asp.net 有刷新实现三级联动

using System;using System.Collections.Generic;using System.Linq;using System.Web;using System.Web.UI;using System.Web.UI.WebControls;using System.Data;using System.Data.SqlClient;using System.Configuration;public partial class _Default : System.Web.UI.Page{    string sqlstr=ConfigurationManager.ConnectionStrings["sqlstr"].ConnectionString;    protected void Page_Load(object sender, EventArgs e)    {        if (!IsPostBack)        {            GetProcince();            DropDownList4.Items.Insert(0, "--请选择省份--");            DropDownList2.Items.Insert(0, "--请选择城市--");            DropDownList3.Items.Insert(0, "--请选择区县--");        }            }    private void GetProcince()    {        using (SqlConnection sqlcnn=new SqlConnection(sqlstr))        {            using (SqlCommand sqlcmm=sqlcnn.CreateCommand())            {                sqlcmm.CommandText = "select * from province";                sqlcnn.Open();                DataTable dt = new DataTable();                SqlDataAdapter adapter = new SqlDataAdapter(sqlcmm);                adapter.Fill(dt);                this.DropDownList4.DataSource = dt;                this.DropDownList4.DataTextField = "province";                this.DropDownList4.DataValueField = "provinceid";                this.DropDownList4.DataBind();            }        }    }    protected void DropDownList4_SelectedIndexChanged(object sender, EventArgs e)    {        this.DropDownList3.Items.Clear();        using (SqlConnection sqlcnn = new SqlConnection(sqlstr))        {            using (SqlCommand sqlcmm = sqlcnn.CreateCommand())            {                sqlcmm.CommandText = "select * from city where father=@father";                sqlcmm.Parameters.AddWithValue("@father", this.DropDownList4.SelectedValue);                sqlcnn.Open();                DataTable dt = new DataTable();                SqlDataAdapter adapter = new SqlDataAdapter(sqlcmm);                adapter.Fill(dt);                this.DropDownList2.DataSource = dt;                this.DropDownList2.DataTextField = "city";                this.DropDownList2.DataValueField = "cityID";                this.DropDownList2.DataBind();                DropDownList2.Items.Insert(0, "--请选择城市--");                DropDownList3.Items.Insert(0, "--请选择区县--");            }        }    }    protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)    {        using (SqlConnection sqlcnn = new SqlConnection(sqlstr))        {            using (SqlCommand sqlcmm = sqlcnn.CreateCommand())            {                sqlcmm.CommandText = "select * from area where father=@father";                sqlcmm.Parameters.AddWithValue("@father", this.DropDownList2.SelectedValue);                sqlcnn.Open();                DataTable dt = new DataTable();                SqlDataAdapter adapter = new SqlDataAdapter(sqlcmm);                adapter.Fill(dt);                this.DropDownList3.DataSource = dt;                this.DropDownList3.DataTextField = "area";                this.DropDownList3.DataValueField = "areaID";                this.DropDownList3.DataBind();                DropDownList3.Items.Insert(0, "--请选择区县--");            }        }    }}