取消任务时的 OperationCanceledException VS TaskCanceledException

Question

以下代码创建了一个正在取消的任务。 await 表达式（案例 1）抛出 System.OperationCanceledException，而同步 Wait()（案例 2）抛出 System.Threading.Tasks.TaskCanceledException（包裹在 System.AggregateException 中）。

using System;
using System.Threading;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        Program.MainAsync().Wait();
    }

    private static async Task MainAsync()
    {
        using(var cancellationTokenSource = new CancellationTokenSource())
        {
            var token = cancellationTokenSource.Token;
            const int cancelationCheckTimeout = 100;

            var task = Task.Run(
                async () => 
                {
                    for (var i = 0; i < 100; i++)
                    {
                        token.ThrowIfCancellationRequested();
                        Console.Write(".");
                        await Task.Delay(cancelationCheckTimeout);  
                    }
                }, 
                cancellationTokenSource.Token
            );

            var cancelationDelay = 10 * cancelationCheckTimeout;
            cancellationTokenSource.CancelAfter(cancelationDelay);

            try
            {
                await task; // (1)
                //task.Wait(); // (2) 
            }
            catch(Exception ex)
            {
                Console.WriteLine(ex.ToString());
                Console.WriteLine($"Task.IsCanceled: {task.IsCanceled}");
                Console.WriteLine($"Task.IsFaulted: {task.IsFaulted}");
                Console.WriteLine($"Task.Exception: {((task.Exception == null) ? "null" : task.Exception.ToString())}");
            }
        }
    }
}

案例 1 输出：

..........System.OperationCanceledException: The operation was canceled.
   at System.Threading.CancellationToken.ThrowIfCancellationRequested()
   at Program.<>c__DisplayClass1_0.<<MainAsync>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Program.<MainAsync>d__1.MoveNext()
Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null

案例 2 输出：

..........System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at Program.<MainAsync>d__1.MoveNext()
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---

Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null

为什么第二种情况下的System.AggregateException 不包含System.OperationCanceledException 作为内部异常？

我知道ThrowIfCancellationRequested() 会抛出OperationCanceledException，我们可以看到在这两种情况下Task 都会进入取消（非故障）状态。

这让我很困惑，因为从 .NET API 中取消方法会在两种情况下产生一致的行为 - 取消的任务只会抛出 TaskCanceledException：

using System;
using System.Threading;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        Program.MainAsync().Wait();
    }

    private static async Task MainAsync()
    {
        using(var cancellationTokenSource = new CancellationTokenSource())
        {
            var token = cancellationTokenSource.Token;

            var task = Task.Delay(1000, token);
            cancellationTokenSource.CancelAfter(100);

            try
            {
                await task; // (1)
                //task.Wait(); // (2)
            }
            catch(Exception ex)
            {
                Console.WriteLine(ex.ToString());
                Console.WriteLine($"Task.IsCanceled: {task.IsCanceled}");
                Console.WriteLine($"Task.IsFaulted: {task.IsFaulted}");
                Console.WriteLine($"Task.Exception: {((task.Exception == null) ? "null" : task.Exception.ToString())}");
            }
        }
    }
}

案例 1 输出：

System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Program.<MainAsync>d__1.MoveNext()
Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null

案例 2 输出：

System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
   at System.Threading.Tasks.Task.Wait()
   at Program.<MainAsync>d__1.MoveNext()
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---

Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null

Answer 1

这里的区别在于使用token.ThrowIfCancellationRequested()。此方法检查取消，如果请求特别抛出OperationCanceledException 而不是TaskCanceledException（可以理解为CancellationToken 不是TPL 独有的）。你可以看看reference source，看看它调用了这个方法：

private void ThrowOperationCanceledException()
{
    throw new OperationCanceledException(Environment.GetResourceString("OperationCanceled"), this);
}

“常规”取消确实会生成TaskCanceledException。您可以通过取消标记在任务有机会开始运行之前看到：

cancellationTokenSource.Cancel();
var task = Task.Run(() => { }, cancellationTokenSource.Token);
try
{
    await task; 
}
catch (Exception ex)
{
    Console.WriteLine(ex.ToString());
    Console.WriteLine($"Task.IsCanceled: {task.IsCanceled}");
    Console.WriteLine($"Task.IsFaulted: {task.IsFaulted}");
    Console.WriteLine($"Task.Exception: {((task.Exception == null) ? "null" : task.Exception.ToString())}");
}

输出：

System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at Sandbox.Program.<MainAsync>d__1.MoveNext()
Task.IsCanceled: True
Task.IsFaulted: False
Task.Exception: null

传统的 .Net 方法通常不将 CancellationToken.ThrowIfCancellationRequested 用于异步 API，因为这仅适用于将工作卸载到另一个线程时。这些方法用于固有的异步操作，因此使用CancellationToken.Register（或内部InternalRegisterWithoutEC）监控取消。

Answer 2

TaskCanceledException 继承自 OperationCanceledException。所以至少有一点一致性。

if( ex is OperationCanceledException)
{
...
}