If you can allow the message to appear when the user attempts to submit (instead of immediately after the edit), add a ValidationSummary control. Set its ShowMessageBox property to true. If you don't want to see the actual ValidationSummary, also set ShowSummary to false.
--- Peter Blum
Creator of Peter's Data Entry Suite (formerly Professional Validation And More and Peter's Date Package) and Peter's Polling Package
www.PeterBlum.com