下面就是在提交按钮的单击事件中填写代码（代码区）（前提是把省市县的数据库建好）

  protected void Page_Load(object sender, EventArgs e)         {             if (!this.Page.IsPostBack)             {                 getddlProvinceDataBind();       //页面首次加载执行省份绑定

            }         }            public void getddlProvinceDataBind()       //省份数据绑定         {             string sqlProvince = "SELECT * FROM province";             DropDownList2.DataSource = getDataSet(sqlProvince);             DropDownList2.DataTextField = "province";             DropDownList2.DataValueField = "provinceID";             DropDownList2.DataBind();

            DropDownList2.Items.Insert(0, new ListItem("--省份--", "0"));         }         protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)         {             //第一层，省份选择事件             {                 int ProvinceID = Convert.ToInt32(DropDownList2.SelectedValue);                 if (ProvinceID > 0)                 {                     string sqlCity = "SELECT * FROM city WHERE father=" + ProvinceID + "";       //根据省份ID找城市                     DropDownList3.DataSource = getDataSet(sqlCity);                     DropDownList3.DataTextField = "city";                     DropDownList3.DataValueField = "cityID";                     DropDownList3.DataBind();

                    DropDownList3.Items.Insert(0, new ListItem("--请选择城市--", "0"));                 }                 else                 {                     DropDownList3.Items.Clear();                     DropDownList3.Items.Insert(0, new ListItem("--请选择城市--", "0"));                     DropDownList3.Items.Clear();                     DropDownList3.Items.Insert(0, new ListItem("--请选择县区--", "0"));                 }             }         }

        protected void DropDownList3_SelectedIndexChanged(object sender, EventArgs e)         {             //第二层，城市件             {                 int CityID = Convert.ToInt32(DropDownList3.SelectedValue);                 if (CityID > 0)                 {                     string sqlDistrict = "SELECT * FROM area  WHERE father=" + CityID + "";       //根据城市ID找县区                     DropDownList4.DataSource = getDataSet(sqlDistrict);                     DropDownList4.DataTextField = "area";                     DropDownList4.DataValueField = "areaID";                     DropDownList4.DataBind();

                    DropDownList4.Items.Insert(0, new ListItem("--请选择县区--", "0"));                 }                 else                 {                     DropDownList4.Items.Clear();                     DropDownList4.Items.Insert(0, new ListItem("--请选择县区--", "0"));                 }             }         }

        protected void DropDownList4_SelectedIndexChanged(object sender, EventArgs e)         {             int ProvinceID = Convert.ToInt32(DropDownList2.SelectedValue);             int CityID = Convert.ToInt32(DropDownList3.SelectedValue);             int DistrictID = Convert.ToInt32(DropDownList4.SelectedValue);             //if (ProvinceID > 0 && CityID > 0 && DistrictID > 0)             //{             //    Response.Write("您选择的省份ID：" + ProvinceID + "城市ID：" + CityID + "县区ID：" + DistrictID + "");             //}         }         public DataSet getDataSet(string sql)       //自定义方法，sql语句参数，返回DataSet数据集         {             string connection = ConfigurationManager.ConnectionStrings["sqlcon"].ConnectionString;             SqlConnection conn = new SqlConnection(connection);             SqlDataAdapter sda = new SqlDataAdapter(sql, conn);             DataSet ds = new DataSet();             sda.Fill(ds);             return ds;         }

这样，就完成我们想要的效果了。如下图所示：

省市县联动dropdownlist