protected void Page_Load(object sender, EventArgs e)    {if (!IsPostBack)        {string fileDependencyPath = Server.MapPath("~/TextFile1.txt");            Response.AddFileDependency(fileDependencyPath);            Response.Cache.SetExpires(DateTime.Now.AddSeconds(10));            Response.Cache.SetCacheability(HttpCacheability.Public);            Response.Cache.SetValidUntilExpires(true);            Cache.Insert("test1", DateTime.Now.ToString(), new System.Web.Caching.CacheDependency(fileDependencyPath));}}            這是一個依賴緩存，如何判斷依賴緩存的失效。