var semaphor = new Semaphore(50, 50);// We allow at most 50 threads for crawlingvar resultPins = new List<Pin>();// Results stored hereforeach (var pin in new HashSet<string>(pinIdList)){  semaphor.WaitOne();  Console.Write(">");  var pinClient = new WebClient();  pinClient.DownloadStringCompleted += (sender, ex) =>  {   var html = ex.Result.Replace("/n", "");   pinClient.Dispose();   lock (pinIdList)   {     // do some post-processing and write back the results   }   Console.Write("<");   semaphor.Release();  };  pinClient.DownloadStringAsync(new Uri(string.Format("http://pinterest.com/pin/{0}/", pin)));}for (int i = 0; i < 50; i++) semaphor.WaitOne();// Wait until the last thread ends.semaphor.Dispose();Console.WriteLine();