private void context_AcquireRequestState(object sender, EventArgs e)
        {
                string validateCode = StringHelper.GetRandNum().ToString();
                context.Session["ValidateCode"] = validateCode;//session为null
                context.Response.ContentType = "image/gif";
                context.Response.Cache.SetCacheability(HttpCacheability.NoCache);
                context.Response.Cache.SetNoStore();
                context.Response.Buffer = true;
                context.Response.ExpiresAbsolute = System.DateTime.Now.AddSeconds(-1);
                context.Response.Expires = -1;
                context.Response.CacheControl = "no-cache";
                context.Response.AddHeader("Pragma", "No-Cache");
                context.Response.BinaryWrite(ValidateHelper.getVerificationImage(validateCode).ToArray());
            }