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📜 WAYPOINT LESSON

Func, Action, Predicate

⭐ beginner⏳ 12 min read📍 Lesson 62 of 85

Three generic delegate types cover almost every callback need — learn their shapes once.

Methods as values

A delegate is a type whose values are methods. You pass a method where data would go, and invoke it later:

static int Apply(int x, Func<int, int> f) => f(x);

Apply(5, n => n * 2);   // 10
Apply(5, n => n + 1);   // 6

The BCL's three workhorses:

  • Action — takes 0–16 arguments, returns nothing: Action<string> log = msg => Console.WriteLine(msg);
  • Func<T, TResult> — takes up to 16 arguments, last type parameter is the return: Func<string, int> len = s => s.Length;
  • Predicate<T> — Func<T, bool> by another name: Predicate<int> isEven = n => n % 2 == 0;

The one syntax rule people trip on: in Func<A, B, C>, A and B are parameters, C is the return type.