International Phone Number Validation: Explained in Detail, Implemented with WPF TextBox

Validating phone numbers is difficult, because the telephony authorities were too creative in how they used the limited available number range and because phone numbers look different in various countries. This article describes how phone number input validation can be reasonably done and provides a Control with production quality code, including letting the user enter only valid keystrokes, controlling if the phone number must be entered (required field) and alerting the user if he tries to close the window without saving the changes made. This control is part of the WpfWindowLib.