c# - 子任务取消，父任务完成？

Question

我试图了解 .net 任务在附加子项时的行为。

我有以下测试代码:

void Test()
{
    var tokenSource = new CancellationTokenSource();
    var token = tokenSource.Token;

    Task child = null;
    var parent = Task.Factory.StartNew(() =>
    {
        child = Task.Factory.StartNew(() =>
        {
            while (!token.IsCancellationRequested)
                Thread.Sleep(100);
            token.ThrowIfCancellationRequested();
        }, token, TaskCreationOptions.AttachedToParent, TaskScheduler.Default);
    }, token);

    Thread.Sleep(500);

    Debug.WriteLine("State of parent before cancel is {0}", parent.Status);
    Debug.WriteLine("State of child before cancel is {0}", child.Status);

    tokenSource.Cancel();
    Thread.Sleep(500);

    Debug.WriteLine("State of parent is {0}", parent.Status);
    Debug.WriteLine("State of child is {0}", child.Status);
}

结果是:

State of parent before cancel is WaitingForChildrenToComplete
State of child before cancel is Running
A first chance exception of type 'System.OperationCanceledException' occurred in mscorlib.dll
State of parent is RanToCompletion
State of child is Canceled

显然父任务状态不是Canceled，即使两个任务共享 token ，并附加子项。

如何让父任务在发生取消时返回状态 Canceled？

注意如果我抛出异常，两个任务都会返回 Faulted。

Answer 1

这是 MSDN 上规定的预期行为。父任务必须 wait (向下滚动到取消部分)子任务。父任务必须处理所有良性故障(如取消)。

要使您的父任务失败，只需等待并传递 token :

Task child = null;
var parent = Task.Factory.StartNew(() =>
{
  child = Task.Factory.StartNew(() =>
  {
    while (!token.IsCancellationRequested) Thread.Sleep(100);
    token.ThrowIfCancellationRequested();
  }, token, TaskCreationOptions.AttachedToParent, TaskScheduler.Default);

  // This is the magic line.
  child.Wait(token);
}, token);

如果您使用这段代码来做一些有成效的事情，而不仅仅是为了测试，您还应该考虑使用支持 async 委托(delegate)的简化的 Task.Run() Task.Factory.StartNew()。这article很有趣。