1     public class CreditCard : EventArgs
 2     {
 3         private float _spendAmount;
 4         public event EventHandler<CreditCard> SpendMoney;
 5 
 6         public float SpendAmount
 7         {
 8             get
 9             {
10                 return _spendAmount;
11             }
12             set
13             {
14                 _spendAmount = value;
15                 Notify();
16             }
17         }
18 
19         private void Notify()
20         {
21             if (SpendMoney != null)
22             {
23                 SpendMoney(this, this);
24             }
25         }
26     }

 1    public interface IObserver<T>
 2     {
 3         void Update(Object sender, T e);
 4     }
 5 
 6     public class SMSNotify : IObserver<CreditCard>
 7     {
 8         public void Update(Object sender, CreditCard e)
 9         {
10             Console.WriteLine("Sms notify.Spend {0}", e.SpendAmount);
11         }
12     }
13 
14     public class Account : IObserver<CreditCard>
15     {
16         private float _accountAmount;
17 
18         public Account(float accountAmount)
19         {
20             _accountAmount = accountAmount;
21         }
22 
23         public void Update(Object sender, CreditCard e)
24         {
25             _accountAmount += e.SpendAmount;
26             Console.WriteLine("Account amount is {0}", _accountAmount);
27         }
28     }

1     static void Main(string[] args)
2     {
3         CreditCard creditCard = new CreditCard();
4         SMSNotify sms = new SMSNotify();
5         Account account = new Account(1000);
6         creditCard.SpendMoney += account.Update;
7         creditCard.SpendMoney += sms.Update;
8         creditCard.SpendAmount = 200;
9     }