This happens if you try to generate your PhoneBook web client application again without removing the existing
PhoneBook.Web directory first. In
uigen.exe is your friend: overwriting your existing project, potentially with weeks or months of invested work by accident is precluded by this simple check. This is admittedly annoying and boring while learning the ropes with
uigen.exe, but it comes in handy if you are developing for production and might want to preserve modifications to the
uigen.exe-generated web application.