c# - 与ManualResetEvent同步时出现问题

我编写了一种下载一些文件的方法，现在我正在尝试使其最多并行下载5个文件，其余的要等待以前的文件完成。
我为此使用了ManualResetEvent，但是当我包含同步化部分时，它不再下载任何内容（没有它可以工作）。

这是方法的代码：

    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();
            }
        }
    }

难道我做错了什么？

它是使用Windows Phone 7的Silverlight 4编写的。

编辑：Silverlight 4中没有Semaphore或SemaphoreSlim。

最佳答案

我在评论中的意思是，在可以使用lock时使用慢速Interlocked的情况。同样，这种方式将使性能更高。

并行最多进行5个下载：

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);
 }
}