This document discusses using C# and Xamarin to build mobile apps for iOS, Android, and Windows. It presents Xamarin.Forms as a way to share UI code and build apps using C# and XAML. It also discusses using Xamarin.iOS and Xamarin.Android to build apps with platform-specific customization while sharing C# business logic. The document recommends Xamarin.Forms for data entry apps and proofs-of-concept, and Xamarin.iOS/Android for apps requiring specialized platform interaction or highly polished design. It provides code samples and links to Xamarin documentation and code repositories.