程序退出时，显示程序运行时间

关闭程序后，显示程序运行时间。代码如下：

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.Diagnostics;
namespace time
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private Stopwatch stop= new Stopwatch();
        private void Form1_Load(object sender, EventArgs e)
        {
            stop.Start();
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            DialogResult dr = MessageBox.Show("真的要退出？", "退出", MessageBoxButtons.YesNo, MessageBoxIcon.Question);
            if (dr == DialogResult.Yes)
            {
                stop.Stop();
                MessageBox.Show("程序共运行时间:" + stop.Elapsed.Seconds.ToString() + "秒");
                e.Cancel = false;
            }
            else
            {
                e.Cancel = true;
            }
        }
    }
}