在列表框中选择的项目为空 [英] Selected item in list box is null
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问题描述
我在列表框中几个项目。当我选择一个项目，我将其存储在一个字符串的进一步使用，但是当我显示该字符串值，其显示为空。
I have few items in the listbox. When I select an item, I store it in a string for further use, but when I display that string value it is shown as null.
下面是code我Invitee.aspx文件：
Below is the code for my Invitee.aspx file:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Invitee.aspx.cs" Inherits="FinalProj2.Invitee" %>
<!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></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="DropDownList1" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList2" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList3" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList4" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList5" runat="server">
</asp:DropDownList>
<asp:DropDownList ID="DropDownList6" runat="server">
</asp:DropDownList>
</div>
<asp:ListBox ID="ListBox1" runat="server" AutoPostBack="true" Height="310px"
onselectedindexchanged="ListBox1_SelectedIndexChanged" Width="271px">
</asp:ListBox>
<asp:TextBox ID="TextBox1" runat="server" Height="217px" Width="544px"></asp:TextBox>
</form>
</body>
</html>
和$ C $下Invitee.aspx.cs：
And code for Invitee.aspx.cs:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace FinalProj2
{
public partial class Invitee : System.Web.UI.Page
{
FinalProj2.Models.DataClasses1DataContext db = new FinalProj2.Models.DataClasses1DataContext();
protected void Page_Load(object sender, EventArgs e)
{
for (int i = 1; i < 13; i++)
{
DropDownList1.Items.Add(new ListItem(i.ToString()));
DropDownList4.Items.Add(new ListItem(i.ToString()));
}
for (int i = 1; i < 32; i++)
{
DropDownList2.Items.Add(new ListItem(i.ToString()));
DropDownList5.Items.Add(new ListItem(i.ToString()));
}
for (int i = 2010; i < 2021; i++)
{
DropDownList3.Items.Add(new ListItem(i.ToString()));
DropDownList6.Items.Add(new ListItem(i.ToString()));
}
var query = from emp in db.Employees
select emp.Employee_Name;
ListBox1.DataSource = query;
ListBox1.DataBind();
}
protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Response.Write("Hello");
string selected = ListBox1.SelectedValue.ToString();
Response.Write("\n Selected Value is" + selected);
var query = from emp in db.Employees
where emp.Employee_Name == selected
select emp.Employee_ID;
Response.Write(query);
//int empid = query.First();
//var query1 = from meet_emp in db.Meet_Emps
// where meet_emp.Employee_ID == empid
// select meet_emp.Meeting_ID;
//int meetid = query1.First();
//Response.Write(meetid);
}
}
}
选择的字符串值是空后，我在列表框中选择一个项目。
Value of string "selected" is null after I select an item in list box.
推荐答案
这是因为你在每个周期中运行 ListBox1.DataBind（）。更改为：
It's because you are running ListBox1.DataBind() on every cycle. Change to:
if (!IsPostBack)
{
var query = from emp in db.Employees
select emp.Employee_Name;
ListBox1.DataSource = query;
ListBox1.DataBind();
}
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