登录判断 之 Global通过Session，统一判断是否是否登录

void Application_PreRequestHandlerExecute(object sender, EventArgs e)
{
    var context = HttpContext.Current;
    if (context.Handler is IRequiresSessionState)
    {
        var passport= context.Session["username"];
        if (passport==null)
        {
            context.Response.Redirect("~/login.aspx");
        }
    }
}