title: Parallel.ForEach - Graceful Cancellation
body: <p>On the subject of waiting until tasks are complete and thread synchronisation.</p>
  
  <p>I currently have an iteration i have enclosed within a Parallel.ForEach. In the Example below I have posed some questions in the comments about how best to handle the graceful termination of the loop (.NET 4.0);</p>
  
  <pre><code>private void myFunction()
      {
  
          IList&lt;string&gt; iListOfItems = new List&lt;string&gt;();
          // populate iListOfItems
  
          CancellationTokenSource cts = new CancellationTokenSource();
  
          ParallelOptions po = new ParallelOptions();
          po.MaxDegreeOfParallelism = 20; // max threads
          po.CancellationToken = cts.Token;
  
          try
          {
              var myWcfProxy = new myWcfClientSoapClient();
  
              if (Parallel.ForEach(iListOfItems, po, (item, loopsate) =&gt;
              {
                  try
                  {
                      if (_requestedToStop)
                          loopsate.Stop();
                      // long running blocking WS call, check before and after
                      var response = myWcfProxy.ProcessIntervalConfiguration(item);
                      if (_requestedToStop)
                          loopsate.Stop();
  
                      // perform some local processing of the response object
                  }
                  catch (Exception ex)
                  {
                      // cannot continue game over.
                      if (myWcfProxy.State == CommunicationState.Faulted)
                      {
                          loopsate.Stop();
                          throw;
                      }
                  }
  
                  // else carry on..
                  // raise some events and other actions that could all risk an unhanded error.
  
              }
              ).IsCompleted)
              {
                  RaiseAllItemsCompleteEvent();
              }
          }
          catch (Exception ex)
          {
              // if an unhandled error is raised within one of the Parallel.ForEach threads, do all threads in the
              // ForEach abort? or run to completion? Is loopsate.Stop (or equivalent) called as soon as the framework raises an Exception?
              // Do I need to call cts.Cancel here?
  
              // I want to wait for all the threads to terminate before I continue at this point. Howe do we achieve that?
  
              // do i need to call cts.Dispose() ?
  
              MessageBox.Show(Logging.FormatException(ex));
          }
          finally
          {
  
              if (myWcfProxy != null)
              {
              // possible race condition with the for-each threads here unless we wait for them to terminate.
                  if (myWcfProxy.State == System.ServiceModel.CommunicationState.Faulted)
                      myWcfProxy.Abort();
  
                  myWcfProxy.Close();
              }
  
              // possible race condition with the for-each threads here unless we wait for them to terminate.
              _requestedToStop = false;
  
          }
  
      }
  </code></pre>
  
  <p>Any help would be most appreciated. The MSDN documentation talks of ManualResetEventSlim's and cancellationToken.WaitHandle's. but not sure how to wire them into this, seem to be struggling understanding the MSDN examples as most dont apply.</p>