Extension Methods

Overview

An extension method in C# is a static method that allows you to add new functionality to an existing class without modifying its source code or inheriting from it.
Extension methods enable you to "extend" the behavior of a class by providing new methods that can be called on instances of that class as if they were part of the class's original API.
To define an extension method, you need to create a static class that contains the extension method as a static method. The first parameter of the extension method must be marked with the this keyword followed by the name of the type you want to extend. This tells the C# compiler that the extension method should be treated as a member of the extended type. You can then call the extension method on an instance of the extended type as if it were a regular member method. Here's an example of an extension method that adds a new method called Increment to the int type:
```
public static class IntExtensions
{
    public static int Increment(this int value)
    {
        return value + 1;
    }
}
```
```
int myInt = 5;
int incrementedInt = myInt.Increment();
Console.WriteLine(incrementedInt); // Outputs: 6
```

Extension Method for List Of Custom Class

namespace ConsoleAppTest
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            try
            {
                var customers = new List<Customer>();
                customers.Add(new Customer(25));
                customers.Add(new Customer(5));
                customers.Add(new Customer(15));

                customers = customers.AboveAge(10);
                foreach (var customer in customers)
                {
                    Console.WriteLine(customer.ToString());
                }
            }
            catch (Exception)
            {
                throw;
            }
            finally
            {
                Console.WriteLine("Press Any Key to Exit");
                Console.ReadLine();
            }
        }
    }

    internal class Customer
    {
        public int Age { get; set; }

        public Customer(int age)
        {
            Age = age;
        }

        public override string ToString()
        {
            return Age.ToString();
        }
    }
    internal static class CustomerHelper
    {
        public static List<Customer> AboveAge(this List<Customer> customers, int age)
        {
            return customers.FindAll(c => c.Age > age);
        }
    }
}