问Google drive api v3响应为空

EN

 public static File UploadFile(DriveService service, string fileName, string filePath, string description, string parent)
{
  var fileMetadata = new File
  {
    Name = fileName,
    MimeType = GetMimeType(fileName),
    Description = description,
    OriginalFilename = fileName,
  };

  FilesResource.CreateMediaUpload request;
  using (var stream = new System.IO.FileStream(filePath, System.IO.FileMode.Open))
  {
      request = service.Files.Create(fileMetadata, stream, GetMimeType(fileName));
      request.Fields = "id";
      request.Alt=FilesResource.CreateMediaUpload.AltEnum.Json;
      request.Upload();
  }
  var file = request.ResponseBody;
  Console.WriteLine("File ID: " + file.Id);


  return null;

}