I have this application that will recurse all folders in a given directory and look for PDF. If a PDF file is found, the application will count its pages using ITextSharp. I did this by using a thread to recursively scan all the folders for pdf, then if then PDF is found, this will be queued to the thread pool. The code looks like this:

//spawn a thread to handle the processing of pdf on each folder.
                var th = new Thread(() =>
                {
                    pdfDirectories = Directory.GetDirectories(pdfPath);
                    processDir(pdfDirectories);
                });
                th.Start();



 private void processDir(string[] dirs)
        {
            foreach (var dir in dirs)
            {
                pdfFiles = Directory.GetFiles(dir, "*.pdf");
                processFiles(pdfFiles);

                string[] newdir = Directory.GetDirectories(dir);
                processDir(newdir);
            }
        }



private void processFiles(string[] files)
        {
            foreach (var pdf in files)
            {
                ThreadPoolHelper.QueueUserWorkItem(
                    new { path = pdf },
                    (data) => { processPDF(data.path); }
                    );
            }
        }

My problem is, how do i know that the thread pool's thread has finished processing all the queued items so i can tell the user that the application is done with its intended task?