两个DropDownList的联动-----ASP.NET

当第一个DropDownList中的数据变动时，触发 事件onselectedindexchanged，在这个事件中完成对第二个DropDownList的数据绑定；

示例：城市和省份的联动

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            BingdingDDLProvince();
            BingdingDDLCity();
        }

    }
    //省份变动事件
    protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
    {
        string id = this.ddlProvince.SelectedValue;
        string sql = "select * from 城市表 where 所在省份=" + id;
        Response.Write(sql);
        this.ddlCity.DataSource = GetDataTable(sql);
        this.ddlCity.DataTextField = "城市名";
        this.ddlCity.DataBind();

    }
    //得到数据表。连接数据库
    public static DataTable GetDataTable(string sql)
    {
        SqlConnection conn = new SqlConnection("server=.;database=wuliu;uid=sa;pwd=ddx");
        conn.Open();
        SqlCommand cmd = new SqlCommand(sql, conn);
        SqlDataReader dr = cmd.ExecuteReader(CommandBehavior.CloseConnection);
        DataTable dt = new DataTable();
        dt.Load(dr);dr放置到dt数据表中
        dr.Close();
        return dt;
    }
    //绑定省份
    protected void BingdingDDLProvince()
    {
        string sql = "select * from 省份";
        this.ddlProvince.DataSource = GetDataTable(sql);
        this.ddlProvince.DataTextField = "省份名";                               DropDownList的数据区域
        this.ddlProvince.DataValueField = "省份编号";                         DropDownList的对应值
        this.ddlProvince.DataBind();
    }
    //绑定城市
    protected void BingdingDDLCity()
    {
        string sql = "select 城市名 from 城市表 where 所在省份='" + this.ddlProvince.SelectedValue + "'";
        this.ddlCity.DataSource = GetDataTable(sql);
        this.ddlCity.DataTextField = "城市名";
        this.ddlCity.DataBind();
    }