using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace PWMS
{
    public partial class F_Login : Form
    {
        DataClass.MyMeans MyClass = new PWMS.DataClass.MyMeans();

        public F_Login()
        {
            InitializeComponent();
        }

        private void textName_TextChanged(object sender, EventArgs e)
        {

        }

        private void textName_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == '\r')
                textPass.Focus();
        }

        private void textPass_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == '\r')
                butLogin.Focus();

        }

        private void butClose_Click(object sender, EventArgs e)
        {
            if ((int)(this.Tag) == 1)
            {
                DataClass.MyMeans.Login_n = 3;
                Application.Exit();
            }
            else
                if ((int)(this.Tag) == 2)
                    this.Close();
        }

        private void butLogin_Click(object sender, EventArgs e)
        {
            if (textName.Text != "" & textPass.Text != "")
            //用自定义方法getcom()在tb_Login数据库表中查找是否有当前登录用户
            {
                SqlDataReader temDR = MyClass.getcom("select * from tb_Login where Name='" + textName.Text.Trim() + "'and Pass='" + textPass.Text.Trim() + "'");
                bool ifcom = temDR.Read();
                if (ifcom)
                {
                    DataClass.MyMeans.Login_Name = textName.Text.Trim();   //将当前用户名记录到全局变量中去
                    DataClass.MyMeans.Login_ID = temDR.GetString(0);
                    DataClass.MyMeans.My_con.Close();    //关闭连接
                    DataClass.MyMeans.My_con.Dispose();   //释放资源
                   [font color=#FF0000] DataClass.MyMeans.Login_n = (int)(this.Tag); [/font]   
                    this.Close();
                }
                else
                {
                    MessageBox.Show("用户名不存在或密码错误!", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                    textName.Text = "";
                    textPass.Text = "";
                }
                MyClass.con_close();
            }
            else 
            {
                MessageBox.Show("用户名和密码不能为空", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }

        private void F_Login_Load(object sender, EventArgs e)
        {
            try
            {
                MyClass.con_open();  //连接数据库
                MyClass.con_close();
                textName.Text = "";
                textPass.Text = "";

            }
            catch
            {
                MessageBox.Show("数据库连接失败。", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                Application.Exit();
            }

        }
    }
}

版主你好，在我程序中的这句
DataClass.MyMeans.Login_n = (int)(this.Tag); 
报出了NullReferenceException
注：这段代码是我模仿你们的源程序写的