manage thread to update UI Control

[dpimental]

All, I'm working on an asp.net app.
I have (C#) pdf page validator that evaluates the number of pages in a pdf file and compares it to the number entered on a form (the form will create an xml file used to configure an order).
My problem is that the process to run the validator against a large pdf file takes long enough that I would need to run it on a separate thread; but I can't seem to update / reference controls on the main UI Thread. The last thing I tried was the following.

Task taskA = Task.Factory.StartNew(() => DoSomeWork(5000));
taskA.Wait();

I have also tried this

MainThread = SynchronizationContext.Current;
        if (MainThread == null) MainThread = new SynchronizationContext();

        MainThread.Send((object state) =>
        {
            //pdfPageStatus.Text = pdfPages.ToString() + " pages in PDF File Uploaded";
            pdfPageStatus.Text = " pages in PDF File Uploaded";
            pdfPageStatus.ForeColor = Color.Blue;
        }, null);

David Pimental
(US, Oh)