下拉列表(DropDownList)

演示一：
数据源使用数组列表的数据，改变选择时，激发一个事件

<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
　　OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
</asp:DropDownList>
<asp:Label ID="Label1" runat="server" Text="Label" Width="125px"></asp:Label>

演示二：两级联动
使用SQL自带的pubs库的authors表，实现选择不同姓名后自动改变城市的联动效果（使用了SqlDataSourse控件）

        请选择姓名：<asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="True" DataSourceID="SqlDataSource1" DataTextField="au_lname" DataValueField="au_id">
        </asp:DropDownList>
        城市：<asp:DropDownList ID="DropDownList3" runat="server" DataSourceID="SqlDataSource2" DataTextField="city" DataValueField="au_id">
        </asp:DropDownList>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:pubsCon %>" SelectCommand="SELECT [au_id], [au_lname] FROM [authors]"></asp:SqlDataSource>
        <asp:SqlDataSource ID="SqlDataSource2" runat="server" ConnectionString="<%$ ConnectionStrings:pubsCon %>" SelectCommand="SELECT [au_id], [city] FROM [authors] WHERE ([au_id] = @au_id)">
            <SelectParameters>
                <asp:ControlParameter ControlID="DropDownList2" Name="au_id" PropertyName="SelectedValue" Type="String" />
            </SelectParameters>
        </asp:SqlDataSource>

Web.config

    <connectionStrings>
        <add name="pubsCon" connectionString="Data Source=.;Initial Catalog=pubs;User ID=i05;Password=i05630229" providerName="System.Data.SqlClient" />
    </connectionStrings>

演示三：动态添加下拉列表的项

        <asp:DropDownList ID="DropDownList4" runat="server">
        </asp:DropDownList>&nbsp;
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />

以下是上面三个演示的后台代码

using System;
using System.Data;
using System.Configuration;
using System.Collections;//因为要使用ArrayList
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class DropDownList : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack) //是否首次加载（是否回发）
        {
            ArrayList arr1 = new ArrayList();
            arr1.Add("请选择");
            arr1.Add("张银");
            arr1.Add("aidd2008");

            DropDownList1.DataSource = arr1;
            DropDownList1.DataBind(); //绑定
        }
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        switch (DropDownList1.SelectedValue) //判断后激发事件
        { 
            case "张银":
                Label1.Text = "选择了本名";
                break;
            case "aidd2008":
                Label1.Text = "选择了其他";
                break;
            default:
                Label1.Text = "没有选择";
                break;
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string aa = TextBox1.Text.ToString();
        DropDownList4.Items.Add(aa); //添加文本框中的内容到下拉列表
    }
}