Design Pattern

using System;
namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            A a = new B();
            a.Say();
            a.SetAction(new ActionB());
            a.Say();
            a = new C();
            a.Say();
            Console.Read();

        }
    }

    public interface TheAction
    {
        void Say();
    }
    public abstract class A
    {
        public TheAction theAction;
        public void Say()
        {
            theAction.Say();
        }
        public void SetAction(TheAction a)
        {
            theAction = a;
        }
    }
    public class B : A
    {
        public B()
        {
            theAction = new ActionA();
        }
    }
    public class C : A
    {
        public C()
        {
            theAction = new ActionB();
        }
    }
    public class ActionA : TheAction
    {
        public void Say()
        {
            Console.WriteLine("Action A");
        }
    }
    public class ActionB : TheAction
    {
        public void Say()
        {
            Console.WriteLine("Action B");
        }
    }
}

using System;
using System.Collections.Generic;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Observer o = new Observer();
            Subject s = new Subject(o);
            o.SetA("Hello World");
            o.SetA("Have a good day");
            Console.Read();
        }
    }

    public class Observer
    {
        public List<Subject> subjects;
        public Observer()
        {
            subjects = new List<Subject>();
        }
        public string A { get; set; }
        public void Regist(Subject subject)
        {
            subjects.Add(subject);
        }
        public void UnRegist(Subject subject)
        {
            subjects.Remove(subject);
        }
        public void notifyObservers()
        {
            foreach (var item in subjects)
            {
                item.Update(A);
            }
        }
        public void SetA(string a)
        {
            A = a;
            notifyObservers();
        }
    }
    public class Subject
    {
        public Observer observer;
        public Subject(Observer o)
        {
            observer = o;
            observer.Regist(this);
        }
        public string A { get; set; }
        public void Update(string a) { A = a;Display(); }
        public void Display()
        {
            Console.WriteLine(A);
        }
    }
    
}