//The Stock class provides an interface for attaching and detaching Investor objects, it knows its Investors.
//Any number of Investor objects may observe.
public class Stock
{
private string _symbol;
private double _price;
private List _investors = new List<IInvestor>();

public Stock(string symbol, double price)
{
_symbol = symbol;
_price = price;
}

public void Attach(IInvestor investor)
{
_investors.Add(investor);
}

public void Detach(IInvestor investor)
{
_investors.Remove(investor);
}

public void Notify()
{
foreach (IInvestor investor in _investors)
{
investor.Update(this);
}
}

public double Price
{
get { return _price; }
set
{
if (_price != value)
{
_price = value;
Notify();
}
}
}

public string Symbol
{
get { return _symbol; }
}
}

//The IBM class stores state of interest to Investor and sends a notification to its observers when its state changes.
public class IBM : Stock
{
public IBM(string symbol, double price) : base(symbol, price)
{
}
}