Delegates

C# / .NET Interview Notes

1. What are Delegates?

A delegate is a type-safe reference to a method.

It allows a method to be:

• Stored in a variable
• Passed as a parameter
• Returned from another method
• Invoked later

2. Basic Delegate

delegate void MyDelegate();

void SayHello() => Console.WriteLine("Hello");

MyDelegate action = SayHello;
action();  // Hello

3. Delegate Signature

4. Why Use Delegates?

delegate int MathOperation(int a, int b);

MathOperation op = Add;
Console.WriteLine(op(10, 20));  // 30

op = Multiply;
Console.WriteLine(op(10, 20));  // 200

5. Passing a Delegate to a Method

void Execute(MathOperation operation)
{
    int result = operation(10, 20);
    Console.WriteLine(result);
}

Execute(Add);     // 30
Execute(Multiply); // 200

6. Anonymous Methods

MathOperation operation = delegate (int a, int b)
{
    return a + b;
};

Console.WriteLine(operation(10, 20));  // 30

7. Lambda Expressions

MathOperation operation = (a, b) => a + b;
MathOperation multiply = (a, b) => a * b;

8. Built-in Delegate — Action

Action<string> print = message => Console.WriteLine(message);
print("Hello");

Action<int, int> printSum = (a, b) => Console.WriteLine(a + b);

9. Built-in Delegate — Func

Func<int, int, int> add = (a, b) => a + b;
int result = add(10, 20);

Func<string, int> getLength = text => text.Length;

In Func<int, int, int>:

• The first two int parameters are the inputs
• The last int is the return type

10. Built-in Delegate — Predicate<T>

Predicate<int> isEven = number => number % 2 == 0;
bool result = isEven(10);  // true

11. Action vs. Func vs. Predicate

12. Delegates in LINQ

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

var evenNumbers = numbers.Where(x => x % 2 == 0);

// x => x % 2 == 0 is a lambda passed to Where()

13. Delegates and Events

Action action;
action = Method1;
action += Method2;
action += Method3;

action();  // Invokes Method1, Method2, Method3

action -= Method2;  // Remove Method2

Action<string> notify;
notify += SendEmail;
notify += SendSms;
notify += SendPushNotification;

notify("Order completed");

// All three methods are called

void Process(Action callback)
{
    Console.WriteLine("Processing...");
    callback();
}

Process(() => Console.WriteLine("Done!"));