C# (.NET) Application Architecture: From Monolith to Microservices

This book is not just a collection of code examples, but a comprehensive guide to building software systems on the C# (.NET) platform. The author, Alexey Smirnov, shares years of experience in designing enterprise solutions, showing how to turn chaotic code into a coherent, scalable, and testable architecture.

The book starts with the basics: you will understand what distinguishes "good" architecture from "bad" and master the key principles of SOLID, DRY, and YAGNI. Then you will dive into the world of design patterns (GoF) and learn how to apply them in the .NET context — from simple factories to complex chains of responsibility.

• Layered Architecture — how to properly divide an application into layers (Presentation, Business, Data Access) and avoid "spaghetti code".
• Dependency Inversion and DI Containers — configuring Autofac, Microsoft.Extensions.DependencyInjection, and testing components.
• Microservices Architecture — when microservices are needed, how to organize their interaction via RabbitMQ, gRPC, or REST.
• Domain-Driven Design (DDD) — modeling complex business processes using aggregates, Value Objects, and repositories.
• Asynchrony and Performance — proper use of async/await, TPL Dataflow, and handling large volumes of data.
• Security and Monitoring — implementing OpenID Connect, Serilog, Application Insights, and Health Checks.

Who will benefit from this book? Primarily, intermediate-level C# developers (2–5 years of experience) who already write code but want to move to the architect level. You will learn not just to implement features, but to make decisions that affect the entire project structure. The book will also be useful for team leads and technical managers looking to standardize development approaches in their team.

After reading, you will be able to:

• Design applications from scratch, choosing the appropriate architectural style (monolith, microservices, modular monolith).
• Refactor legacy code without disrupting business processes.
• Write tests (Unit, Integration, End-to-End) for complex architectural solutions.
• Justify your architectural choices to the client and team.

The book contains numerous practical examples in C# (.NET 8), ready-made diagrams (C4 model), and code templates. All source code is available in a GitHub repository. This is not just a textbook — it is a desktop reference for those building systems that will live for years.