Lambda Expressions

Syntax

(parameters) => expression

Examples

// Old way: a named method
int SquareMethod(int x) => x * x;

// New way: a lambda assigned to a Func delegate
Func<int, int> square = x => x * x;

Console.WriteLine(square(5));  // 25
Console.WriteLine(SquareMethod(5));  // 25

Func<int, int, int> add = (a, b) => a + b;       // takes 2 ints, returns an int
Console.WriteLine(add(3, 4));  // 7

Action<string> print = message => Console.WriteLine($"Log: {message}");
print("Hello");                                    // takes a string, returns nothing

Predicate<int> isEven = n => n % 2 == 0;
Console.WriteLine(isEven(4));  // true            // takes a value, returns bool

Func<int, string> classify = n =>
{
    if (n < 0) return "negative";
    else if (n == 0) return "zero";
    else return "positive";
};

Console.WriteLine(classify(-5));  // negative
Console.WriteLine(classify(0));    // zero

List<string> names = new List<string> { "Charlie", "Alice", "Bob" };

names.Sort((a, b) => a.CompareTo(b));
Console.WriteLine(string.Join(", ", names));  // Alice, Bob, Charlie

names.Sort((a, b) => b.Length - a.Length);  // longest name first
Console.WriteLine(string.Join(", ", names));

Best practices

• Use lambdas for short, simple implementations passed as arguments - switch to a full local function or method once the logic gets long or complex
• Use Func<> when the lambda returns a value, Action<> when it does not, and Predicate<> specifically for a single bool-returning test
• Keep lambda parameter names short but meaningful, especially in LINQ chains where context is often clear from position
• Remember lambdas capture variables from their enclosing scope (closures) - be mindful of this when a lambda outlives the loop iteration that created it

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation