Observer Design Pattern

ObserverDesignPattern.cs

public enum ProductStatus 
{
    Approvaled,
    ExistInStock,
    NotExistInStock,
    SoldOut,
    Delivered
}

public class Product
{
    public int ID { get; set; }
    public string CreatedBy { get; set; }
    public DateTime CreatedDate { get; set; }
    public string UpdatedBy { get; set; }
    public DateTime UpdatedDate { get; set; }

    public long UnitPrice { get; set; }
    public int StockCount { get; set; }
    public ProductStatus Status{ get; set; }
}

public interface IProductObserver
{
    void Notify(Product product);
}

public class StakeHolderObserver
    : IProductObserver
{
    public void Notify(Product product)
    {
        if(product.StockCount < 10)
            Console.WriteLine(); //........ for example

        Console.WriteLine("Prodcut StockCount : {0} ", product.StockCount);
    }
}

public class ManagerObserver
    : IProductObserver
{
    public void Notify(Product product)
    {
        Console.WriteLine("Prodcut StockCount : {0} ", product.StockCount);
    }
}

public class CustomerObserver
    : IProductObserver
{
    public void Notify(Product product)
    {
        Console.WriteLine("Prodcut StockCount : {0} ", product.StockCount);
    }
}

public abstract class ProductBaseHandler
{
    protected readonly List<IProductObserver> _list = null;
    public ProductBaseHandler()
    {
        this._list = new List<IProductObserver>();
    }

    public abstract void Notify();

    public void Subscribe(IProductObserver observer) 
    {
        if (!_list.Contains(observer))
            _list.Add(observer);
    }

    public void UnSubscribe(IProductObserver observer) 
    {
        if (_list.Contains(observer))
            _list.Remove(observer);
    }
}

public class ProductChangeHandler
    : ProductBaseHandler
{
    private Product product;
    public Product Product
    {
        get { return product; }
        set
        {
            product = value;
            Notify();
        }
    }

    public override void Notify()
    {
        foreach (var item in _list)
            item.Notify(product);
    }
}

//using
class Program
{
    static void Main(string[] args)
    {
        var sbj = new ProductChangeHandler();
        sbj.Subscribe(new ManagerObserver());
        sbj.Subscribe(new StakeHolderObserver());
        sbj.Subscribe(new CustomerObserver());

        Product Original = new Product()
        {
            CreatedBy = "EMRE",
            CreatedDate = DateTime.Now,
            Status = ProductStatus.Delivered,
            ID = 1,
            StockCount = 15,
            UnitPrice = 23,
            UpdatedBy = "EMRE",
            UpdatedDate = DateTime.Now
        };

        sbj.Product = Original;

        // process for product object

        Original.Status = ProductStatus.NotExistInStock;
        Original.StockCount = 5;

        sbj.Product = Original;

        Console.ReadLine();
    }
}