Functional Programming in C# (.NET): A Practical Guide

This book is a practical guide to introducing a functional approach into everyday C# development within the .NET ecosystem. Unlike academic works, the material is focused on solving real-world problems: you will learn to write more expressive, concise, and, most importantly, predictable code.

What is this book about?

The book systematically reveals the key concepts of functional programming (FP) and shows how to apply them in C#:

• FP Basics: function purity, data immutability, absence of side effects. Why this is important for multithreaded applications and testing.
• Lambda Expressions and Delegates: from syntax to creating closures. You will learn to pass behavior as data.
• LINQ — the Heart of Functional C#: a deep dive into deferred execution, operator chains, custom providers, and performance.
• FP Patterns: Option (Maybe), Either, Try (Railway Oriented Programming). How to handle errors without exceptions.
• Function Composition: partial application, currying, pipe composition. Building complex logic from simple blocks.
• Reactive Programming: connection with Rx.NET (IObservable) and functional data streams.

Who is this book for?

• C# developers at Junior+ and Middle levels who want to elevate code quality to a new level.
• Architects of .NET systems striving to create testable and maintainable applications.
• Java programmers transitioning to C# and wanting to master its functional capabilities.
• Anyone who has heard about functional programming but doesn't know how to apply it in an imperative language.

What will you learn?

• How to replace loops and conditional statements with expressive method chains.
• How to design APIs using monads for safe null and error handling.
• How to use immutable types (record) for thread safety.
• How to write higher-order functions to encapsulate repetitive logic.
• How to combine OOP and FP — without abandoning classes, but adding functional purity.

Each chapter contains code examples that can be run immediately in Visual Studio or Rider. You will get not just theory, but ready-made tools for refactoring legacy code and writing new code in a functional style.

Note: The book assumes basic knowledge of C# syntax (classes, methods, types). Familiarity with LINQ is welcome but not required.