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1. selectedIndex——指的是dropdownlist中选项的索引，为int，从0开始，可读可写
2. selectedItem——指的是选中的dropdownlist中选项，为ListItem，只读不写
3. selectedValue——指的是选中的dropdownlist中选项的值，为string， 只读不写
4. selectedItem.Text——指的是选中的dropdownlist中选项的文本内容，与selectedItems的值一样为string，可读可写
5. selectedItem.value——指的是选中的dropdownlist中选项的值，与selectedValue的值一样，为string，可读可写
光看文字可能不太理解，我也是通过程序来加深理解的，下面举个例子：
前台代码：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="dropdown.aspx.cs" Inherits="dropdown" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server"><title>无标题页</title>
</head>
<body><form id="form1" runat="server"><div><asp:DropDownList ID="DropDownList1" runat="server"><asp:ListItem Value="1">北京</asp:ListItem><asp:ListItem Value="2">上海</asp:ListItem><asp:ListItem Value="3">广州</asp:ListItem></asp:DropDownList><asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="check" /><br /><asp:Label ID="Label1" runat="server" Text=""></asp:Label><br /><asp:Label ID="Label2" runat="server" Text=""></asp:Label><br /><asp:Label ID="Label3" runat="server" Text=""></asp:Label><br /><asp:Label ID="Label4" runat="server" Text=""></asp:Label><br /><asp:Label ID="Label5" runat="server" Text=""></asp:Label></div></form>
</body>
</html>后台代码：
using System;
using System.Data;
using System.Configuration;
using System.Collections;
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 dropdown : System.Web.UI.Page
{protected void Page_Load(object sender, EventArgs e){}protected void Button1_Click(object sender, EventArgs e){Label1.Text = "selectedIndex=" + DropDownList1.SelectedIndex;Label2.Text = "selectedItem=" + DropDownList1.SelectedItem;Label3.Text = "selectedValue=" + DropDownList1.SelectedValue;Label4.Text = "selectedItem.text=" + DropDownList1.SelectedItem.Text;Label5.Text = "selectedItem.value=" + DropDownList1.SelectedItem.Value;}
}运行效果如下：
https://blog.csdn.net/iqv520/article/details/4419186/?utm_medium=distribute.pc_relevant_bbs_down.none-task-blog-baidujs-2.nonecase&depth_1-utm_source=distribute.pc_relevant_bbs_down.none-task-blog-baidujs-2.nonecase