登录窗口登陆成功但是进入主窗口打开后登录窗口不关闭怎么回事

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Data.SqlClient;namespace YFanDriver
{
    public partial class LoginForm : Form
    {        public LoginForm()
        {
            InitializeComponent();        }        private void button1_Click(object sender, EventArgs e)
        {
            string Id = textBox1.Text;
            string LoginPwd = textBox2.Text;
            if (Id == string.Empty)
            {
                MessageBox.Show("请输入用户名！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }
            else if (LoginPwd == string.Empty)
            {
                MessageBox.Show("请输入密码！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
                return;
            }
            SqlConnection conn = new SqlConnection("Data Source=.;Initial Catalog=YFanDriver;Integrated Security=True");
            conn.Open();
            string sql = string.Format("select count(*) from users where Id='{0}' and LoginPwd='{1}'", Id, LoginPwd);
            SqlCommand comm = new SqlCommand(sql, conn);
            int tag = 0;
            if ((int)comm.ExecuteScalar() > 0)
                tag = 1;
            conn.Close();            if (tag == 1)
            {
                this.DialogResult = DialogResult.OK;
                MainForm form = new MainForm();
                form.ShowDialog();
                LoginForm login = new LoginForm();
                login.Close();
            }
            else
            {
                MessageBox.Show("用户名或密码错误！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }        }        private void button2_Click(object sender, EventArgs e)
        {
            this.Close();
        }
    }
}