ASP.NET中DropDownList和ListBox实现两级联动功能

<span style="font-family:KaiTi_GB2312;font-size:18px;"> protected void Page_Load(object sender, EventArgs e) 
 { 
  if (!Page.IsPostBack ) //判断页面是否第一次加载 
  { 
  SqlConnection con = DB.createConnection(); //此方法在上一篇文章中已经介绍，调用一个已经编写好的创建数据库连接的方法。 
  SqlCommand cmd = new SqlCommand("select  from province",con); 
  SqlDataReader sdr = cmd.ExecuteReader(); 
  this.DropDownList1.DataTextField = "proName"; 
  this.DropDownList1.DataValueField = "proID"; //主键字段 
  this.DropDownList1.DataSource = sdr; 
  this.DropDownList1.DataBind(); 
  sdr.Close(); 
 
  } 
 
 }</span> 

<span style="font-family:KaiTi_GB2312;font-size:18px;"> protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e) 
 { 
  this.ListBox1.Items.Clear(); 
  SqlConnection con2 = DB.createConnection(); 
  SqlCommand cmd1 = new SqlCommand("select  from city where proID=" + this.DropDownList1.SelectedValue, con2); 
  SqlDataReader sdr1 = cmd1.ExecuteReader(); 
  while (sdr1.Read()) 
  { 
  this.ListBox1.Items.Add(new ListItem(sdr1.GetString(2),sdr1.GetInt32(0).ToString())); 
  } 
 }</span>