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Lesson 3615 min read

Lambda Expressions

Learn the => syntax for writing compact, inline anonymous functions in C#.

What is a Lambda Expression?

A lambda expression is a compact way to write an anonymous (unnamed) function inline, most often assigned to a delegate type like Func or Action, or passed directly to a method.

Lambda Syntax

// Traditional method
static int Square(int x) => x * x;


// Equivalent lambda expression
Func<int, int> square = x => x * x;


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


// Multiple parameters need parentheses
Func<int, int, int> add = (a, b) => a + b;


// Multi-statement lambdas use braces and an explicit return
Func<int, string> classify = n =>
{
    if (n % 2 == 0) return "even";
    return "odd";
};

Lambdas with Collections

Lambdas shine when passed to collection methods that accept a delegate — you'll see this pattern constantly once you learn LINQ in the next lesson.

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6 };


List<int> evens = numbers.Where(n => n % 2 == 0).ToList();
bool anyBig = numbers.Any(n => n > 10);


Console.WriteLine(string.Join(", ", evens));
Console.WriteLine(anyBig);
Output

Click Run to see what this code prints.

Captured Variables

A lambda can reference variables from its enclosing scope — this is called "closing over" a variable, and the lambda keeps that variable alive as long as it needs it.

int threshold = 3;
Func<int, bool> isAboveThreshold = n => n > threshold;


Console.WriteLine(isAboveThreshold(5)); // True
threshold = 10;
Console.WriteLine(isAboveThreshold(5)); // False — uses the updated threshold

FAQs

They're closely related — lambdas are a more concise syntax that largely replaced the older `delegate(...) { ... }` anonymous method syntax.

Summary

  • Lambda expressions write small, inline functions using =>.
  • They're commonly assigned to Func/Action or passed to collection methods.
  • Lambdas can capture variables from their enclosing scope.
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LINQ