "Id", "Name");            ViewBag.user = list2;            return View();        }        /// 
        /// 获取城市信息        /// 
        ///         /// 
              public string GetCity(int id)        {            var list = bll.getList().Where(m => m.PId == id).ToList();            return JsonConvert.SerializeObject(list);        }