如何在Amazon S3存储桶中创建子目录

我正在使用Amazon S3 Web服务，但无法在我的存储桶上创建子目录。

我想要这样的东西

用户上载文件时，会在S3 Bucket上创建一个名为user id的子目录，并将文件存储在该子目录中。

我正在使用以下代码

 AmazonS3Client s3Client = new AmazonS3Client("AWSAccessKey", "AWSSecretKey", Amazon.RegionEndpoint.APSoutheast1);  

    protected void btnUpload_Click(object sender, EventArgs e)
    {

        if (!string.IsNullOrEmpty(fileUpload.FileName))
        {
            //Saving File to local disk folder.
            string filePath = Server.MapPath("aa") + "\\" + fileUpload.FileName;
            string fileExtension = fileUpload.FileName.Substring(fileUpload.FileName.LastIndexOf(".") + 1);
            fileUpload.SaveAs(filePath);
            string contentType = GetContentType(fileExtension);    

            //Push the given object into S3 Bucket
            PutObjectRequest objReq = new PutObjectRequest
            {
                Key = fileUpload.FileName,
                FilePath =  filePath,
                ContentType = contentType,
                BucketName = "datastore.MyData",
                CannedACL = S3CannedACL.Private,               

            };

            PutObjectResponse response = s3Client.PutObject(objReq);
            if (response.ETag != null)
            {
                string etag = response.ETag;
                string versionID = response.VersionId;
                Response.Write("");
            }
            //Deleting Localy Saved File
            if (File.Exists(filePath))
            {
                File.Delete(filePath);
            }
        }
    }
    private string GetContentType(string fileExtension)
    {
        string contentType = string.Empty;
        switch (fileExtension)
        {
            case "bmp": contentType = "image/bmp"; break;
            case "jpeg": contentType = "image/jpeg"; break;
            case "jpg": contentType = "image/jpg"; break;
            case "gif": contentType = "image/gif"; break;
            case "tiff": contentType = "image/tiff"; break;
            case "png": contentType = "image/png"; break;
            case "plain": contentType = "text/plain"; break;
            case "rtf": contentType = "text/rtf"; break;
            case "msword": contentType = "application/msword"; break;
            case "zip": contentType = "application/zip"; break;
            case "mpeg": contentType = "audio/mpeg"; break;
            case "pdf": contentType = "application/pdf"; break;
            case "xgzip": contentType = "application/x-gzip"; break;
            case "xcompressed": contentType = "applicatoin/x-compressed"; break;
        }
        return contentType;
    }

请帮助我实现这一目标。