public void ProcessDirectories(string[] directories)
        {
            try
            {
                foreach (var path in directories)
                {
                    string capId = ExtractCapId(path);

                    ProcessDirectory(capId, path);
                }

                if (OnDirectoriesProcessed != null) OnDirectoriesProcessed(this, new EventArgs());
            }
            catch (Exception ex)
            {
                if (OnDirectoriesProcessingException != null) OnDirectoriesProcessingException(this, ex);
            }
        }

ThreadPool.UnsafeQueueUserWorkItem(new WaitCallback(worker.ProcessDirectories), directories.Take(3).ToArray());

        private void DirectoriesProcessingException(object self, Exception exc)
        {
            textBox1.Text = exc.ToString();
        }

        private void DirectoriesProcessed(object self, EventArgs e)
        {
            label1.Content = "Finished!";
        }

        private void DirectoriesProcessingException(object self, Exception exc)
        {
            textBox1.Dispatcher.Invoke( (Action)delegate() {textBox1.Text = exc.ToString();}, null);
        }