NotSupportedException on WaitHandle.WaitAll

Question

I am trying to execute the following code. The code tries to parallely download and save images. I pass a list of images to be downloaded. I wrote this in C# 3.0 and compiled it using .NET Framework 4 (VS.NET express edition). The WaitAll operation is resulting in a NotSupportedException (WaitAlll for multiple handles on a STA thread is not supported) everytime I try to run my program. I tried removing SetMaxThreads, but that didn't do any difference.

public static void SpawnThreads(List<string> imageList){
    imageList = new List<string>(imageList);
    ManualResetEvent[] doneEvents = new ManualResetEvent[imageList.Count];
    PicDownloader[] picDownloaders = new PicDownloader[imageList.Count];
    ThreadPool.SetMaxThreads(MaxThreadCount, MaxThreadCount);
    for (int i = 0; i < imageList.Count; i++) {
        doneEvents[i] = new ManualResetEvent(false);
        PicDownloader p = new PicDownloader(imageList[i], doneEvents[i]);
        picDownloaders[i] = p;
        ThreadPool.QueueUserWorkItem(p.DoAction);
    }
    // The following line is resulting in "NotSupportedException"     
    WaitHandle.WaitAll(doneEvents);
    Console.WriteLine("All pics downloaded");
}

Can you please let me understand what is the issue I am running into?

Thank you

Answer 1

I advise against using multiple WaitHandle instances to wait for completion. Use the CountdownEvent class instead. It results in more elegant and scalable code. Plus, the WaitHandle.WaitAll method only supports up to 64 handles and cannot be called on an STA thread. By refactoring your code to use the canonical pattern I came up with this.

public static void SpawnThreads(List<string> imageList)
{ 
  imageList = new List<string>(imageList); 
  var finished = new CountdownEvent(1);
  var picDownloaders = new PicDownloader[imageList.Count]; 
  ThreadPool.SetMaxThreads(MaxThreadCount, MaxThreadCount); 
  for (int i = 0; i < imageList.Count; i++) 
  { 
    finished.AddCount();    
    PicDownloader p = new PicDownloader(imageList[i]); 
    picDownloaders[i] = p; 
    ThreadPool.QueueUserWorkItem(
      (state) =>
      {
        try
        {
          p.DoAction
        }
        finally
        {
          finished.Signal();
        }
      });
  } 
  finished.Signal();
  finished.Wait();
  Console.WriteLine("All pics downloaded"); 
}