三级联动方法二

 string sqlcon = ConfigurationManager.ConnectionStrings["sqlservercon"].ConnectionString;
        protected void Page_Load(object sender, EventArgs e)
        {
            if(!IsPostBack)
            {
                GetProvince();
            }
        }
        public void GetProvince()
        {
            SqlConnection conn = new SqlConnection(sqlcon);
            conn.Open();
            SqlCommand cmd=new SqlCommand();
            cmd.Connection=conn;
            cmd.CommandText = "select provincename,provinceID from province";
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            adapter.Fill(dt);
            cmd.Dispose();
            conn.Dispose();
            this.ddlProvince.DataSource=dt;
            this.ddlProvince.DataTextField = "provincename";
            this.ddlProvince.DataValueField = "provinceID";
            this.ddlProvince.DataBind();
        }

        protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
        {
            ////每次加载的时候清空一下
            ddlCity.Items.Clear();
            int pid = Convert.ToInt32(ddlProvince.SelectedValue);

            SqlConnection conn = new SqlConnection(sqlcon);
            conn.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = conn;
            cmd.CommandText = "select cityname,cityID from city where father=@father";
            cmd.Parameters.AddWithValue("@father",pid);
            //DataSet dt = new DataSet();
            DataTable dt = new DataTable();
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            adapter.Fill(dt);
            cmd.Dispose();
            conn.Dispose();
            this.ddlCity.DataSource = dt;
            this.ddlCity.DataTextField = "cityname";
            this.ddlCity.DataValueField = "cityID";
            this.ddlCity.DataBind();
        }

        protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
        {
            ddlArea.Items.Clear();
            int cid = Convert.ToInt32(ddlCity.SelectedValue);
            SqlConnection conn = new SqlConnection(sqlcon);
            conn.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = conn;
            cmd.CommandText = "select areaname,areaID from area where father=@father";
            cmd.Parameters.AddWithValue("@father", cid);
            DataTable dt = new DataTable();
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            adapter.Fill(dt);
            cmd.Dispose();
            conn.Dispose();
            this.ddlArea.DataSource = dt;
            this.ddlArea.DataTextField = "areaname";
            this.ddlArea.DataValueField = "areaID";
            this.ddlArea.DataBind();
        }