请教获取OSS文件列表的错误

下面的函数是用来获取OSS上的文件列表的，但在执行时，经常出现下面的错误（一会行，一会不行的，头疼），恭请老大们指点





        /// <summary>
        /// 返回文件及MD5列表
        /// </summary>
        /// <param name="client"></param>
        /// <param name="bucketName">oss空间的名称</param>
        /// <param name="subdir">OSS上的子文件夹名称（末尾不带斜杠）</param>
        /// <param name="All">是否列出所有子文件夹的文件</param>
        /// <returns>“List<string> 文件及MD5列表”</returns>


        public static List<List<string>> DirFileMD5lst(OssClient client, String bucketName, string subdir, bool All)
        {
            List<List<string>> LSTZ = new List<List<string>>();


            if (subdir.Trim() != "")
            {
                subdir = subdir.Replace("\\", "/"); //子文件夹绝对不能用“\\”
                string k = subdir.Substring(subdir.Length - 1, 1);
                if (k != "/")
                {
                    subdir += "/";
                }
            }
            ListObjectsRequest req = new ListObjectsRequest(bucketName);


            req.Prefix = subdir;
            if (All) req.Delimiter = "";
            else req.Delimiter = "/";


            req.MaxKeys = 1000;

            ObjectListing objs = client.ListObjects(req); // ★提示错误的就是这一句 ★

            List<string> lst = new List<string>();




            foreach (OssObjectSummary obj in objs.ObjectSummaries)
            {
                string key = obj.Key; //文件名称
                string MD5 = obj.ETag;
                LSTZ.Add(new List<string> { key, MD5 });
            }






            return LSTZ;
        }