三级联动---DropDownList控件

AutoPostBack属性：意思是自动回传，也就是说此控件值更改后是否和服务器进行交互
比如Dropdownlist控件，若设置为True，则你更换下拉列表值时会刷新页面（如果是网页的话），设置为flase就不会刷新了（也就是false时不和服务器交互）

列如：要操作ChinaStates表，  先连接数据库--三大类，ChinaDA类：如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.SqlClient;

/// <summary>
/// ChinaDA 的摘要说明
/// </summary>
public class ChinaDA
{
  private SqlConnection _conn ;
  private SqlCommand _cmd;
  private SqlDataReader _dr;
    public ChinaDA()
    {
        _conn = new SqlConnection("server=.;database=mydb;user=sa;pwd=100867");
        _cmd = _conn.CreateCommand();
    }

    public List<ChinaStates> Select(string PC)//只查询 父级（parentareacode）
    {
        _cmd.CommandText = "select * from ChinaStates where ParentAreaCode=@parentareacode";
        _cmd.Parameters.Add("@parentareacode",PC);
        _conn.Open();
        _dr = _cmd.ExecuteReader();
        List<ChinaStates> list = new List<ChinaStates>();
        if (_dr.HasRows)
        {
            while (_dr.Read())
            {
                ChinaStates cs = new ChinaStates();
                cs.AreaCode=_dr[].ToString();
                cs.AreaName=_dr[].ToString();
                cs.ParentAreaCode=_dr[].ToString();
                list.Add(cs);
            }
        }
        _conn.Close();
        return list;
    }
}

 aspx.cs里：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class sanji : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {//下面做了方法来这里调用,调用三次
            Bind(DropDownList1,new ChinaDA().Select(""));//中国下的
            Bind(DropDownList2,new ChinaDA().Select(DropDownList1.SelectedValue));//取省下面的值
            Bind(DropDownList3,new ChinaDA().Select(DropDownList2.SelectedValue));//取市下面的值

        }
        DropDownList1.SelectedIndexChanged += DropDownList1_SelectedIndexChanged;//做委托
        DropDownList2.SelectedIndexChanged += DropDownList2_SelectedIndexChanged;
    }

    void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
    {
       // 列表控件里的值在信息发往服务器时发生的变化，
        Bind(DropDownList3, new ChinaDA().Select(DropDownList2.SelectedValue));//取市下面的值-区
    }

    void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        Bind(DropDownList2, new ChinaDA().Select(DropDownList1.SelectedValue));//取省下面的值-市
        Bind(DropDownList3, new ChinaDA().Select(DropDownList2.SelectedValue));//取市下面的值-区
    }

    //填充三个下拉的内容，做集合   绑定数据
    private void Bind(DropDownList dd1, List<ChinaStates> list)
    {
        dd1.DataSource = list;
        dd1.DataTextField = "AreaName";
        dd1.DataValueField = "AreaCode";
        dd1.DataBind();//绑定到...
    }

}