在没有使用令牌初始化和在 lambda 中调用 ThrowIfCancellationRequested 的情况下，任务中的状态被取消

Question

我认为如果(source):

设置了任务的取消状态

The task acknowledged cancellation by throwing an OperationCanceledException with its own CancellationToken while the token was in signaled state, or the task's CancellationToken was already signaled before the task started executing.

但是如果你在任务的 lambda 中调用 ThrowIfCancellationRequested 而没有用令牌初始化，那么状态会变为已取消。这是一个错误还是请解释这是怎么回事？

在 C# 上针对 VS 2019 上的 .NET 5 和 VS 2022 上的 .NET 6 进行了测试。

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

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            CancellationTokenSource cancellationTokenSource = new CancellationTokenSource(500);

            void MyMethod()
            {
                while (true)
                {
                    Thread.Sleep(100);
                    cancellationTokenSource.Token.ThrowIfCancellationRequested();
                }
            }

            Task t1 = new Task(MyMethod);
            t1.Start();

            Task<int> t2 = new Task<int>(() => { MyMethod(); return 1; });
            t2.Start();

            Task t3 = Task.Factory.StartNew(MyMethod);

            Task t4 = Task.CompletedTask.ContinueWith((t) => { MyMethod(); });

            Task t5 = Task.Run(MyMethod);

            Task t6 = Task.Run(() =>
           {
               while (true)
               {
                   Thread.Sleep(100);
                   cancellationTokenSource.Token.ThrowIfCancellationRequested();
               }
           });

            try
            {
                Task.WaitAll(t1, t2, t3, t4, t5, t6);
            }
            catch
            {

            }

            Console.WriteLine($"Status t1 from new Task:                            {t1.Status}");  //Faulted
            Console.WriteLine($"Status t2 from new Task<:                           {t2.Status}");  //Faulted
            Console.WriteLine($"Status t3 from Task.Factory.StartNew:               {t3.Status}");  //Faulted
            Console.WriteLine($"Status t4 from Task.CompletedTask.ContinueWith:     {t4.Status}");  //Faulted
            Console.WriteLine("\n");
            Console.WriteLine($"Status t5 from Task.Run:                            {t5.Status}");  //Faulted
            Console.WriteLine($"Status t6 from Task.Run with lambda:                {t6.Status}");  //Canceled
        }
    }
}

Faulted 而不是 Canceled.

Online demo.

输出：

Status t1 from new Task:                        Faulted
Status t2 from new Task<int>:                   Faulted
Status t3 from Task.Factory.StartNew:           Faulted
Status t4 from Task.CompletedTask.ContinueWith: Faulted
Status t5 from Task.Run:                        Faulted
Status t6 from Task.Run with lambda:            Canceled

Answer 1

这可能是 C# 编译器中的错误。最小复制：

using System;
using System.Threading.Tasks;

static class Program
{
    static async Task Main()
    {
        {
            var task = Task.Run(() => // Resolved to Task.Run(Action)
            {
                bool condition = true;
                if (condition) throw new OperationCanceledException();
            });
            try { await task; } catch { }
            Console.WriteLine($"if (condition): {task.Status}");
        }
        {
            var task = Task.Run(() => // Resolved to Task.Run(Func<Task>)
            {
                if (true) throw new OperationCanceledException();
            });
            try { await task; } catch { }
            Console.WriteLine($"if (true): {task.Status}");
        }
    }
}

输出：

if (condition): Faulted
if (true): Canceled

Online demo.

Task.Run 有许多重载，其中一个带有 Action 参数，一个带有 Func<Task> 参数。似乎无条件地在 lambda 中的 while (true) 或 if (true) 或 for (; ; ) 块内抛出，会触发异步解析。

Task.Run(Func<Task>) 重载导致 Canceled 而不是 Faulted 任务的原因是因为 Task.Run(Func<Task>) is essentially a shortcut 为此：

Task.Factory.StartNew<Task>(function,
    CancellationToken.None,
    TaskCreationOptions.DenyChildAttach,
    TaskScheduler.Default).Unwrap();

当 function 是一个 async 方法时，它总能成功生成 Task。此任务被传递给 Unwrap，后者按原样传播其状态（CancellationToken 未传递给 Unwrap）。此外，async 方法在遇到 OperationCanceledException 时会按设计转换为取消状态。这两个因素的组合导致观察到的不一致行为。这种行为可能是疏忽而不是故意的。但由于 Task.Run 已经生产了 10 年，现在更改它可能为时已晚。

在没有使用令牌初始化和在 lambda 中调用 ThrowIfCancellationRequested 的情况下，任务中的状态被取消

Status is canceled in task without initialization with token and with ThrowIfCancellationRequested call in lambda

c#

multithreading

task