Delegates — C# Action, Func, and Lambda

S7.0 What This Teaches

• Delegates: type-safe function pointers
• Built-in delegate types: Action, Func, Predicate
• Lambda expressions: (x) => expression
• Closures: lambdas capturing outer variables
• Passing delegates as callbacks and higher-order functions
• Multicast delegates

S7.1 Delegate Basics

// Declare a delegate type
delegate int MathOp(int a, int b);

// Assign a method
MathOp add = (a, b) => a + b;
MathOp mul = (a, b) => a * b;

Console.WriteLine(add(3, 4));   // 7
Console.WriteLine(mul(3, 4));   // 12

// Store and pass as argument
static int Apply(int x, int y, MathOp op) => op(x, y);
Console.WriteLine(Apply(5, 6, add));   // 11

S7.2 Action and Func

// Action<T> - takes arguments, returns void
Action<string> greet = name => Console.WriteLine($"Hello, {name}!");
greet("Alice");

// Func<T, TResult> - takes arguments, returns TResult
Func<int, int, int> add = (a, b) => a + b;
Console.WriteLine(add(3, 4));   // 7

// Predicate<T> - returns bool (shorthand for Func<T, bool>)
Predicate<int> isEven = n => n % 2 == 0;
Console.WriteLine(isEven(4));   // True

// Higher-order function using Func
static List<T> Filter<T>(List<T> items, Func<T, bool> predicate) =>
    items.Where(predicate).ToList();

S7.3 Lambda Expressions

// Expression lambda - single expression, implicit return
Func<int, int> square = x => x * x;

// Statement lambda - multiple statements in braces
Func<int, int> safeSqrt = x =>
{
    if (x < 0) return 0;
    return (int)Math.Sqrt(x);
};

// No parameters
Action printTime = () => Console.WriteLine(DateTime.Now);

// Multiple parameters
Func<int, int, int> clamp = (value, max) => Math.Min(value, max);

S7.4 Closures

int multiplier = 3;
Func<int, int> triple = x => x * multiplier;
Console.WriteLine(triple(5));   // 15

multiplier = 10;
Console.WriteLine(triple(5));   // 50 - sees the changed value!

// Factory function returning a closure
static Func<int, int> MakeAdder(int delta) => x => x + delta;

var addFive = MakeAdder(5);
var addTen  = MakeAdder(10);
Console.WriteLine(addFive(3));  // 8
Console.WriteLine(addTen(3));   // 13

S7.5 Multicast Delegates

Action<string> log = msg => Console.WriteLine($"[Log] {msg}");
Action<string> alert = msg => Console.WriteLine($"[ALERT] {msg}");

Action<string> notify = log;
notify += alert;   // both will be called

notify("Server started");
// [Log] Server started
// [ALERT] Server started

notify -= log;   // remove from invocation list

S7.6 Example - All Together

// Delegates - pipeline of transformation steps with Func composition.

var pipeline = Compose<string, string>(
    s => s.Trim(),
    s => s.ToLower(),
    s => s.Replace(" ", "_")
);

string[] inputs = { "  Hello World  ", " Foo Bar ", "  C Sharp  " };
foreach (string s in inputs)
    Console.WriteLine(pipeline(s));

static Func<T, T> Compose<T, T2>(params Func<T, T>[] steps)
    where T2 : T
{
    return input => steps.Aggregate(input, (acc, f) => f(acc));
}

hello_world
foo_bar
c_sharp

S7.7 Exercise

S7.8 Common Mistakes

var actions = new List<Action>();
for (int i = 0; i < 3; i++)
    actions.Add(() => Console.WriteLine(i));  // captures i, not a copy

actions.ForEach(a => a());   // prints 3 3 3 - all see i=3!

// Fix: capture a local copy
for (int i = 0; i < 3; i++)
{
    int copy = i;
    actions.Add(() => Console.WriteLine(copy));  // prints 0 1 2
}

Action<string> handler = null;
handler("test");   // NullReferenceException
handler?.Invoke("test");   // safe - no-ops if null

S7.9 Key Terms