extension-methods.cs

public class Human
    {
        public string Name { get; set; }
        public DateTime DateTimeOfBirth { get; set; }
    }

    public class NewPerson : Human
    {
        public int CalculateAge()
        {
            return 1;
        }
    }


    public static class HumanExtensions
    {
        public static int CalculateAge(this Human human)
        {
            return 7331;
        }
    }


    public static class StringExtensions
    {
        public static string HelloWorld(this string str)
        {
            return "Hello World";
        }
    }

    public static class HumanHelper
    {
        public static int CalculateAge(Human human)
        {
            return 1337;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Human person = new Human() { Name = "Ellie", DateTimeOfBirth = DateTime.Parse("2019-12-07 01:37") };

            string name = "Robert";
            Console.WriteLine(name.HelloWorld());


            Console.WriteLine("Age: " + person.CalculateAge());
            Console.WriteLine("Age: " + HumanHelper.CalculateAge(person));

            List<string> names = new List<string>() { "Robert", "Mayer" };


            Console.ReadLine();