我们可以通过引用每个观察者拥有的Subject，从Observer* a通知Subject * a。
让我通过C#展示一个例子。这是我们的抽象：

public interface IMyObserver
{
    public void Update(string myMessage);

    public void NotifySubject(string message);
}

public interface IMySubject
{
    public void RegisterObserver(IMyObserver o);

    public void RemoveObserver(IMyObserver o);

    public void NotifyObservers();

    public void NotifyMe(string messageFromObserver);
}

及其MyObserver的具体实现：

public class MyObserver : IMyObserver
{
    private MySubject _mySubject;
    private string _myMessage;

    public MyObserver(MySubject mySubject)
    {
        _mySubject = mySubject;
        _mySubject.RegisterObserver(this);
    }

    public void NotifySubject(string message)
    {
        _mySubject.NotifyMe(message);
    }

    public void Update(string myMessage)
    {
        _myMessage = myMessage;
        Console.WriteLine($"Observer have seen this message: {_myMessage}");
    }
}

MySubject看起来像这样：

public class MySubject : IMySubject
{
    private List<IMyObserver> _observers;
    private string _myMessage;
    private string _messageFromObserver;

    public MySubject() => _observers = new List<IMyObserver>();

    public void NotifyObservers()
    {
        foreach (IMyObserver observer in _observers)
            observer.Update(_myMessage);
    }

    public void RegisterObserver(IMyObserver o) => _observers.Add(o);

    public void RemoveObserver(IMyObserver o) => _observers.Remove(o);

    public void MyMessageChanged() => NotifyObservers();

    public void SetMessage(string message)
    {
        _myMessage = message;
        MyMessageChanged();
    }

    public string GetMessage() => _myMessage;

    public void NotifyMe(string messageFromObserver)
    {
        _messageFromObserver = messageFromObserver;
        Console.WriteLine($"Subject received this message from observer: 
            {_messageFromObserver}");
    }
}

上面的代码可以这样调用：

MySubject mySubject = new ();
MyObserver myObserver = new (mySubject);

// message from subject
mySubject.SetMessage("Hello World!"); // OUTPUT: Observer have seen this message: 
    // Hello World!

// message from observer to subject
myObserver.NotifySubject("We are notifying subject"); ; // OUTPUT: Subject 
    // received this message from observer: We are notifying subject