當任務運行異步方法時，等待所有任務

Question

我一直在嘗試在等待多個線程時合並幾個問題的結果，但是我的電話等待Task.WhenAll(Tasks); 在所有任務完成之前通過！

List<Task> tasks = new List<Task>();
foreach (DownloadingFile file in downloadingFiles)
{
    Task t = new Task(async delegate { await file.PollDownload(); } );
    tasks.Add(t);
    t.Start();
}
await Task.WhenAll(tasks);

internal async Task PollDownload()
{
    string localFile = "pathToDestination";
    WebRequest headerRequest = WebRequest.Create(uri);
    headerRequest.Method = "HEAD";
    HttpWebResponse response = null;
    try
    {
        response = (HttpWebResponse) await headerRequest.GetResponseAsync();
    }
    catch
    {
        Thread.Sleep(500);
        try
        {
            response = (HttpWebResponse)await headerRequest.GetResponseAsync();
        }
        catch
        {
            response = null;
        }
    }
    if (response == null)
    {
        state = DOWNLOAD_STATE.FAILED;
        return;
    }
    totalBytes = response.ContentLength;
    state = DOWNLOAD_STATE.WAITING_START;
    //Check to see whether a local copy of the remote file exists
    if (File.Exists(localFile))
    {
        DateTime localFileUpdated = File.GetCreationTimeUtc(localFile);
        DateTime onlineFileUpdated = response.LastModified;
        // If the download date of the local file comes after the modification date of the remote copy
        // then we do not need to take any further action here.
        if (onlineFileUpdated <= localFileUpdated)
        {
            state = DOWNLOAD_STATE.SKIPPED;
        }
        //Otherwise we should remove the local copy and proceed with the download
        else
        {
            File.Delete(localFile);
        }
    }
    return;
}

我認為這是因為任務包含異步委托方法？ 我不希望它們是異步的，但是它們必須是因為PollDownload()等待其他異步方法（盡管在這里可以將它們更改為同步方法，但在此問題的另一個實例中不能將它們更改為同步方法，所以我仍然需要解決方案）。

Answer 1

new Task()接受一個Action ，因此您傳遞了一個不等待其結果的async void 。

您應該改用Task.Run（），它也接受Func<Task> 。