This book is a comprehensive guide to mastering the C# 13 programming language and the .NET 9 SDK, providing a thorough learning experience for both beginners and experienced developers. With 31 detailed chapters, 30 online knowledge quizzes, and access to downloadable project code, this guide is structured to support your journey from basic concepts to advanced programming techniques, all within the Visual Studio Code environment. We will start by introducing the foundational components of the C# ecosystem, providing an overview of the Common Language Infrastructure (CLI), the Common Intermediate Language (CIL), and the Virtual Execution System (VES). These initial sections will offer a deeper understanding of the platform's architecture. Following that, we will provide step-by-step instructions for installing and using the Visual Studio Code environment. As you advance, you will learn about the foundational principles of the C# language. This will include exploring topics such as