await Task.WhenAll 看似完成，但它永远不会到达下一行代码

Question

我有一个简单的文件下载器，我从服务器上获取一些文件，然后显示一条消息，告知它是成功还是失败。这是代码：

public async Task GetFiles(IEnumerable<string> urlList)
{
    //some variable declaring, setting etc.

    await Task.WhenAll(urlList.Select(url => Task.Run(() => DownloadFile(url, dir, count++))).ToList());
    //problem lies here, the WhenAll never completes
    if (fileDownloadError)
    {
        Directory.Delete(Path.Combine(AppDomain.CurrentDomain.BaseDirectory, "downloads"));
        MessageBox.Show("Fail");
        return;
    }
    MessageBox.Show("Success");
}

internal async Task DownloadFile(string url, string dir, int count)
{
    string filename = HelperClass.GetFilenameFromUrl(url);
    long a = 0;
    newList.Add(a);
    using (var client = new WebClient())
    {
        TempDownload tp = new TempDownload()
        {
            id = count
        };

        client.DownloadProgressChanged += new DownloadProgressChangedEventHandler((sender, e) => ProgressChanged(sender, e, tp));
        client.DownloadFileCompleted += new AsyncCompletedEventHandler(Completed);

        await client.DownloadFileTaskAsync(url, string.Concat(dir, "\", filename));
    }
}

ProgressBar 我在 DownloadProgressChanged 事件中更改为 100%，文件已正确下载，但 WhenAll 之后的代码从未执行，即它从未命中if (fileDownloadError) 上的断点，未显示 MessageBox 等。我在这里遗漏了什么吗？

Answer 1

由于死锁，断点永远不会命中 if 语句。将 ConfigureAwait(false) 添加到所有等待。前-

await Task.WhenAll(urlList.Select(url => Task.Run(() => DownloadFile(url, dir, count++))).ToList()).ConfigureAwait(false);

还有

await client.DownloadFileTaskAsync(url, string.Concat(dir, "\", filename)).ConfigureAwait(false);

解决这个问题的另一个选择是，不在 UI 线程上使用 Wait 或 Result。使用异步等待

ex-

private async void button1_Click(object sender, EventArgs e)
{
            await GetFiles();
}

详情，

https://msdn.microsoft.com/en-us/magazine/gg598924.aspx

https://blog.stephencleary.com/2012/07/dont-block-on-async-code.html

await Task.WhenAll 看似完成，但它永远不会到达下一行代码

await Task.WhenAll seemingly completes, but it never reaches next line of code

c#

task

multitasking