列表框中的选定项为空-C#/.NET问题

Selected item in list box is null(列表框中的选定项为空)

本文介绍了列表框中的选定项为空的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

列表框中的项目很少.当我选择一个项目时，我将它存储在一个字符串中以供进一步使用，但是当我显示该字符串值时，它显示为 null.

下面是我的 Invitee.aspx 文件的代码:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Invitee.aspx.cs" Inherits="FinalProj2.Invitee" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 过渡//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head runat="服务器"><标题></标题></头><身体><form id="form1" runat="server"><asp:DropDownList ID="DropDownList1" runat="server"></asp:DropDownList><asp:DropDownList ID="DropDownList2" runat="server"></asp:DropDownList><asp:DropDownList ID="DropDownList3" runat="server"></asp:DropDownList>&nbsp;&nbsp;&nbsp;&nbsp;<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" 宽度="271px"></asp:ListBox><asp:TextBox ID="TextBox1" runat="server" Height="217px" Width="544px"></asp:TextBox></表格></身体></html>

Invitee.aspx.cs 的代码:

使用系统；使用 System.Collections.Generic；使用 System.Linq；使用 System.Web；使用 System.Web.UI；使用 System.Web.UI.WebControls；命名空间 FinalProj2{公共部分类受邀者: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 = 来自 db.Employees 中的 emp选择emp.Employee_Name；ListBox1.DataSource = 查询；ListBox1.DataBind();}protected void ListBox1_SelectedIndexChanged(对象发送者，EventArgs e){Response.Write("你好");字符串选择 = ListBox1.SelectedValue.ToString();Response.Write("
 选择的值是" + selected);var query = 来自 db.Employees 中的 emp其中 emp.Employee_Name == 已选择选择emp.Employee_ID；Response.Write(查询);//int empid = query.First();//var query1 = from meet_emp in db.Meet_Emps//其中 meet_emp.Employee_ID == empid//选择 meet_emp.Meeting_ID;//int meetid = query1.First();//Response.Write(meetid);}}}

在列表框中选择一个项目后，字符串selected"的值为空.

解决方案

这是因为你在每个周期都运行 ListBox1.DataBind().改为:

if (!IsPostBack){var query = 来自 db.Employees 中的 emp选择emp.Employee_Name；ListBox1.DataSource = 查询；ListBox1.DataBind();}

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.

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>
&nbsp;&nbsp;&nbsp;&nbsp;
        <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>

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("
 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);
        }
    }
}

Value of string "selected" is null after I select an item in list box.

解决方案

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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