ObserverPatternWithNetFramework.cs

//interface the all observer classes should implement
public interface IObserver<TObject>
{
    void Notify(TObject obj);
}

//interface that all observable classes should implement
public interface IObservable<TObject>
{
    void Register(IObserver<TObject> observer);
    void UnRegister(IObserver<TObject> observer);
    void Notify(TObject obj);
}

public class ObservableImpl<TObject>
    : IObservable<TObject>
{
    protected readonly List<IObserver<TObject>> _list = null;
    public ObservableImpl()
    {
        _list = new List<IObserver<TObject>>();
    }

    public void Register(IObserver<TObject> observer)
    {
        if (!_list.Contains(observer))
            _list.Add(observer);
    }

    public void UnRegister(IObserver<TObject> observer)
    {
        if (_list.Contains(observer))
            _list.Remove(observer);
    }

    public void Notify(TObject obj)
    {
        foreach (IObserver<TObject> item in _list)
            item.Notify(obj);
    }
}

public class Stock
    :ObservableImpl<long>
{
    private long price;
    public long Price
    {
        get { return price; }
        set 
        { 
            price = value;
            base.Notify(price);
        }
    }
}

public class StockMonitor
    : IObserver<long>
{
    public void Notify(long obj)
    {
        Console.WriteLine(obj);
    }
}

// using structure
public class MainClass
{
    public static void Main()
    {
        StockMonitor stockDisplay = new StockMonitor();
        Stock stock = new Stock();

        stock.Register(stockDisplay);

        //loop 100 times and modify the ask price
        for (int looper = 0; looper < 100; looper++)
        {
            stock.Price = looper;
            if (looper == 10) stock.UnRegister(stockDisplay); // for example
        }

        //unregister the display
        stock.UnRegister(stockDisplay);

        Console.ReadLine();
    }
}