To fully appreciate the power of functional programming, it’s essential to understand how it differs from other paradigms, such as imperative and object-oriented programming.

Imperative programming

Imperative programming is the traditional approach in many languages. It focuses on explicitly specifying the sequence of steps to solve a problem. This style relies heavily on mutable state and side effects, which can make code more prone to bugs and harder to understand as the code base grows.

Object-oriented programming

Object-oriented programming (OOP) organizes code around objects, which encapsulate data and behavior. OOP is great for modeling real-world entities and promoting encapsulation. However, it can sometimes lead to complex hierarchies and tight coupling between objects, making code harder to modify and test.

Functional programming

Functional programming, in contrast, emphasizes pure functions...

C# has evolved significantly over the years, incorporating a range of functional programming features that make it a powerful language for this paradigm. Let’s take a closer look at some of these features and how they support functional programming:

• Lambda expressions: These provide a concise syntax for creating anonymous functions, enabling easy creation of higher-order functions
• LINQ: This provides a set of extension methods that enable functional-style operations such as filtering, mapping, and reducing collections
• Immutable data types: Data types such as strings and tuples guarantee that once created, their values cannot be changed
• Pattern matching: This allows us to test values against patterns and extract data based on those patterns
• Delegates and events: These allow you to treat functions as first-class citizens, passing them as arguments and storing them in variables

Throughout this book, we...

Writing functional code in C# means the implementation of functional concepts and techniques that will help us write functional code:

• Expressions: By favoring expressions over statements, we can write more declarative code that focuses on the desired result rather than the steps to achieve it.
• Pure functions: A pure function always produces the same output for a given input and has no side effects. It relies solely on its input parameters and does not modify any external state. Using pure functions, we can create easier code to reason about, test, and parallelize.
• Honest functions: Honest functions are an extension of pure functions that provide a clear and unambiguous contract. They explicitly communicate their input requirements and potential output scenarios, including error cases. Honest functions enhance code readability, maintainability, and error handling.
• Higher-order functions: These functions can accept other functions...

Let’s examine an example that demonstrates functional programming concepts using a book publishing system scenario:

public record Book(string Title, string Author, int Year, string Content);
// Pure function to validate a book
private static bool IsValid(Book book) =>
     !string.IsNullOrEmpty(book.Title) &&
     !string.IsNullOrEmpty(book.Author) &&
     book.Year > 1900 &&
     !string.IsNullOrEmpty(book.Content);
// Pure function to format a book
private static string FormatBook(Book book) =>
     $"{book.Title} by {book.Author} ({book.Year})";
// Higher-order function for processing books
private static IEnumerable<T> ProcessBooks<R>(
     IEnumerable<Book> books,
     Func<Book...

One of the strengths of C# is its ability to seamlessly combine functional and object-oriented programming paradigms. By leveraging the best of both worlds, we can create code that is modular, reusable, and expressive. Here are some strategies for combining functional and object-oriented programming:

• Immutable objects: Immutable objects are thread-safe, easier to reason about, and align well with functional programming principles
• Extension methods: These allow us to enhance the functionality of types without modifying their original implementation, promoting a more functional and compositional approach
• Higher-order functions as instance methods: This approach helps us to encapsulate behavior and provide a fluent and expressive API for working with objects in a functional manner
• Dependency injection and composition: By injecting functional dependencies and composing objects based on their behavior, you can achieve...

To make our journey through functional programming in C# more engaging and relatable, let’s introduce our main characters: Steve and Julia.

Steve is a middle-level C# software developer who has heard that functional programming can help him write better code, become more valuable at his current job, and gain an advantage over other candidates if he decides to pursue a new opportunity. He’s eager to learn but unsure where to start.

Julia, on the other hand, is already an expert in functional programming in C#. She’s passionate about the paradigm and enjoys sharing her knowledge with others. Throughout the book, Julia will provide Steve with advice, guidance, and practical examples to help him master functional programming concepts.

As we progress through the chapters, we’ll follow Steve’s journey as he learns from Julia and applies functional programming techniques to real-world scenarios.

Congratulations on taking the first step toward mastering functional programming in C#! In this chapter, we’ve explored the differences between functional, imperative, and object-oriented programming paradigms. We’ve also delved into the functional features of C#, such as lambda expressions, LINQ, immutable data types, pattern matching, and delegates, and how they support functional programming.

Furthermore, we’ve introduced the concepts and techniques for writing functional code in C# such as expressions, pure functions, honest functions, higher-order functions, functors, and monads. Finally, we’ve discussed strategies for combining functional and object-oriented programming in C#, allowing us to leverage the best of both paradigms in our projects.

As we progress through the next chapters, we’ll dive deeper into each of these concepts, learning how to write cleaner, more modular, and more testable code using functional principles.