Functional Programming in C# / Learn how to write better C# code

Functional Programming in C#, Second Edition Out now
1. Author: Enrico Buonanno
2. Date of Publication: February 2022
Functional Programming in C# teaches you to apply functional thinking to real-world problems using the C# language. The book, with its many practical examples, is written for proficient C# programmers with no prior FP experience. It will give you an awesome new perspective.

Updated for .NET 6 and C# 10.
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Functional Programming in C#: Classic Programming Techniques for Modern Projects
1. Author: Oliver Sturm
2. Date of Publication: May 2011
Take advantage of the growing trend in functional programming. C# is the number-one language used by .NET developers and one of the most popular programming languages in the world. It has many built-in functional programming features, but most are complex and little understood.
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Real World Functional Programming: With Examples in F# and C#
1. Author: Tomas Petricek
2. Date of Publication: December 2009
Real-World Functional Programming is a unique tutorial that explores the functional programming model through the F# and C# languages. The clearly presented ideas and examples teach readers how functional programming differs from other approaches. It explains how ideas look in F#-a functional language-as well as how they can be successfully used to solve programming problems in C#. Readers build on what they know about .NET and learn where a functional approach makes the most sense and how to apply it effectively in those cases.
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Why Functional Programming Matters

Benefits of functional programming in general

Pure functions are easier to reason about
Testing is easier, and pure functions lend themselves well to techniques like property-based testing
Debugging is easier
Programs are more bulletproof
Programs are written at a higher level, and are therefore easier to comprehend
Function signatures are more meaningful
Parallel/concurrent programming is easier

Referential Transparency

A function should operate only on the values that we pass and it shouldn't refer to the global state.

// This function is not referentially transparent // because it refers to the global DateTime.Now property public int ElapsedDays(DateTime from) { DateTime now = DateTime.Now; return (now - from).Days; } // Doesn't depend on the global state public static int ElapsedDays(DateTime from, DateTime now) { return (now - from).Days; }
Function Honesty

A function signature should convey all information about the possible input that it takes and the possible output that it produces.

// Dishonest signature public static int Divide(int x, int y) { // If y is 0 it will throw DivideByZero exception. // The function's signature doesn't convey enough // information about the output of the operation. return x / y; } // Honest signature public static int Divide(int x, NonZeroInteger y) { // NonZeroInteger is a custom type that can // hold any integer except zero. return x / y.Value; } // or public static int? Divide(int x, int y) { if (y == 0) return null return x / y; }
Functions as first-class citizens

Functions can be assigned to variables, stored to collections, just like values of other type.

// Assign function (x => x % 2 == 0) to a variable isEven Func<int, bool> isEven = x => x % 2 == 0; var list = Enumerable.Range(1, 10); // Pass the function as an argument to Where var evenNumbers = list.Where(isEven);
Higher-order functions (HOF)

Functions that take one or more functions as arguments or returns a function as a result or both.

public static IEnumerable<T> Where<T> (this IEnumerable<T> ts, Func<T, bool> predicate) { foreach (T t in ts) if (predicate(t)) yield return t; }
Pure functions

Functions that follows referential transparency and function honesty principles and don't cause any side effects.

public static class Multpilication { static int counter // An impure function as it mutates the global state static string Counter(int val) { return $"{++counter} x 2 = {val}"; } } // A pure function public static int MultiplyByTwo(int value) { return value * 2; }