public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            Application["NumeroSessioni"]=0;
        }

        protected void Application_End(object sender, EventArgs e)
        {
            Application["NumeroSessioni"]=0;
        }

        protected void Session_Start(object sender, EventArgs e)
        {
            Application.Lock();            
            Application["NumeroSessioni"]=int.Parse(Application["NumeroSessioni"].ToString())+1;
            Application.UnLock();
        }

        protected void Session_End(object sender, EventArgs e)
        {
            Application.Lock();
            Application["NumeroSessioni"]=int.Parse(Application["NumeroSessioni"].ToString())-1;
            Application.UnLock();
        }
    }