using System.IO;
protected void Application_Start(object sender, EventArgs e)
        {
            FileStream fs = File.Open(Server.MapPath("counter.txt"),FileMode.OpenOrCreate);
            StreamReader sr = new StreamReader(fs);
            Application["count"] = Convert.ToInt32(sr.ReadLine());
            sr.Close();
            fs.Close();
        }
//counter.txt会自动创建
        protected void Session_Start(object sender, EventArgs e)
        {
            Application.Lock();
            Application["count"] = (int)Application["count"] + 1;
            FileStream fs = File.Open(Server.MapPath("counter.txt"), FileMode.OpenOrCreate,FileAccess.ReadWrite);
            StreamWriter sw = new StreamWriter(fs);
            sw.WriteLine(Application["count"]);
            sw.Close();
            fs.Close();
            Application.UnLock();
        }