Task.ContinueWith 在任务完成之前触发

Question

我正在尝试在注册延续任务后开始一项任务。但在await Task.Delay()调用后，Continue 会立即触发。

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

namespace ConsoleApplication30
{
    class Program
    {
        static void Main(string[] args)
        {
            var task = new Task(async delegate {
                Console.WriteLine("Before delay");
                await Task.Delay(1000);
                Console.WriteLine("After delay");
            });

            task.ContinueWith(t => {
                Console.WriteLine("ContinueWith");
            });

            task.Start();

            Console.ReadLine();
        }
    }
}

输出：

Before delay
ContinueWith
After delay

这里有什么问题吗？

Answer 1

使用Task.Run

void Main()
{
    Task.Run(async()=>{
                Console.WriteLine("Before delay");
                await Task.Delay(1000);
                Console.WriteLine("After delay");
            }).ContinueWith(t => {
                // do somthing with your Task t here
                Console.WriteLine("ContinueWith");
            });

           // task.Start();

            Console.ReadLine();
}

// Define other methods and classes here

输出：

延迟前
延迟后
继续

为什么你的代码不起作用：看看你的例子（有点修改）：

static void Main(string[] args)
{
    var outerTask = new Task(async delegate {
        Console.WriteLine("Before delay");
         await Task.Delay(1000); // inner task
        Console.WriteLine("After delay");
    },"Outertask");

    outerTask.ContinueWith(t => {
        Console.WriteLine("ContinueWith");
    });

    outerTask.Start();
    outerTask.Wait(); // wait for outerTask to finish
    var breakHere = 0; // set a brakpoint here
}

您将获得对outerTask 的引用。outerTask 与innerTask 没有任何连接。调用 start 后，outerTasks 会触发委托的执行，并立即继续执行“ContinueWith”委托。延续连接到了outerTask！

OP的评论：

我正在尝试创建一个任务，在完成后将其自身从任务列表中删除。为了安全地执行此操作，我需要确保在启动之前将其添加到列表中。我正在尝试执行以下操作：创建一个任务，将其添加到列表中，注册删除它的延续，启动任务。有没有更好的办法？

虽然以下代码可以工作，但您必须首先证明使用的合理性！代码没有优化或者什么的。那里有或者肯定有更好的模式。做你的研究。

ConcurrentBag<AsyncLazy<Task>> taskList = new ConcurrentBag<AsyncLazy<Task>>();
void Main()
{
    int v = 3242;
    AsyncLazy<Task> objAsyncLazy = null;
    objAsyncLazy = new AsyncLazy<Task>(new Func<Task<Task>>(async () =>
   {
       return await Task.FromResult<Task>(doLongRunningAsync(v).
                    ContinueWith<Task>(async (doLongRunningAsyncCompletedTask) =>
                   {
                       Console.WriteLine(doLongRunningAsyncCompletedTask.IsCompleted); // 
                       await removeMeFromListAsync(objAsyncLazy);
                   }));
   }));

    taskList.Add(objAsyncLazy);
    Console.WriteLine("al added");
    Console.WriteLine("ConcurrentBag.Count: " + taskList.Count);
    // execute the task
    var t = objAsyncLazy.GetValueAsync(System.Threading.CancellationToken.None);

    // wait for the first task "t" to finish or not, 
    // ContinueWith will execute after first task "t" has finished anyways. 
    t.Wait(); 
    // ContinueWith is executing now
    Console.ReadLine();
}

public async Task doLongRunningAsync(int val)
{
    Console.WriteLine("Before delay" + val);
    await Task.Delay(1000);
    Console.WriteLine("After delay");
}

public async Task removeMeFromListAsync(AsyncLazy<Task> al) //(AsyncLazy<Task> t)
{
    Console.WriteLine("continue start");
    taskList.TryTake(out al);
    Console.WriteLine("al removed");
    Console.WriteLine("ConcurrentBag.Count: " + taskList.Count);
    await Task.Delay(1000);
    Console.WriteLine("continue end");
}
}