任务取消时OperationCanceledException与TaskCanceledException的比较
任务取消时OperationCanceledException与TaskCanceledException的比较
提问于 2015-12-18 15:40:07
下面的代码创建一个正在被取消的任务。await表达式(案例1)抛出System.OperationCanceledException，而同步Wait() (案例2)抛出System.Threading.Tasks.TaskCanceledException (包装在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产出：
..........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产出：
..........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 System.OperationCanceledException 不包含System.OperationCanceledException作为内部例外？
我知道ThrowIfCancellationRequested()抛出OperationCanceledException，我们可以看到，在这两种情况下，Task都会被取消(而不是出错)状态。
这让我感到困惑，因为从.NET API中取消一个方法会在两种情况下产生一致的行为--取消的任务只抛出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产出：
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产出：
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回答 2
Stack Overflow用户
回答已采纳
发布于 2015-12-18 16:03:51
这里的区别来自于使用token.ThrowIfCancellationRequested()。此方法检查是否取消，如果请求，则抛出OperationCanceledException，而不是TaskCanceledException (可以理解，因为CancellationToken并不是第三方物流独有的)。您可以查看参考源并看到它调用此方法：
private void ThrowOperationCanceledException()
{
throw new OperationCanceledException(Environment.GetResourceString("OperationCanceled"), this);
}不过，“常规”取消确实会生成一个TaskCanceledException。您可以看到，通过在之前取消令牌，任务有机会开始运行：
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方法通常不为异步API使用CancellationToken.ThrowIfCancellationRequested，因为只有在将工作卸载到另一个线程时才适合使用异步API。这些方法用于固有的异步操作，因此使用CancellationToken.Register (或内部InternalRegisterWithoutEC)监视取消。
Stack Overflow用户
发布于 2019-03-13 14:39:01
TaskCanceledException继承自OperationCanceledException。所以它至少有一点一致性。
if( ex is OperationCanceledException)
{
...
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/34359129
复制相关文章
相似问题
腾讯云开发者
