下拉列表联动显示(中国省市)三级联动

最主要的就是要判断谁根据谁编写的！！！理清思路！！！
1、Models
namespace 下拉列表联动显示_中国省市_.Models
{
    public class ChinaBF
    {
        private MyDBDataContext _context = new MyDBDataContext();
        public List<ChinaStates> Select()
        {
            return _context.ChinaStates.ToList();       
        }
       //因为所有数据都在一个表里， 都是根据ParentAreaCode与AreaCode比较
        public List<ChinaStates> SelectPro(string AreaCode) //省份
        {

            var query = _context.ChinaStates.Where(P => P.ParentAreaCode == AreaCode);
            if (query.Count() > 0)
            {
                return query.ToList();
            }
            else
            {
                return null;
            }

        }

        public List<ChinaStates> SelectCity1(string AreaCode)//市辖
        {
            var query = _context.ChinaStates.Where(P => P.ParentAreaCode == AreaCode);
            if (query.Count() > 0)
            {
                return query.ToList();
            }
            else
            {
                return null;
            }
        }

        public List<ChinaStates> SelectCity2(string AreaCode)//市区
        {
            var query = _context.ChinaStates.Where(P => P.ParentAreaCode == AreaCode);
            if (query.Count() > 0)
            {
                return query.ToList();
            }
            else
            {
                return null;
            }
        }

    }
}


2、Controllers
namespace 下拉列表联动显示_中国省市_.Controllers
{
    public class HomeController : Controller
    {
        
        [HttpGet] //刚开始加载页面的时候
        public ActionResult Index()
        {   

            //省份  根据编号0001查的就是省份
            string ParentAreaCodepro;
            ParentAreaCodepro = "0001";
            List<ChinaStates> listpro = new ChinaBF().SelectPro(ParentAreaCodepro);
            SelectList bb = new SelectList(listpro, "AreaCode", "AreaName");
            ViewBag.databb = bb;

            //市辖  这里默认显示的是北京
            string AreaCode1;
            AreaCode1 = "11";
            List<ChinaStates> listcity1 = new ChinaBF().SelectCity2(AreaCode1);
            SelectList cc1 = new SelectList(listcity1, "AreaCode", "AreaName");
            ViewBag.datacc1 = cc1;
         


            //市区 这里默认显示的是北京的数据
            string AreaCode;
            AreaCode = "1101";
            List<ChinaStates> listcity = new ChinaBF().SelectCity2(AreaCode);
            SelectList cc = new SelectList(listcity, "AreaCode", "AreaName");
            ViewBag.datacc = cc;
            return View();
        }

        [HttpPost]  //当页面提交后，数据便会改变，联动显示
        public ActionResult Index(string pro,string city,string city1)三个参数很重要，名字必须和Views视图里数据的name值一样
        {
            
            //省份  还是加载编号0001 这样加载的全是省份
            string AreaCodepro;
            AreaCodepro = "0001";
            List<ChinaStates> listpro = new ChinaBF().SelectPro(AreaCodepro);
            
            SelectList bb = new SelectList(listpro, "AreaCode", "AreaName",pro); //第四个参数是选定的值，通过选定 的这个值来筛选市辖
            ViewBag.databb = bb;

            //市辖
            
            List<ChinaStates> listcity1 = new ChinaBF().SelectCity1(pro);//由上一步选定的值来筛选市辖
            SelectList cc1 = new SelectList(listcity1, "AreaCode", "AreaName",city);
            ViewBag.datacc1 = cc1;
     
            //市区
            var b = listcity1.Exists(P => P.AreaCode == city) ? city : listcity1[0].AreaCode;//两种情况，需要判断一下 看看市辖是不是当前的
            List<ChinaStates> listcity = new ChinaBF().SelectCity2(b);
            SelectList cc = new SelectList(listcity, "AreaCode", "AreaName");
            ViewBag.datacc = cc;
            return View();
       
        }

    }
}

3、Views
@using 下拉列表联动显示_中国省市_.Models;

@{
    Layout = null;
}

<!DOCTYPE html>

<html>
<head>
    <meta name="viewport" content="width=device-width" />
    <title>Index</title>
</head>
<body>
    @{
        SelectList bb = ViewBag.databb;
        SelectList cc = ViewBag.datacc;
        SelectList cc1 = ViewBag.datacc1;
        }
     @using (Html.BeginForm("Index", "Home", FormMethod.Post))
       {
    <div>

        
        省份： @Html.DropDownList("pro",bb,new { onchange="document.forms[0].submit();"})
        市辖：@Html.DropDownList("city",cc1,new { onchange="document.forms[0].submit();"})
         市区：@Html.DropDownList("city1",cc)
    </div>
     }
</body>
</html>
效果图：