与 ManualResetEvent 同步时出现问题

Question

我已经编写了一个下载一些文件的方法，现在我正试图让它并行下载多达 5 个文件，其余的等待前面的文件完成。 我为此使用了 ManualResetEvent，但是当我包含同步部分时，它不再下载任何东西（没有它就可以工作）。

这里是方法的代码：

    static readonly int maxFiles = 5;
    static int files = 0;
    static object filesLocker = new object();
    static System.Threading.ManualResetEvent sync = new System.Threading.ManualResetEvent(true);

    /// <summary>
    /// Download a file from wikipedia asynchronously
    /// </summary>
    /// <param name="filename"></param>
    public void DoanloadFileAsync(string filename)
    {
        ...
        System.Threading.ThreadPool.QueueUserWorkItem(
            (o) =>
            {
                bool loop = true;
                while (loop)
                    if (sync.WaitOne())
                        lock (filesLocker)
                        {
                            if (files < maxFiles)
                            {
                                ++files;
                                if (files == maxFiles)
                                    sync.Reset();
                                loop = false;
                            }
                        }
                try
                {
                    WebClient downloadClient = new WebClient();
                    downloadClient.OpenReadCompleted += new OpenReadCompletedEventHandler(downloadClient_OpenReadCompleted);
                    downloadClient.OpenReadAsync(new Uri(url, UriKind.Absolute));
                    //5 of them do get here
                }
                catch
                {
                    lock (filesLocker)
                    {
                        --files;
                        sync.Set();
                    }
                    throw;
                }
            });
    }

    void downloadClient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
    {
        try
        {
            //but none of the 5 get here
            ...Download logic... //works without the ManualResetEvent
        }
        finally
        {
            lock (filesLocker)
            {
                --files;
                sync.Set();
            }
        }
    }

我是不是做错了什么？

它是用 Silverlight 4 for Windows Phone 7 编写的。

编辑：Silverlight 4 中没有 Semaphore 或 SemaphoreSlim。

Answer 1

我在评论中的意思是当你可以使用Interlocked 时使用慢速lock。这种方式的性能也会更高。

最多有 5 个并行下载：

public class Downloader
{
 private int fileCount = 0;
 private AutoResetEvent sync = new AutoResetEvent(false);

 private void StartNewDownload(object o)
 {
  if (Interlocked.Increment(ref this.fileCount) > 5) this.sync.WaitOne();

  WebClient downloadClient = new WebClient();
  downloadClient.OpenReadCompleted += downloadClient_OpenReadCompleted;
  downloadClient.OpenReadAsync(new Uri(o.ToString(), UriKind.Absolute));
 }

 private void downloadClient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
 {
  try
  {
   // Download logic goes here.
  }
  catch {}
  finally
  {
   this.sync.Set();
   Interlocked.Decrement(ref this.fileCount);
  }
 }

 public void Run()
 {
  string o = "url1";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);
  Thread.Sleep(100);

  o = "url2";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);

  o = "url3";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);
  Thread.Sleep(200);

  o = "url4";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);

  o = "url5";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);

  o = "url6";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);

  o = "url7";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);
  Thread.Sleep(200);

  o = "url8";
  System.Threading.ThreadPool.QueueUserWorkItem(this.StartNewDownload, o);
  Thread.Sleep(400);
 }
}

Answer 2

您似乎正在尝试限制一次可以进入您的关键部分（文件下载）的线程数。与其尝试手工制作，不如使用System.Threading.Semaphore - 这就是它的作用！

Answer 3

从外观上看，WaitOne() 在您创建 WebClient 之前已被命中。由于所有调用Set() 的代码都在事件处理程序或异常处理程序中，因此它永远不会命中。

也许您错误地将WebClient 代码包含在线程池线程方法中，而该方法应该在它之外

    System.Threading.ThreadPool.QueueUserWorkItem(
        (o) =>
        {
            bool loop = true;
            while (loop)
                if (sync.WaitOne())
                    lock (filesLocker)
                    {
                        if (files < maxFiles)
                        {
                            ++files;
                            if (files == maxFiles)
                                sync.Reset();
                            loop = false;
                        }
                    }

        });

//Have the try catch OUTSIDE the background thread.
            try
            {
                WebClient downloadClient = new WebClient();
                downloadClient.OpenReadCompleted += new OpenReadCompletedEventHandler(downloadClient_OpenReadCompleted);
                downloadClient.OpenReadAsync(new Uri(url, UriKind.Absolute));
                //5 of them do get here
            }
            catch
            {
                lock (filesLocker)
                {
                    --files;
                    sync.Set();
                }
                throw;
            }