A widely-used open-source library, libphonenumber-csharp, provides extensive resources for validating and manipulating phone numbers of all types. It's derived from an open source library created by Google. The .NET frameworks provide limited capabilities for validating phone numbers, particularly with respect to internationalization ("i18n"). A common design pattern in software built with C# and the .NET frameworks uses class objects to store data. Data validation is abstracted into validation attributes which are used to "decorate" the properties of entity classes. The libphonenumber-csharp library provides a convenient way to implement phone number validation in .NET projects, making it easier to handle standard data types more robustly. Phone numbers are permitted to be expressed with digits and characters like "-", ".", "(", and ")", plus an extension marked with "ext.", "ext", or "x". The library can validate whether the potential phone number is valid in a specific country. It's also possible to use the PhoneNumberUtil class to determine the type of phone number, format for dialing from a mobile phone in another country, and more. The libphonenumber-csharp library provides robust functionality for handling standard data types like phone numbers, making it easier to build secure and user-friendly software applications.