使用Service Account从Google Drive下载Update.exe文件失败问题

我尝试用Google Drive结合服务账号实现应用自动更新功能：Google Drive里有个名为Update的文件夹已共享给该服务账号，文件夹内包含记录最新版本号的Version.txt文件和约1MB的Update.exe可执行文件。目前能正常读取Version.txt，但下载Update.exe时，看似执行了下载流程，内存流ms2却始终为空，怀疑和服务账号认证有关，求解决。

相关代码如下：

public class GoogleDriveAccess
{
    //google drive file IDs
    private readonly static string versionFileID = @"blahblahblahblah";
    private readonly static string updateFileID = @"blehblehblehbleh";

    //check for updated assembly on google drive and install if authorised
    public async static void CheckForUpdate()
    {
        try
        {
            //load json key file from resources into a byte array
            var jsonFile = Properties.Resources.drive_access;

            //create service account keyfile from json byte array
            var serviceAccountKeyfile = System.Text.Encoding.Default.GetString(jsonFile);

            //create Google credentials with full access from service account keyfile
            GoogleCredential credential = GoogleCredential.FromJson(serviceAccountKeyfile)
                .CreateScoped(new[] { DriveService.ScopeConstants.Drive });

            //create Google drive service using credentials
            DriveService service = new DriveService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential
            });

            //get version file metadata
            var getRequest = service.Files.Get(versionFileID);

            //download version file to memory
            await using var ms = new MemoryStream();
            await getRequest.DownloadAsync(ms);

            //read memory stream into a string
            ms.Position = 0;
            var sr = new StreamReader(ms);
            string textUpdateVersion = sr.ReadToEnd().Trim();

            //obtain our current assembly version
            Version currentVersion = Assembly.GetEntryAssembly().GetName().Version;

            //create an assembly version from the downloaded version text file
            Version updateVersion = new Version(textUpdateVersion);

            //if downloaded version is higher than our current version then request to download new assembly
            if (updateVersion > currentVersion)
            {
                //prompt user to commence update
                DialogResult result = MessageBox.Show($"Would you like to update to the latest version {updateVersion}?", Properties.Resources.AppName, MessageBoxButtons.YesNo);
                if (result == DialogResult.Yes)
                {
                    //get update file metadata
                    getRequest = service.Files.Get(updateFileID);

                    //download update file to memory
                    await using var ms2 = new MemoryStream();
                    await getRequest.DownloadAsync(ms2);

                    //***ms2 is always empty here***

                    //convert file to binary
                    ms2.Position = 0;
                    var br = new BinaryReader(ms2);
                    byte[] bin = br.ReadBytes(Convert.ToInt32(ms2.Length));

                    //rest of code

                }
            }
        }
        catch
        {
           //deal with any exceptions
        }
    }
}

排查与解决建议 #

• 验证文件权限与ID：确认Update.exe的文件ID完全正确，且该文件已明确共享给服务账号的邮箱地址，权限至少设为“查看”（下载需要此权限）。
• 修正异步方法签名：将async static void CheckForUpdate()改为async static Task CheckForUpdate()，调用时使用await GoogleDriveAccess.CheckForUpdate();。async void会导致异步操作未完成就继续执行后续代码，可能出现流未写入就读取的情况。
• 添加异常详情捕获：把空的catch块改为捕获具体异常并输出详情，比如：

  catch (Exception ex)
  {
      MessageBox.Show($"更新出错：{ex.Message}\n{ex.StackTrace}");
  }
  

  这能快速定位是权限问题、API调用错误还是文件不存在等问题。
• 检查API作用域：确认使用的Scope是DriveService.Scope.DriveReadonly（只读权限足够下载文件）或DriveService.Scope.Drive，避免权限不足导致下载失败。
• 下载前验证流状态：在操作ms2前先判断ms2.Length是否大于0，若为0则直接提示下载失败，避免后续无效操作：

  await getRequest.DownloadAsync(ms2);
  if (ms2.Length == 0)
  {
      MessageBox.Show("更新文件下载失败，流为空");
      return;
  }
  ms2.Position = 0;
  //后续二进制读取代码
  

内容的提问来源于stack exchange，提问作者Deolus