ObserverDesignPattern.cs

using System;
using System.Collections.Generic;

namespace ObserverDesignPattern
{
    public class Program 
    {
        public static void Main()
        {
            // our two users
            User firstUser = new User("First");
            User secondUser = new User("Second");
            
            // our channel
            YoutubeChannel learnDotnet = new YoutubeChannel();
            
            // Both user subscribe channed
            learnDotnet.Subscribe(firstUser);
            learnDotnet.Subscribe(secondUser);
            
            // uploaded new video
            learnDotnet.UploadNewVideo();
            
            Console.WriteLine("=============================================");
            
            //second user is no more intrested so he unscribe. 
            learnDotnet.UnSubscribe(secondUser);
            
            // uploads new video but secondUser wont be notified as he is no more subscriber
            learnDotnet.UploadNewVideo();

        }
    }
    
    public interface ISubject {}
    
    public interface IObserver
    {
        void Notify();
    }
    
    public class YoutubeChannel : ISubject
    {
        private List<IObserver> subscribersList = new List<IObserver>();
        
        public void Subscribe(IObserver observer) => subscribersList.Add(observer);
        
        public void UnSubscribe(IObserver observer) => subscribersList.Remove(observer);
        
        public void UploadNewVideo() 
        {
            Console.WriteLine("New video has been uploaded");
            foreach(var o in subscribersList) 
            {
                o.Notify();
            }
        }
    }
    
    public class User : IObserver
    {
        private string _name { get; set; }
        public User(string username)
        {
            _name = username;
        }
    
        public void Notify()
        {
            Console.WriteLine($"User {_name} has been notified for new video"); 
        }
    }

}

​ObserverDesignPattern.output.txt

New video has been uploaded
User First has been notified for new video
User Second has been notified for new video
=============================================
New video has been uploaded
User First has been notified for new video

​​.sharplab.json

{
    "version": 1,
    "target": "Run",
    "mode": "Debug"
}