Very simple observer pattern

IObserver.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Patterns.Observer
{
    public interface IObserver
    {
        void Update();
    }
}

Observable.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Patterns.Observer
{
    public abstract class Observable
    {
        private IList<IObserver> _observers;


        public void AddObserver(IObserver observer)
        {
            _observers = _observers ?? new List<IObserver>();
            _observers.Add(observer);
        }


        public void RemoveObserver(IObserver observer)
        {
            _observers = _observers ?? new List<IObserver>();
            if (_observers.Contains(observer))
                _observers.Remove(observer);
        }


        public void NotifyObservers()
        {
            _observers = _observers ?? new List<IObserver>();
            foreach (var observer in _observers)
                observer.Update();
        }
    }
}