（一）数据库操作方法：public class mydb
{private MyDBDataContext context = new MyDBDataContext();public List<ChinaStates> Select(string AreaCode)//将中国的表全部查询
    {return  context.ChinaStates.Where(p=>p.ParentAreaCode==AreaCode).ToList();}

}

（二） 操作方法
public partial class yemian : System.Web.UI.Page
{protected void Page_Load(object sender, EventArgs e)//显示界面{if (!IsPostBack){fangfa();}}protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)//根据第一个来操作第二个和第三个{string code = DropDownList1.SelectedItem.Value;List<ChinaStates> diqu1 = new mydb().Select(code);DropDownList2.DataSource = diqu1;DropDownList2.DataTextField = "AreaName";DropDownList2.DataValueField = "AreaCode";DropDownList2.DataBind();List<ChinaStates> diqu3= new mydb().Select(code);DropDownList3.DataSource = diqu3;DropDownList3.DataTextField = "AreaName";DropDownList3.DataValueField = "AreaCode";DropDownList3.DataBind();}private void fangfa()//写了一个方法{List<ChinaStates> list = new mydb().Select("0001");DropDownList1.DataSource = list;//制定数据源DropDownList1.DataTextField = "AreaName";DropDownList1.DataValueField = "AreaCode";DropDownList1.DataBind();//最后绑定一下List<ChinaStates> shi = new mydb().Select("11");DropDownList2.DataSource = shi;DropDownList2.DataTextField = "AreaName";DropDownList2.DataValueField = "AreaCode";DropDownList2.DataBind();List<ChinaStates> diqu = new mydb().Select("1101");DropDownList3.DataSource = diqu;DropDownList3.DataTextField = "AreaName";DropDownList3.DataValueField = "AreaCode";DropDownList3.DataBind();}protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)//根据第二个改变第三个{string code = DropDownList2.SelectedItem.Value;List<ChinaStates> diqu2 = new mydb().Select(code);DropDownList3.DataSource = diqu2;DropDownList3.DataTextField = "AreaName";DropDownList3.DataValueField = "AreaCode";DropDownList3.DataBind();}
}

点击改变事件一定要将AutoPostBack 的属性改成true