"ThrowIfCancellationRequested"能不被TaskScheduler_UnobservedTaskException抓到吗？

Question

我有一个非常非常奇怪的问题，现在这是我的代码：

namespace TaskParallelTest
{
    using System.Threading;
    using System.Threading.Tasks;
    using System;
    using System.IO;

    public class Program
    {
        static Program()
        {
            TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;
        }
        private static void DoPrint(int id, CancellationToken cToken)
        {
            Thread.Sleep(100);
            if (!cToken.IsCancellationRequested)
            {
                Console.WriteLine("Id is:" + id + ";Current State:" + cToken.IsCancellationRequested);
                cToken.Register(() => Console.WriteLine("Rollback for:" + id));
            }
        }
        static void Main(string[] args)
        {
            CancellationTokenSource cTokenSource = new CancellationTokenSource();

            Task.Run(() =>
            {
                for (int i = 1; i < 6; i++)
                {
                    cTokenSource.Token.ThrowIfCancellationRequested();
                    DoPrint(i, cTokenSource.Token);
                }
            }, cTokenSource.Token);

            Random r = new Random();

            Thread.Sleep(400);
            cTokenSource.Cancel(true);
            Thread.Sleep(10000);
            GC.Collect();
            GC.WaitForPendingFinalizers();
            Console.WriteLine("OK");
            Console.ReadLine();
        }

        private static void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
        {
            File.WriteAllText("C:\Resume\Error.txt", e.Exception.StackTrace);
            e.SetObserved();
        }
    }
}

气死我的是为什么"UnobservedTaskException"的事件抓不到？我用了GC.Collect()和Thread.Sleep()，但是没有任何帮助……?

有时 "Error.txt" 没有创建，有时创建的文件什么也没有......?

【已解决——现在根据提示给出答案】

1) 请注意，我应该删除 "Cancellation" 并在此处模拟异常：

static void Main(string[] args)
        {
            CancellationTokenSource cTokenSource = new CancellationTokenSource();

            Task.Run(() =>
            {
                for (int i = 1; i < 6; i++)
                {
                    if (i==5)
                    {
                        throw new Exception("Error occured!");
                    }
                    DoPrint(i, cTokenSource.Token);
                }
            },cTokenSource.Token)
            .ContinueWith
            (
                t =>
                {
                    Console.WriteLine("Error has happened now.");
                    Console.WriteLine(t.IsFaulted);
                },
                TaskContinuationOptions.OnlyOnFaulted
            );

            Thread.Sleep(400);
            //cTokenSource.Cancel();
            //Thread.Sleep(2000);
            GC.Collect();
            GC.WaitForPendingFinalizers();
            //Thread.Sleep(6000);
            Console.WriteLine("OK");
            Console.ReadLine();
        }

2) 然后展平异常（因为这是一个聚合异常）：

 private static void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
        {
            foreach (var item in e.Exception.Flatten().InnerExceptions)
            {
                Console.WriteLine(item.StackTrace);
            }
            e.SetObserved();
        }

Answer 1

如果针对特定令牌抛出 OperationCanceledException，并且这与您在创建任务时传递的令牌相同 - 它不会被视为 unhandled\unobserved 异常，因为它只是正常的预期取消流程.此异常只会将任务状态设置为已取消。你的情况也是如此：

var task = Task.Run(() =>
{
   for (int i = 1; i < 6; i++)
   {
       // this exception is associated with cTokenSource.Token
       cTokenSource.Token.ThrowIfCancellationRequested();
       DoPrint(i, cTokenSource.Token);
   }
}, cTokenSource.Token); // and this is the same token you pass when creating a task

如果不是这种情况（例如，您在创建任务时传递了不同的令牌）- 异常将被 UnobservedTaskException 处理程序拦截。

问题是：您为什么要将此异常视为未观察到？您希望任务可以取消，然后您取消它，它现在处于 Canceled 状态。没有 unobserved\unhandled.

"ThrowIfCancellationRequested"能不被TaskScheduler_UnobservedTaskException抓到吗？

Can "ThrowIfCancellationRequested" not be caught by TaskScheduler_UnobservedTaskException？

c#

unobserved-exception