This book is for Computer Science students who use the C# language and who wish to be able to develop cross-platform desktop applications with a graphical interface. To write the code, a software that is also cross-platform will be used, operating with Windows, MacOS and Linux operating systems. Therefore, we will be able to work without the need to resort to Windows virtual machines to run Visual Studio in environments such as MacOS and Linux. The goal is to use AvaloniaUI to build desktop applications for Windows, Linux and macOS. The reader is accompanied step by step in the practical creation of some example applications that allow you to proceed gradually in the development of typical features of a desktop application.