以下代码创建一个正在取消的任务. await表达式(情况1)抛出System.OperationCanceledException,而同步Wait()(情况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($"Task.Exception: {((task.Exception == null) ? "null" : task.Exception.ToString())}");
}
}
}
}
案例1输出：
..........System.OperationCanceledException: The operation was canceled. at System.Threading.CancellationToken.ThrowIfCancellationRequested() at Program.<>c__DisplayClass1_0.<<MainAsync>b__0>d.MoveNext() --- End of stack trace from prevIoUs location where exception was thrown --- at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Program.<MainAsync>d__1.MoveNext() Task.IsCanceled: True Task.IsFaulted: False Task.Exception: null
案例2输出：
..........System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled. --- End of inner exception stack trace --- at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout,CancellationToken cancellationToken) at System.Threading.Tasks.Task.Wait() at Program.<MainAsync>d__1.MoveNext() ---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<--- Task.IsCanceled: True Task.IsFaulted: False Task.Exception: null
为什么第二种情况下的System.AggregateException不包含System.OperationCanceledException作为内部异常？
我知道ThrowIfCancellationRequested()抛出OperationCanceledException,我们可以看到在两种情况下Task都被取消(没有错误)状态.
这让我很困惑,因为从.NET API中取消一个方法会在两种情况下都产生一致的行为 – 取消任务只会抛出TaskCanceledException：
using System;
using System.Threading;
using System.Threading.Tasks;
public class Program
{
public static void Main()
{
Program.MainAsync().Wait();
}
private static async Task MainAsync()
{
using(var cancellationTokenSource = new CancellationTokenSource())
{
var token = cancellationTokenSource.Token;
var task = Task.Delay(1000,token);
cancellationTokenSource.CancelAfter(100);
try
{
await task; // (1)
//task.Wait(); // (2)
}
catch(Exception ex)
{
Console.WriteLine(ex.ToString());
Console.WriteLine($"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
解决方法
这里的区别来自使用token.ThrowIfCancellationRequested().此方法检查取消,如果请求专门抛出OperationCanceledException而不是TaskCanceledException(可理解为CancellationToken不是TPL独有的).您可以查看
reference source并看到它调用此方法：
private void ThrowOperationCanceledException()
{
throw new OperationCanceledException(Environment.GetResourceString("OperationCanceled"),this);
}
“常规”取消虽然确实会生成TaskCanceledException.您可以通过在任务有机会开始运行之前取消令牌来查看：
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方法通常不对异步API使用CancellationToken.ThrowIfCancellationRequested,因为这仅适用于将工作卸载到另一个线程.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.
