Task.WaitAll(...)阻止MVC 4应用程序

Question

在MVC 4应用程序中,我有一个将文件复制到目标文件夹的任务.因为我有多个文件,所以我为每个文件创建一个任务,我想等到它们都完成了.问题是我的代码块Task.WaitAll(copyingTasks.ToArray())好像任务永远不会结束,所以它永远不会通过那行代码.下面是示例代码:

    private void CopyFilesFromWorkingCopyForProject(string projectName)
    {
        var copyingTasks = new List<Task>
            {
                CopyAllFromDirectoryToDirectory(FilesUtils.AndroidConfigsPath(), FilesUtils.AndroidPathForProject(projectName)),
                CopyAllFromDirectoryToDirectory(FilesUtils.AndroidValuesPath(), FilesUtils.AndroidPathForProject(projectName)),
                CopyFileToDirectory(FilesUtils.AndroidManifestPath(), FilesUtils.AndroidPathForProject(projectName)),
                CopyAllFromDirectoryToDirectory(FilesUtils.IosConfigsPath(), FilesUtils.IosPathForProject(projectName))

            };
        Task.WaitAll(copyingTasks.ToArray());
    }

    private async Task CopyAllFromDirectoryToDirectory(string sourceDirectory, string destinationDirectory)
    {
        foreach (string filename in Directory.EnumerateFiles(sourceDirectory))
        {
            await CopyFileToDirectory(filename, destinationDirectory);
        }
    }

    private async Task CopyFileToDirectory(string filename, string destinationDirectory)
    {
        using (FileStream sourceStream = File.Open(filename, FileMode.Open))
        {
            using (FileStream destinationStream = File.Create(destinationDirectory + filename.Substring(filename.LastIndexOf('\\'))))
            {
                await sourceStream.CopyToAsync(destinationStream);
            }
        }
    }

如果我评论 Task.WaitAll(copyingTasks.ToArray());它不再阻止,但我想等待所有文件被复制.

Answer 1

组合await和同步等待会导致死锁,因为async方法会尝试在当前被等待阻止的上下文中恢复.

你应该做的,而不是是让CopyFilesFromWorkingCopyForProject()也async(和调用该方法,并调用该方法,...):

private async Task CopyFilesFromWorkingCopyForProject(string projectName)
{
    var copyingTasks = new List<Task>
        {
            CopyAllFromDirectoryToDirectory(FilesUtils.AndroidConfigsPath(), FilesUtils.AndroidPathForProject(projectName)),
            CopyAllFromDirectoryToDirectory(FilesUtils.AndroidValuesPath(), FilesUtils.AndroidPathForProject(projectName)),
            CopyFileToDirectory(FilesUtils.AndroidManifestPath(), FilesUtils.AndroidPathForProject(projectName)),
            CopyAllFromDirectoryToDirectory(FilesUtils.IosConfigsPath(), FilesUtils.IosPathForProject(projectName))

        };
    await Task.WhenAll(copyingTasks);
}

如果您不能或不想这样做,则需要确保async方法不会在当前上下文中恢复.为此,您可以使用ConfigureAwait(false)所有awaits,或者您可以async使用后台线程调用方法Task.Run().