这个怎么解决，谢谢

Question

编译时没有报错
但输入用户名和密码时，提示如图

Answer 1

using System;
using System.Collections.Generic;
using System.Text;
using System.Data;
using System.Data.OleDb;
using System.Configuration;
using System.Web;namespace gzsoft.myclass
{
    
    ///<summary>
    ///conn的摘要说明
    ///</summary>
public class mycon
{
        /// <summary>
        /// 连接数据库字符串
        /// </summary>
        private string connectionString;        /// <summary>
        /// 存储数据库连接（保护类，只有由它派生的类才能访问）
        /// </summary>
        protected OleDbConnection Connection;        /// <summary>
        /// 构造函数：数据库的默认连接
        /// </summary>
        public void Conn()
        {
            string connStr;
            connStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString.ToString();
            connectionString = connStr;
            //OleDbConnection cn = new OleDbConnection();
        }          /// <summary>
         /// 获得连接字符串`
        /// </summary>
        public string ConnectionString
        {
            get
            {
                return ConnectionString;
            }
        }    }
}

Answer 2

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Data.OleDb;
using gzsoft.myclass;
namespace gzsoft.windows
{
    public partial class Login : Form
    {
        public Login()
        {
            InitializeComponent();        }        mycon Mycon = new mycon();
    
        /// <summary>
        /// 退出登陆
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btn_exit_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }        /// <summary>
        /// 登陆系统
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btn_ok_Click(object sender, EventArgs e)
        {
            string Username = txt_yhm.Text.Trim();
            string Userpsword = txt_mm.Text.Trim();            try
            {
                if (txt_yhm.Text.Trim() != ""||txt_mm.Text.Trim()!="")
                {
                       string queryString = "select username,psword from tb_user where Username='" +
                          Username + "' and Userpsword='" + Userpsword + "'";
                        OleDbConnection cn = new OleDbConnection();
                        cn.Open();
                        OleDbCommand cmd = new OleDbCommand(queryString, cn);
                        OleDbDataReader reader = cmd.ExecuteReader();
                        reader.Read();
                        //reader.Close();
                   
                    if (reader.HasRows)
                    {
                        MessageBox.Show("登陆成功！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                    }
                    else
                    {
                        txt_yhm.Text="";
                        txt_mm.Text = "";
                        MessageBox.Show("用户名或者密码错误！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);                    }
                  
                }                else if(txt_mm.Text.Trim() == "")
                {
                    MessageBox.Show("请输入密码！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                }
            }            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}

Answer 3

 connStr = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString.ToString();
connStr is null or Empty

Answer 4

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="myconn" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=data\gzsoft.mdb"/> 
    </connectionStrings>
</configuration>
//app.config

Answer 5

 connStr = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString.ToString();
            connectionString = connStr;

Answer 6

connStr = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString.ToString(); 自己看你4楼的web.config怎么配置的...先打好基础别急着写代码...

Answer 7

connStr = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString.ToString(); 
--------
connStr = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString;还有...ConnectionString属性已经是string了...不要没事儿多穿条裤子...

Answer 8

public string ConnectionString
        {
            get
            {
                return ConnectionString;
            }
        }
你这里的C到底是大写还是小写啊？如果是大写。当然不对了