.net sdk 图片上传oss后，如何获取原图的缩略图

        /// <summary>
        /// 获取下载链接
        /// 算法参考：https://help.aliyun.com/document_detail/27135.html?spm=5176.8232292.domaindetail.19.IN2iTT
        /// </summary>
        public string GetDownloadUrl(string bucketName, string key, string cdnDomain,
             DateTime expireDate, string privateKey, out string msg)
        {
            msg = string.Empty;
            try
            {
                var ossUri = Clinet.GeneratePresignedUri(bucketName, key, expireDate);
                var cdnUrl = BuildCdnRequestUrl(_endpoint, bucketName, ossUri.AbsoluteUri, cdnDomain);
                var timestamp = (int)(expireDate - new DateTime(1970, 1, 1)).TotalSeconds;
                var rand = new Random().Next();
                var md5Value = GetMd5(string.Format("/{0}-{1}-{2}-{3}-{4}", key, timestamp, rand, 0, privateKey));
                cdnUrl += string.Format("&auth_key={0}-{1}-{2}-{3}", timestamp, rand, 0, md5Value);
                return cdnUrl;
            }
            catch (Exception ex)
            {
                msg = ex.Message;
                return null;
            }
        }