CODESAMPLE

Observer - C#

Observer C_sharp

The Observer pattern defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. Here, an ISubject interface declares the Attach, Detach, and Notify methods for managing observers. A concrete Subject publishes events containing data. Multiple Observer classes ConcreteObserverA and ConcreteObserverB subscribe to these events and respond to changes in the subject’s state. This implementation uses C#’s events and delegates for a type-safe and concise way to manage the observer relationships, fitting the language’s event-driven programming style.

// ISubject interface
public interface ISubject
{
    void Attach(IObserver observer);
    void Detach(IObserver observer);
    void Notify();
}

// Observer interface
public interface IObserver
{
    void Update(string data);
}

// Subject class
public class Subject : ISubject
{
    private readonly List<IObserver> _observers = new();
    private string _state;

    public string State
    {
        get { return _state; }
        set
        {
            _state = value;
            Notify();
        }
    }

    public void Attach(IObserver observer)
    {
        _observers.Add(observer);
    }

    public void Detach(IObserver observer)
    {
        _observers.Remove(observer);
    }

    public void Notify()
    {
        foreach (var observer in _observers)
        {
            observer.Update(_state);
        }
    }
}

// Concrete Observer A
public class ConcreteObserverA : IObserver
{
    public void Update(string data)
    {
        Console.WriteLine($"Observer A: Received update - {data}");
    }
}

// Concrete Observer B
public class ConcreteObserverB : IObserver
{
    public void Update(string data)
    {
        Console.WriteLine($"Observer B: Received update - {data.ToUpper()}");
    }
}

// Usage
public class Example
{
    public static void Main(string[] args)
    {
        var subject = new Subject();
        var observerA = new ConcreteObserverA();
        var observerB = new ConcreteObserverB();

        subject.Attach(observerA);
        subject.Attach(observerB);

        subject.State = "Initial state";
        subject.State = "Another state";

        subject.Detach(observerA);

        subject.State = "Final state";
    }
}