为什么任务在主线程中运行？答案

【问题标题】：Why tasks runned in main thread?为什么任务在主线程中运行？
【发布时间】：2013-09-25 18:18:22
【问题描述】：

我编写了我的自定义 TaskScheduler：

public class LimitedConcurrencyLevelTaskScheduler : TaskScheduler
{

    private BlockingCollection<Task> _tasks = new BlockingCollection<Task>();
    private List<Thread> _threads;
    private bool work = true;

    public LimitedConcurrencyLevelTaskScheduler(int maxConcurrencyLevel)
    {

        _threads = new List<Thread>();
        for (int i = 0; i < maxConcurrencyLevel; i++)
        {
            _threads.Add(new Thread(() =>
                                        {
                                            while (work)
                                            {
                                                TryExecuteTask(_tasks.Take());
                                            }
                                        }) { IsBackground = true, Name = "TaskShedulerThread#" + i });
        }
        foreach (var thread in _threads)
        {
            thread.Start();
        }

    }

    protected override void QueueTask(Task task)
    {
        _tasks.Add(task);
    }

    protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
    {
        return !_threads.Contains(Thread.CurrentThread) && TryExecuteTask(task);
    }

    public override int MaximumConcurrencyLevel { get { return 1; } }

    protected override IEnumerable<Task> GetScheduledTasks()
    {
        return _tasks.ToArray();
    }

    public void Dispose()
    {
        if (_threads != null)
        {
            _tasks.CompleteAdding();
            work = false;

            _tasks.Dispose();
            _tasks = null;
            _threads = null;
        }
    }
}

并以这种方式使用它：

    static void Main(string[] args)
    {
        var taskScheduller = new LimitedConcurrencyLevelTaskScheduler(1);
        Thread.CurrentThread.Name = "MainThread";
        var taskFactory = new TaskFactory(taskScheduller);
        var tasks = new Task[100];
        for (int i = 0; i < 100; i++)
        {
            tasks[i] = taskFactory.StartNew(() => Console.WriteLine(String.Format("Call in {0}", Thread.CurrentThread.Name)));
        }
        Task.WaitAll(tasks);
    }

程序的输出：

Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in MainThread
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in MainThread
Call in MainThread
Call in MainThread
Call in MainThread
Call in MainThread
Call in MainThread
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
Call in TaskShedulerThread#0
...

为什么我的任务在主线程中运行？

【问题讨论】：

标签： c# task

【解决方案1】：

由于称为task inlining 的 TPL 功能，任务正在主线程上执行。当线程在尚未开始执行的任务上调用WaitAll（或任何类似方法）时，TPL 可能允许所述线程尝试执行挂起的任务本身，而不是阻塞直到它被其中一个任务执行调度程序的工作线程。

可以通过覆盖任务调度程序的TryExecuteTaskInline 方法来控制此行为，禁止它执行任何任务，除非当前线程恰好是工作线程。

protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
    return !_threads.Contains(Thread.CurrentThread) && TryExecuteTask(task);
}

在您的实现中，您只需要在 Contains 检查之前删除否定：

protected override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued)
{
    return _threads.Contains(Thread.CurrentThread) && TryExecuteTask(task);
}

【讨论】：