如何等待所有后台线程完成（在 C# 中）？

Question

如何让前台线程等待所有后台（子）线程在 C# 中完成？我需要从队列（数据库）中获取待处理作业的列表，启动一个新线程来执行它们中的每一个，最后等待所有子线程完成。如何在 C# 中做到这一点？提前致谢。

Answer 1

创建一个结构来跟踪您的工作线程

private struct WorkerThreadElement
{
    public IAsyncResult WorkerThreadResult;
    public AsyncActionExecution WorkerThread;
}

您还需要跟踪预期创建的线程总数和当前已完成的线程数

private int _TotalThreads = 0;
private int _ThreadsHandled = 0;
private List<WorkerThreadElement> _WorkerThreadElements = new List<WorkerThreadElement>();

然后创建一个自动重置句柄以等待线程完成。

// The wait handle thread construct to signal the completion of this process
private EventWaitHandle _CompletedHandle = new AutoResetEvent(false);

您还需要一个委托来创建新线程 - 有多种方法可以做到这一点，但为了这个示例，我选择了一个简单的委托

// Delegate to asynchronously invoke an action
private delegate void AsyncActionExecution();

假设 Invoke 方法是创建所有线程并等待它们执行的入​​口点。所以我们有：

public void Invoke()
{ 
    _TotalThreads = N; /* Change with the total number of threads expected */

    foreach (Object o in objects) 
    {
        this.InvokeOneThread();
    }             

    // Wait until execution has been completed
    _CompletedHandle.WaitOne();

    // Collect any exceptions thrown and bubble them up
    foreach (WorkerThreadElement workerThreadElement in _WorkerThreadElements)
    {
        workerThreadElement.WorkerThread.EndInvoke(workerThreadElement.WorkerThreadResult);
    }
}         

InvokeOneThread 是用于为一个操作创建单个线程的方法。这里我们需要创建一个工作线程元素并调用实际的线程。

 private void InvokeOneThread()
 {
     WorkerThreadElement threadElement = new WorkerThreadElement();
     threadElement.WorkerThread = new AsyncActionExecution();
     threadElement.WorkerThreadResult = threadElement.WorkerThread.BeginInvoke(actionParameters, InvokationCompleted, null);

     _WorkerThreadElements.Add(threadElement);
 }

线程完成回调

private object _RowLocker = new object();

/// <summary>
/// Increment the number of rows that have been fully processed
/// </summary>
/// <param name="ar"></param>
private void InvokationCompleted(IAsyncResult ar)
{
    lock (_RowLocker) 
    {
        _RowsHandled++; 
    }

    if (_TotalThreads == _ThreadsHandled) 
        _CompletedHandle.Set();
}

完成

Answer 2

这是不完整的代码，但 ManualResetEvent 适合你

var waitEvents = new List<ManualResetEvent>();
foreach (var action in actions)
{
    var evt = new ManualResetEvent(false);
    waitEvents.Add(evt);
    ThreadPool.RegisterWaitForSingleObject(asyncResult.AsyncWaitHandle, TimeoutCallback, state, 5000, true);
}

if (waitEvents.Count > 0)
    WaitHandle.WaitAll(waitEvents.ToArray());

Answer 3

使用动态数据，您可以传递您的对象和 WaitHandle (ActionResetEvent)，让您等待所有后台线程完成而无需声明额外的类：

static void Main(string[] args)
{
    List<AutoResetEvent> areList = new List<AutoResetEvent>();
    foreach (MyObject o in ListOfMyObjects)
    {
        AutoResetEvent are = new AutoResetEvent(false);
        areList.Add(are);
        ThreadPool.QueueUserWorkItem(DoWork, new { o, are });
    };

    Console.WriteLine("Time: {0}", DateTime.Now);
    WaitHandle.WaitAll(areList.ToArray());
    Console.WriteLine("Time: {0}", DateTime.Now);
    Console.ReadKey();
}

static void DoWork(object state)
{
    dynamic o = state;
    MyObject myObject = (MyObject)o.o;
    AutoResetEvent are = (AutoResetEvent)o.are;

    myObject.Execute();
    are.Set();
}

Answer 4

考虑使用线程池。你想要的大部分已经完成了。有一个example from Microsoft 几乎可以完成您的全部任务。将“fibonacci”替换为“数据库任务”，这听起来像是你的问题。

Answer 5

您可以将每个启动的线程存储在一个数组中。然后当你需要全部等待时，在循环中对数组中的每个线程调用 Join 方法。

Thread child = new Thread(...);
Threads.Add(child);
child.Start()

...

foreach(Thread t in Threads)
{
   t.Join();
}

HTH