// "Subject"
abstract class Event
{
// Fields
protected string description;
private ArrayList receivers = new ArrayList();

// Constructor
public Event( string d ) {this.description = d; }

// Methods
public void Attach( Receiver receiver ) { receivers.Add( receiver ); }
public void Detach( Receiver receiver ) { receivers.Remove( receiver ); }
public void Notify()
{
  foreach( Receiver r in receivers )
    r.Notify( this );
}

// Properties
public string Description
{
  get{ return description; }
  set
  {
    description = value;
    Notify(); }
  }
}

//--------------------------------------------
// "ConcreteSubject"
class BirthDay : Event
{
// Constructor
public BirthDay( string descr )
  : base( descr )
    {}
}
//--------------------------------------------

// "Observer"
interface IReceiver
{
// Methods
void Notify( Event e );
}

//--------------------------------------------
// "ConcreteObserver"

class Receiver : IReceiver
{
// Fields
  private string name;
  private Event e;

// Constructors
public Receiver( string name ) { this.name = name; }

// Methods
public void Notify( Event e )
{
  Console.WriteLine( "Notified receiver {0} about {1} ", name, e.Description );
}

// Properties
public Event Event
{
  get{ return e; }
  set{ e = value; }
}
}
//--------------------------------------------

public class ObserverTest
{
public static void Main( string[] args )
{
// Create list of receivers
  Receiver r1 = new Receiver( "Jerry" );
  Receiver r2 = new Receiver( "Tom" );

// Create "Birth Day" event and attach the receivers
  BirthDay b1 = new BirthDay( "Bill's birth day." );
  b1.Attach( r1 );
  b1.Attach( r2 );

// Change "Birth Day" description, which notifies receivers
  b1.Description = "Sam's birth day.";
  b1.Description = "Mom's birth day.";

  Console.Read();
}
}