Lambda Expressions, Func & Action

A lambda expression is a compact, inline way to write a method, most useful when you need to pass a small piece of behavior somewhere, like a callback or a LINQ predicate.

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

Action<string> log = message => Console.WriteLine($"[LOG] {message}");
log("Server started");

List<int> scores = new List<int> { 55, 82, 91, 40 };
List<int> passing = scores.Where(s => s >= 70).ToList();

Func and Action

• Func<T1, ..., TResult> is a built-in delegate type for a method that returns a value, the last type parameter is always the return type. Func<int, int, int> takes two ints and returns an int.
• Action<T1, ...> is the same idea for a method that returns nothing (void). Action<string> takes one string and returns nothing, plain Action takes no parameters at all.
• Both are just delegate types the .NET library already defines, so you don't need your own delegate declaration for common shapes.

Lambda syntax

• (a, b) => a + b is shorthand for a method taking a and b, and returning a + b.
• A single parameter can drop the parentheses: s => s >= 70.
• A lambda can capture variables from its surrounding scope (a closure), a lambda defined inside a method can still use that method's local variables even after being passed elsewhere.