未附加VS调试器时Task.WhenAll()异常：HttpClient超时设置解决

在异步编程中使用Task.WhenAll()批量下载Google Drive文件夹时遇到以下异常问题：

• 附加Visual Studio调试器运行：下载约1000个文件耗时22-25秒，全程正常
• 未附加调试器运行：耗时增至2分钟以上，同时抛出System.Threading.Tasks.TaskCanceledException异常

文件夹下载核心代码 #

public static async Task<bool> DownloadFolder(CloudDataModel.File file, string path, params string[] exclude)
{
    try
    {
        if (file != null && !string.IsNullOrEmpty(file.id))
        {
            List<string> toExclude = new List<string>();
            if(exclude != null)
            {
                toExclude = exclude.ToList();
            }
           
            List<Task> downloadFilesTask = new List<Task>();
            var files = await file.GetFiles();
            foreach (var f in files)
            {
                var task = f.Download(path);
                downloadFilesTask.Add(task);
            }
            
            var folders = await file.GetFoldersAsync();
            foreach (var folder in folders)
            {
                if (toExclude.Contains(folder.name))
                {
                    continue;
                }
                Task task = null;
                if (path.Equals(Statics.ProjectFolderName))
                {
                    task = DownloadFolder(folder, folder.name);
                }
                else
                {
                    task = DownloadFolder(folder, Path.Combine(path, folder.name));
                }
              
                downloadFilesTask.Add(task);
            }
            var array = downloadFilesTask.ToArray();
            await Task.WhenAll(array);
            return true;
        }                
    }
    catch (Exception e)
    {
        Crashes.TrackError(e);
    }
    return false;
}

问题根源：单个文件下载方法 #

经排查，异常由文件下载方法DownloadFile引发，核心代码如下：

public static async Task<StorageFile> DownloadFile(CloudDataModel.File file, string destinationFolder)
{
    try
    {
        
        if (file != null)
        {
            Debug.WriteLine($"start download {file.name}");
            if (file.mimeType == Statics.GoogleDriveFolderMimeType)
            {
                Debug.WriteLine($"did not download resource,  resource was folder instead of file. mimeType: {file.mimeType}");
                return null;
            }

            var endpoint = Path.Combine(DownloadFileEndpoint, $"{file.id}?alt=media");
            // 此处引发异常行为
            HttpResponseMessage response = await client.GetAsync(endpoint);
            
           
     
            StorageFile downloadedFile;
            using (Stream streamToReadFrom = await response.Content.ReadAsStreamAsync())
            {
                var memstream = new MemoryStream();
                StreamReader reader = new StreamReader(streamToReadFrom);
                streamToReadFrom.Position = 0;
                await streamToReadFrom.CopyToAsync(memstream);
                downloadedFile = await fileHandler.SaveDownloadedCloudFile(memstream, file, destinationFolder);
                Debug.WriteLine($"download finished {file.name}");
            }

            return downloadedFile;
        }

        return null;
    }

    catch (Exception e)
    {
        Crashes.TrackError(e);
        return null;
    }
}

解决方案 #

为System.Net.Http.HttpClient设置超时时间后，代码执行恢复正常：

client.Timeout = new TimeSpan(0,0,5);

内容的提问来源于stack exchange，提问作者Rick Bieze