Delegate in C#

What is a Delegate?

• A delegate in C# is a type that represents references to methods with a specific parameter list and return type.

• When you create an instance of a delegate, you can associate it with any method that has a compatible signature (matching parameters and return type).

• Delegates allow you to pass methods as arguments to other methods.

Code Example

using System;
using System.Collections.Generic;

public class Program
{
    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }

    // Define a delegate to represent filtering methods
    public delegate bool FilterDelegate(Person p);

    public static void Main()
    {
        // Create some Person objects
        Person p1 = new Person() { Name = "Rohit", Age = 26 };
        Person p2 = new Person() { Name = "Lal", Age = 40 };
        Person p3 = new Person() { Name = "Prashant", Age = 65 };

        // Populate a list with Person objects
        List<Person> people = new List<Person>() { p1, p2, p3 };

        // Display people based on different filters
        DisplayPeople("Children", people, IsChild);
        DisplayPeople("Adults", people, IsAdult);
        DisplayPeople("Seniors", people, IsSenior);

        Console.Read();
    }

    // Display people based on a specified filter
    static void DisplayPeople(string title, List<Person> people, FilterDelegate filter)
    {
        Console.WriteLine(title + ":");
        foreach (Person p in people)
        {
            if (filter(p))
            {
                Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
            }
        }
        Console.WriteLine();
    }

    // Filter methods
    static bool IsChild(Person p) => p.Age < 18;
    static bool IsAdult(Person p) => p.Age >= 18 && p.Age < 65;
    static bool IsSenior(Person p) => p.Age >= 65;
}

Key Concepts Explained

Delegate Declaration:

• We define a FilterDelegate to represent filtering methods that take a Person object and return a boolean value.
• This delegate allows us to dynamically choose different filters for our list of people.

Method Definitions:

• We have three filter methods: IsChild, IsAdult, and IsSenior.
• These methods determine whether a person falls into the child, adult, or senior category based on their age.

Displaying People:

• The DisplayPeople method takes a title, a list of people, and a filter delegate.
• It iterates through the list and displays relevant information based on the specified filter.

Output:

Children:
Rohit, 26 years old

Adults:
Rohit, 26 years old
Lal, 40 years old

Seniors:
Prashant, 65 years old