Dependency Injection in .NET

Introduction

The book «Dependency Injection in .NET» by Mark Seemann and Steven van Deursen is arguably the most authoritative and in-depth work on the topic of dependency injection (DI) in the .NET ecosystem. The second edition, published in 2019, has been completely revised to reflect modern realities: .NET Core, ASP.NET Core, Microsoft.Extensions.DependencyInjection containers, and the best practices of recent years. The book not only teaches how to use DI containers but also fosters the right architectural mindset for creating loosely coupled, testable, and maintainable applications.

What This Book Is About

The book is divided into three logical parts. The first part introduces the basics: what dependencies are, why injecting them improves architecture, and what patterns exist (constructor, property, method). The second part is dedicated to antipatterns and trade-offs — the authors thoroughly analyze common mistakes such as Service Locator, Ambient Context, and excessive use of static classes. The third part is a practical guide to selecting and configuring DI containers (Autofac, Simple Injector, Microsoft.Extensions.DependencyInjection), as well as integrating DI into real projects: web applications, desktop applications, services, and tests.

Who This Book Is For

The book is intended for intermediate and advanced .NET developers who are already familiar with C# and want to deepen their knowledge of application architecture. For beginners just starting to learn DI, it may seem complex due to the abundance of terms and examples, but with due diligence, it will become an excellent textbook. The book is especially useful for those transitioning from .NET Framework to .NET Core who want to understand how to properly use the built-in DI container.

What the Reader Will Learn

• DI Basics: what dependencies are, Inversion of Control (IoC), and how dependency injection helps create loosely coupled code.
• Injection Patterns: constructor, property, method — when and which to use.
• Antipatterns: how to recognize and avoid Service Locator, Control Freak, Bastard Injection, and other mistakes.
• Working with Containers: configuring Autofac, Simple Injector, Castle Windsor, and the built-in Microsoft.Extensions.DependencyInjection container.
• Object Lifetimes: Singleton, Scoped, Transient — how to choose and avoid mistakes.
• Integration with Frameworks: ASP.NET Core MVC, Web API, Entity Framework Core, WPF, Xamarin.
• Testing: how DI simplifies unit testing and dependency mocking.
• Architectural Trade-offs: when DI is excessive and how to find a balance.

Strengths

The first thing that impresses is the depth of topic coverage. The authors don't just list patterns; they analyze them from the perspective of maintainability, testability, and performance. Each antipattern is accompanied by a real code example that is then refactored. The second is relevance: the book is written for .NET Core 3.0 and ASP.NET Core 3.0, making it applicable right now. The third is practical focus: each chapter ends with exercises, and the appendices contain complete project examples. Finally, the writing style: the authors write clearly, with humor and without excessive academicism, making the reading engaging.

Who Should Read It

I highly recommend this book to everyone who writes in C# and wants their code to be not just functional, but flexible, testable, and ready for change. If you are an architect, team lead, or senior developer — this is a must-read. If you are just starting your journey in .NET — read it after mastering the basics of the language. The book will be an excellent addition to such classic works as «Clean Code» by Robert Martin and «CLR via C