//The IInvestor interface defines an updating interface for objects that should be notified of changes in an Stock's status.
public interface IInvestor
{
void Update(Stock stock);
}

//The Investor class maintains a reference to a Stock object, stores state that should stay consistent with the Stock's state
//and implements the IInvestor updating interface to keep its state consistent with the Stock's state.
public class Investor : IInvestor
{
private string _name;
private Stock _stock;

public Investor(string name)
{
_name = name;
}

public void Update(Stock stock)
{
Console.WriteLine("Notified {0} of {1}'s change to {2:C}", _name, stock.Symbol, stock.Price);
}

public Stock Stock
{
get { return _stock; }
set { _stock = value; }
}
}