如何从取消的任务中获取内部异常？

Question

我有一个任务，如果发生异常，它会取消正在使用 sme 取消令牌的任务并引发异常：

var cancellationTokenSource = new CancellationTokenSource();

var task1 = Task.Factory.StartNew(
                () =>
                    {
                        try
                        {
                            // Do work 
                        }
                        catch (Exception exception)
                        {                              
                            cancellationTokenSource.Cancel();

                            // rethrow the error
                            throw;
                        }     
                    },
                cancellationTokenSource.Token,
                TaskCreationOptions.None,
                taskScheduler); 

我有另一个 task3（返回任务）在 task1 和 task2 上继续，并使用相同的取消令牌：

return task3 =
                Task.Factory.ContinueWhenAll(
                    new[] { task1, task2 },
                    tasks =>
                        {                           
                           // Do work
                        },
                cancellationTokenSource.Token,
                TaskContinuationOptions.None,
                this.taskScheduler).Unwrap();

一旦 task1 被取消，所有具有相同取消令牌的任务也会被取消。 需要获取task1抛出的内部异常，怎么做？

Answer 1

如何使用 async/await 和常规异常处理？不过它需要.NET 4.5。像这样的东西（我不知道任务必须如何相互交互，所以你的代码最终可能看起来完全不同）：

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

namespace ConsoleApplication1
{

    class Program
    {
        static void Main(string[] args)
        {
            new MyClass().RunAsyncMethods();
            Console.ReadLine();

        }

        public class MyClass
        {
            public async void RunAsyncMethods()
            {
                try
                {
                    var cancellationTokenSource = new CancellationTokenSource();

                    var task1 = RunFirstTaskAsync(cancellationTokenSource);
                    var task2 = RunSecondTaskAsync(cancellationTokenSource);

                    await Task.WhenAll(task1, task2);
                    await RunThirdTaskAsync(cancellationTokenSource);
                    Console.WriteLine("Done");
                }
                catch (Exception exc)
                {
                    Console.WriteLine(exc.Message);
                }
            }

            public Task RunFirstTaskAsync(CancellationTokenSource cancelSource)
            {
                return Task.Run(() =>
                {
                    try
                    {
                        Console.WriteLine("First Task is Running");
                        throw new Exception("Error happened in first task");
                    }
                    catch (Exception exception)
                    {
                        cancelSource.Cancel();

                        throw;
                    }
                },
                    cancelSource.Token);
            }

            public Task RunSecondTaskAsync(CancellationTokenSource cancelSource)
            {
                return Task.Run(
                    () =>
                    {
                        Console.WriteLine("Second Task is Running");
                    },
                    cancelSource.Token);
            }

            public Task RunThirdTaskAsync(CancellationTokenSource cancelSource)
            {
                return Task.Run(
                    () =>
                    {
                        Console.WriteLine("Third Task is Running");
                    },
                    cancelSource.Token);
            }
        }
    }
}

更新：我更新了代码以使用WhenAll。

更新：更新了代码以使用 Task.Run 方法创建任务，而不是使用冷任务。

更新：您可以省略 async/await（尽管我认为这样更好）并进行一些老式的 TPL 错误处理。根本不使用 task3 的取消令牌（返回任务），仅用于它正在等待的任务。当它们完成时（正常或通过异常/取消），您可以像这样处理 task3 中的异常：

// don't use the cancellation token for the third task as you used for the previous ones
var task3 = Task.Factory.ContinueWhenAll(
                    new[] { task1, task2 },
                    tasks =>
                    {
                        if (tasks[0].Exception != null)
                        {
                            tasks[0].Exception.Handle(exc =>
                            {
                                Console.WriteLine("First task failed :(");
                                return false; // signal that exception was handled, so it won't propagate
                            });
                            // add additional code here, or inside the Handle method above
                        }

                        if (tasks[1].Exception != null)
                        {
                            tasks[1].Exception.Handle(exc =>
                            {
                                Console.WriteLine("Second task failed :(");
                                return false; // signal that exception was handled, so it won't propagate
                            });
                            // add additional code here, or inside the Handle method above
                        }

                        // do the same for the rest of the tasks or iterate throught them with a foreach loop...
                    });