OSS如何获取私有bucket的object URL

OssClient ossClient = new OssClient(OssManager._http, response.Credentials.AccessKeyId, response.Credentials.AccessKeySecret, response.Credentials.SecurityToken);
这个使用临时的ak ,sk ，SecurityToken 新建一个ossClient 客户端，可以遍历它里面的文件，然后我用

var req = new GeneratePresignedUriRequest(OssManager._bucket, obj.Key, SignHttpMethod.Get);

            {
                //DateTime Expiration = DateTime.Now.AddHours(1);
            };
            //生成URL
            var uri = ossClient.GeneratePresignedUri(req);

这两句话，得到url ，用浏览器访问这个url 就会报SignatureDoesNotMatch 错误！！！
很郁闷，好几天了，