如何让我的方法等待所有线程完成？

Question

我有一个方法可以触发线程来做一些工作。会有2个线程异步运行一段时间，当他们的回调方法get被调用时，回调会触发另一个线程，直到所有工作完成。如何让我的方法等待所有这些线程完成并被触发？

Answer 1

如果这是 .Net 4.0，您可以使用 CountdownEvent

const int threads = 10;
using( CountdownEvent evt = new CountdownEvent(threads) )
{
    for( int x = 0; x < threads; ++x )
    {
        ThreadPool.QueueUserWorkItem((state) =>
            {
                // Do work here
                ((CountdownEvent)state).Signal();
            }, evt);
    }

    evt.Wait();
}
Console.WriteLine("Everyone finished!");

这具有在Thread.Join 不是一个选项时工作的优势（例如，如果您正在使用线程池），并且比使用等待句柄更好地扩展（因为WaitHandle.WaitAll 最多有 64 个句柄，并且您也不需要分配尽可能多的对象）。

请注意，如果您使用的是 .Net 4，您还可以使用 Task Parallel Library，这使得这种事情变得更容易。

更新：

既然你说这不是 .Net 4.0，这里有一个简单版本的 CountdownEvent，可以在 .Net 3.5 中使用。我最初编写它是因为我需要一个可以在 Mono 中使用的 CountdownEvent，而当时 Mono 还不支持 .Net 4。它不像真正的那样灵活，但它可以满足您的需求：

/// <summary>
/// Represents a synchronization primitive that is signaled when its count reaches zero.
/// </summary>
/// <remarks>
/// <para>
///   This class is similar to but less versatile than .Net 4's built-in CountdownEvent.
/// </para>
/// </remarks>
public sealed class CountdownEvent : IDisposable
{
    private readonly ManualResetEvent _reachedZeroEvent = new ManualResetEvent(false);
    private volatile int _count;
    private volatile bool _disposed;

    /// <summary>
    /// Initializes a new instance of the <see cref="CountdownEvent"/> class.
    /// </summary>
    /// <param name="initialCount">The initial count.</param>
    public CountdownEvent(int initialCount)
    {
        _count = initialCount;
    }

    // Disable volatile not treated as volatile warning.
#pragma warning disable 420

    /// <summary>
    /// Signals the event by decrementing the count by one.
    /// </summary>
    /// <returns><see langword="true" /> if the count reached zero and the event was signalled; otherwise, <see langword="false"/>.</returns>
    public bool Signal()
    {
        CheckDisposed();

        // This is not meant to prevent _count from dropping below zero (that can still happen due to race conditions),
        // it's just a simple way to prevent the function from doing unnecessary work if the count has already reached zero.
        if( _count <= 0 )
            return true;

        if( Interlocked.Decrement(ref _count) <= 0 )
        {
            _reachedZeroEvent.Set();
            return true;
        }
        return false;
    }

#pragma warning restore 420

    /// <summary>
    /// Blocks the calling thread until the <see cref="CountdownEvent"/> is set.
    /// </summary>
    public void Wait()
    {
        CheckDisposed();
        _reachedZeroEvent.WaitOne();
    }

    /// <summary>
    /// Blocks the calling thread until the <see cref="CountdownEvent"/> is set, using a <see cref="TimeSpan"/> to measure the timeout.
    /// </summary>
    /// <param name="timeout">The timeout to wait, or a <see cref="TimeSpan"/> representing -1 milliseconds to wait indefinitely.</param>
    /// <returns><see langword="true"/> if the <see cref="CountdownEvent"/> was set; otherwise, <see langword="false"/>.</returns>
    public bool Wait(TimeSpan timeout)
    {
        CheckDisposed();
        return _reachedZeroEvent.WaitOne(timeout, false);
    }

    /// <summary>
    /// Blocks the calling thread until the <see cref="CountdownEvent"/> is set, using a 32-bit signed integer to measure the timeout.
    /// </summary>
    /// <param name="millisecondsTimeout">The timeout to wait, or <see cref="Timeout.Infinite"/> (-1) to wait indefinitely.</param>
    /// <returns><see langword="true"/> if the <see cref="CountdownEvent"/> was set; otherwise, <see langword="false"/>.</returns>
    public bool Wait(int millisecondsTimeout)
    {
        CheckDisposed();
        return _reachedZeroEvent.WaitOne(millisecondsTimeout, false);
    }

    /// <summary>
    /// Performs application-defined tasks associated with freeing, releasing, or resetting unmanaged resources.
    /// </summary>
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    private void Dispose(bool disposing)
    {
        if( !_disposed )
        {
            if( disposing )
                ((IDisposable)_reachedZeroEvent).Dispose();
            _disposed = true;
        }
    }

    private void CheckDisposed()
    {
        if( _disposed )
            throw new ObjectDisposedException(typeof(CountdownEvent).FullName);
    }
}

Answer 2

在所有线程上简单调用Join。因此，如果您只有两个线程变量：

thread1.Join();
thread2.Join();

或者，如果您有收藏：

foreach (Thread thread in threads)
{
    thread.Join();
}

线程完成的顺序无关紧要；只有在所有线程都完成后，代码才会继续。

但是，如果您一直在创建新线程，这可能无济于事......您可能需要一些只能在内部访问的集合（例如队列）一个锁，并让每个线程生成活动将新线程添加到队列中......然后迭代（小心！）直到队列为空：

while (true)
{
    Thread nextThread;
    lock (collectionLock)
    {
        if (queue.Count == 0)
        {
            break;
        }
        nextThread = queue.Dequeue();
    }
    nextThread.Join();
}

不过，如果您在 .NET 4 上，请尝试使用任务并行库 - 它让很多事情变得更容易:)

Answer 3

Interlocked. 在启动任何线程之前增加一个初始为零的计数器。互锁。在退出/环回之前在每个线程中减少一个计数器。如果任何线程将计数器减为零，则 Set() 一个 AutoResetEvent。在 AutoResetEvent 上的 WaitOne()。

Rgds, 马丁

Answer 4

使用WaitHandles，每个线程都应该有一个WaitHandle，如ManualResetEvent，完成后对事件调用Set()。

main 方法应该使用WaitHandle.WaitAll 传递每个线程的句柄。

        IList<WaitHandle> waitHandles = new List<WaitHandle>();
        var newThread = new Thread(new ParameterizedThreadStart((handle) =>
        {
            // thread stuff goes here

            ((ManualResetEvent)handle).Set();
        }));
        var manualResetEvent = new ManualResetEvent(false);
        waitHandles.Add(manualResetEvent);
        newThread.Start(manualResetEvent);

        // create other threads similarly

        // wait for all threads to complete - specify a timeout to prevent a deadlock if a thread fails to set the event
        WaitHandle.WaitAll(waitHandles.ToArray());

Answer 5

在最简单的情况下，您可以使用 Join

    Threading.Thread myThread1 = new Thread(new ThreadStart(Worker1));
    Threading.Thread myThread2 = new Thread(new ThreadStart(Worker2));
    myThread1.Start();
    myThread2.Start();
    myThread1.Join();
    myThread2.Join();