ASP.NET实现省市县三级联动

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                GetProvince();
            }

        }
        private void LoadData(string id, DropDownList ddl)
        {
            string strcon = @"Data Source=;Initial Catalog=;Persist Security Info=True;User ID=sa;Password=";
            SqlConnection conn = new SqlConnection(strcon);
            SqlCommand cmd = conn.CreateCommand();
            cmd.CommandText = "SELECT A_Id,A_Name FROM T_Arear WHERE A_ParentId=@parentid ORDER BY A_Id";
            cmd.Parameters.AddWithValue("@parentid", id);
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            adapter.Fill(dt);
            cmd.Dispose();
            conn.Dispose();

            ddl.DataSource = dt;
            //指定下拉列表要显示的字段的值
            ddl.DataTextField = "A_Name";
            //指定下拉列表的value属性要绑定的字段的值
            ddl.DataValueField = "A_Id";
            ddl.DataBind();
            //插入默认选中项
            if (id == "0")
            {
                ListItem item = new ListItem("---请选择---", "0");
                //this.ddlProvince.Items.Add(item);
                ddl.Items.Insert(0, item);
            }
        }
        private void GetProvince()
        {
            LoadData("0", this.ddlProvince);
        }

        private int Sum(int x, int y)
        {
            int num = x + y;
            return num;
        }

        protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
        {
            string id = this.ddlProvince.SelectedItem.Value;
            if (id != "0")
            {
                LoadData(id, this.ddlCity);
                string selectid = this.ddlCity.SelectedItem.Value;
                LoadData(selectid, this.ddlArear);
            }
            else
            {
                ddlCity.Items.Clear();
                ddlArear.Items.Clear();
            }
        }

        protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
        {
            string id = this.ddlCity.SelectedItem.Value;
            LoadData(id, this.ddlArear);
        }

    }