为什么 Task.IsCanceled 不正确？

Question

我这里有一个简单的程序

private static void CancellingSingleTask()
{
    DateTime whenStarted = DateTime.Now;

    Console.WriteLine("[{0}] - Main: Started", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));

    CancellationTokenSource cts = new CancellationTokenSource();
    CancellationToken ct = cts.Token;

    Task task = Task.Factory.StartNew(() => 
    {
        int? taskId = Task.CurrentId;

        Console.WriteLine("[{0}] - Task - [{1}]:  Started", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks), taskId);

        Thread.Sleep(2000);

        if (ct.IsCancellationRequested)
        {
            Console.WriteLine("[{0}] - Task - [{1}]:  Cancellation Requested", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks), taskId);
            throw new OperationCanceledException();
        }
        Console.WriteLine("[{0}] - Task - [{1}]:  No Cancellation Requested", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks), taskId);
    }, ct);

    Action Print = () =>
    {
        Console.WriteLine("[{0}] - Main: Task.IsCanceled = [{1}]  Task.IsFaulted = [{2}] Task.IsCompleted = [{3}] Task.Status = [{4}]", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks),
            task.IsCanceled, task.IsFaulted, task.IsCompleted, task.Status);
    };

    Console.WriteLine("[{0}] - Main: Started New Task", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));
    Print();

    Thread.Sleep(1000);

    Console.WriteLine("[{0}] - Main: Cancelling Task", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));

    cts.Cancel();

    Thread.Sleep(2000);

    Console.WriteLine("[{0}] - Main: After Cancelling Task", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));
    Print();

    try
    {
        Console.WriteLine("[{0}] - Main: Waiting For Task", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));

        task.Wait();

        Console.WriteLine("[{0}] - Main: After Waiting For Task", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));
        Print();
    }
    catch (AggregateException aggregateException)
    {
        Thread.Sleep(2000);
        Console.WriteLine("[{0}] - Main: In Catch Block", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks));
        Print();

        foreach (var exception in aggregateException.InnerExceptions)
        {
            Console.WriteLine("[{0}] - Main: Received Exception In Task [{1}]", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks), exception.Message);
        }
    }

} 

样本输出
[00:00:00.0010000] - 主要：开始
[00:00:00.0040002] - 主要：开始新任务
[00:00:00.0060003] - 主要：IsCanceled = [False] IsFaulted = [False] IsCompleted = [False] 状态 = [Running]
[00:00:00.0070004] - 任务 - [1]：已开始
[00:00:01.0070576] - 主要：取消任务
[00:00:02.0071148] - 任务 - [1]：请求取消
[00:00:03.0111722] - 主要：取消任务后
[00:00:03.0111722] - 主要：IsCanceled = [False] IsFaulted = [True] IsCompleted = [True] 状态 = [Faulted]
[00:00:03.0111722] - 主要：等待任务
[00:00:05.0112866] - 主要：在 Catch 块中
[00:00:05.0112866] - 主要：IsCanceled = [False] IsFaulted = [True] IsCompleted = [True] 状态 = [Faulted]
[00:00:05.0112866] - 主要：在任务中收到异常 [操作已取消。]

我从来没有看到 Task.IsCanceled 设置为 true，我是在犯错还是遗漏了一些明显的东西。 我已经对此问题进行了一些研究/搜索，但无法找到确凿的答案。

注意：StackOverFlow 上的相关问题 Cancellation of a tasktask IsCanceled is false, while I canceledTask.IsCancelled doesn't work

Answer 1

我想你应该将CancellationToken 传递给OperationCanceledException 的构造函数。

  if (ct.IsCancellationRequested)
    {
        Console.WriteLine("[{0}] - Task - [{1}]:  Cancellation Requested", TimeSpan.FromTicks(DateTime.Now.Ticks - whenStarted.Ticks), taskId);
        throw new OperationCanceledException(ct);
    }

TPL 将检查两个CancellationToken 是否相同，如果是，它将把任务标记为Cancelled，在你的情况下它不是，所以 TPL 假设你的任务没有被取消。

或者更好的使用ThrowIfCancellationRequested方法，就这么简单

ct.ThrowIfCancellationRequested();