Question

我使用以下方法创建新任务并长时间在后台执行操作。如果满足某个条件，我需要完全停止所有任务并向用户显示消息。

dowork()
{

    mylist = new List<DataModel.CheckData>();
    int index = 0;
    foreach (var line in mylist)
    {
        mylist.Add(new DataModel.CheckData() { RawLine = line, data = line,FileName=virtualfilelist[index].ToString() });
        index++;
    }

    BlockingCollection<DataModel.CheckData> ujobs = new BlockingCollection<DataModel.CheckData>();
    timerRefreshUi.Start();

    Task.Factory.StartNew(() =>
    {
        _dtRows.Clear();
        uiQueue.Clear();
        uiQueueBad.Clear();
        uiQueueGood.Clear();

        for (int i = 0; i < mylist.Count; i++)
        {
             AddResultRow(mylist[i].data, "Waiting...",mylist[i].FileName, Color.White);
             ujobs.TryAdd(new DataModel.CheckData() { RowId = i, data = mylist[i].data }, 1000);

         }
         List<Task> openCheckTasks = new List<Task>();

         while (ujobs.Count > 0)
         {
             while (openCheckTasks.Where(task => task.Status == TaskStatus.Running).ToList().Count >= threadcount)
              System.Threading.Thread.Sleep(250);

              Task t = new Task(new Action(() =>
              {

              }));

              openCheckTasks.Add(t);
              t.Start();
         }
         Task.WaitAll(openCheckTasks.ToArray());

    }).ContinueWith(task => {
        _benchmark.Stop();
        this.BeginInvoke(new Action(() =>
        {

        }));

    });

}

我尝试在while循环中使用Cancellation Token和break。但是它无法正常工作。请建议停止所有线程的最佳方法。我对多线程编程的经验很少

Answer 1

CancellationToken是正确的方法。

您是否尝试控制一次运行的任务数量？这就是TPL已经做的事情，而且做得很好。

请参阅此示例，启动许多CPU密集型任务，然后在三秒后取消所有这些任务：

public static void Main()
{
    var delay = TimeSpan.FromSeconds(1);
    var cts = new CancellationTokenSource();
    var tasks = Enumerable.Range(0, 100).Select(i => Task.Run(() => SlowSqrt/*Async*/(i, delay, cts.Token), cts.Token)).ToArray();
    Thread.Sleep(3000);
    cts.Cancel();
}

public static double SlowSqrt(double arg, TimeSpan delay, CancellationToken token)
{
    Console.WriteLine($"Calculating Sqrt({arg})...");
    var burnCpuTimeUntil = DateTime.Now + delay;
    while (DateTime.Now < burnCpuTimeUntil) token.ThrowIfCancellationRequested();
    var result = Math.Sqrt(arg);
    Console.WriteLine($"Sqrt({arg}) is {result}.");
    return result;
}

public static async Task<double> SlowSqrtAsync(double arg, TimeSpan delay, CancellationToken token)
{
    Console.WriteLine($"Calculating Sqrt({arg})...");
    await Task.Delay(delay, token);
    var result = Math.Sqrt(arg);
    Console.WriteLine($"Sqrt({arg}) is {result}.");
    return result;
}

其输出为：

Calculating Sqrt(1)...
Calculating Sqrt(2)...
Calculating Sqrt(0)...
Calculating Sqrt(3)...
Sqrt(2) is 1.4142135623731.
Calculating Sqrt(4)...
Sqrt(0) is 0.
Calculating Sqrt(5)...
Sqrt(3) is 1.73205080756888.
Calculating Sqrt(6)...
Sqrt(1) is 1.
Calculating Sqrt(7)...
Sqrt(5) is 2.23606797749979.
Calculating Sqrt(8)...
Sqrt(4) is 2.
Calculating Sqrt(9)...
Sqrt(6) is 2.44948974278318.
Calculating Sqrt(10)...
Sqrt(7) is 2.64575131106459.
Calculating Sqrt(11)...

由于我的计算机上有四个核心，因此一次只能激活4个任务。取消令牌时（12..99）尚未启动的任务甚至都没有开始。已启动的任务（8..11）在token.ThrowIfCancellationRequested()中出错。所有这些都以TaskStatus.Canceled状态结束。

现在，如果您更改上面的代码以调用SlowSqrtAsync，则1秒延迟不会使用CPU，因此TPL会激活所有100个任务，尝试最大化CPU使用率。大约一秒钟后，您将获得所有100个结果。如果您在Task.Delay内部取消了任务，则会OperationCanceledException抛出任务，就像Token.ThrowIfCancellationRequested()那样。

Calculating Sqrt(0)...
Calculating Sqrt(1)...
:
:
Calculating Sqrt(92)...
Calculating Sqrt(89)...
(about 1 second later:)
Sqrt(19) is 4.35889894354067.
Sqrt(5) is 2.23606797749979.
:
:
Sqrt(99) is 9.9498743710662.
Sqrt(92) is 9.59166304662544.

Answer 2

要停止多个正在进行的任务，您可以使用await Task.WhenAll(...)，传入一系列任务。每个任务都获得对CancellationToken（System.Threading）的引用。它由每个任务决定（因为他们正在做自己的工作）每隔一段时间检查一次令牌，看看是否已经请求取消 - 如果是的话，取消就可以了，抛出一个OperationCanceledException - 等待线程将相应地捕获。

此外，但不是必须取消，如果要将每个任务的进度报告回等待线程，则可以传入System.IProgress。

使用CancellationToken和IProgress（.NET 4.5.2控制台应用）查看下面的工作演示。

快乐的编码。

private static CancellationTokenSource _tokenSource;
private static Progress<string> _progress = new Progress<string>((msg) => Console.WriteLine(msg));
private static IProgress<string> _progressReporter = _progress as IProgress<string>;

static void Main(string[] args)
{
  Task.Run(async () => await Run()).GetAwaiter().GetResult();
}

private static async Task Run()
{
  try
  {
    Console.CancelKeyPress += Console_CancelKeyPress;

    using (_tokenSource = new CancellationTokenSource())
    {
      _progressReporter.Report("Running 3 async tasks... press [Ctrl+C] to cancel.\r\n");

      await Task.WhenAll
      (
          DoWorkAsync(1, 6, _tokenSource.Token, _progress),
          DoWorkAsync(2, 3, _tokenSource.Token, _progress),
          DoWorkAsync(3, 4, _tokenSource.Token, _progress)
      );

      _progressReporter.Report($"\r\nRun complete: All tasks finished successfully (without user cancelling)");
    }
  }
  catch (OperationCanceledException)
  {
    _progressReporter.Report($"\r\nRun complete: User cancelled one or more ongoing tasks");
  }
  finally
  {
    Console.ReadLine(); // pause
  }
}

private static void Console_CancelKeyPress(object sender, ConsoleCancelEventArgs e)
{
  _progressReporter.Report("\r\nCancelling tasks...\r\n");
  _tokenSource.Cancel();
  e.Cancel = true;
}

private static async Task DoWorkAsync(int taskNumber, int durationInSeconds, CancellationToken ct, IProgress<string> progress)
{
  await Task.Run(() => 
  {
    if (ct.IsCancellationRequested)
    {
      progress.Report($"Task {taskNumber} cancelled before it started");
      return;
    }
    else
      progress.Report($"Started task {taskNumber} (duration: {durationInSeconds} sec. steps)");

    for (int i = 0; i < durationInSeconds; i++)
    {
      progress.Report($"Task {taskNumber} is working (on step {i + 1}/{durationInSeconds})");

      Thread.Sleep(1000); // simulate work in 1 sec steps

      if (ct.IsCancellationRequested)
      {
        progress.Report($"Task {taskNumber} cancelled (upon request) after it started (on step {i + 1}/{durationInSeconds})");
        ct.ThrowIfCancellationRequested();
      }
    }

    progress.Report($"Finished task {taskNumber}");
  });
}

停止多个任务

2 个答案: