C#

WebClient laddaner = new WebClient();
laddaner.DownloadStringCompleted += new DownloadStringCompletedEventHandler(laddaner_DownloadStringCompleted);
laddaner.DownloadStringAsync(new Uri("http://thisurldoesnotexist.com"));

WebClient laddaner = new WebClient();
laddaner.DownloadStringCompleted += new DownloadStringCompletedEventHandler(laddaner_DownloadStringCompleted);

try
{
   laddaner.DownloadStringAsync(new Uri("http://thisurldoesnotexist.com"));
}
catch(WebException)
{
   MessageBox.Show("Error!");
}

private static bool WaitForCompletionOrTimerExpiration(Function method, int timeout)
     {
         using (ManualResetEvent threadWaitHandle = new ManualResetEvent(false))
         {
             bool hasFinishedExecutingMethod = false;
             int hasSignalled = 0;

             // Create a timer callback. When the timer callback hits, the wait handle will be set.
             TimerCallback timerCallback = delegate(object state)
             {
                 int originalValueOfHasSignalled = Interlocked.CompareExchange(ref hasSignalled, 1, 0);
                 if (originalValueOfHasSignalled == 0)
                 {
                     threadWaitHandle.Set();
                 }
             };

             // Create a method to be called on the threadpool that will invoke the method passed to us and set the handle when done executing.
             Exception methodException = null;
             WaitCallback threadPoolMethod = delegate(object state)
             {
                 try
                 {
                     method();
                 }
                 finally
                 {
                     int originalValueOfHasSignalled = Interlocked.CompareExchange(ref hasSignalled, 1, 0);
                     if (originalValueOfHasSignalled == 0)
                     {
                         hasFinishedExecutingMethod = true;
                         threadWaitHandle.Set();
                     }
                 }
             };

             // Create and run our timer, start running the method on the thread pool, wait on the wait handler until someone signals us.
             // We will be signaled when either the timer expires or when the method finishes executing.
             using (Timer timer = new Timer(timerCallback, null, timeout, Timeout.Infinite))
             {
                 ThreadPool.QueueUserWorkItem(threadPoolMethod);

                 //
                 // THIS IS THE LINE I'M HAVING TROUBLE WITH
                 //
                 threadWaitHandle.WaitOne(); // BUG! Sometimes this will wait forever!
             }

             return hasFinishedExecutingMethod;
         }
     }

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