Concrete Subject (NewsPublisher):?

Short answer: The NewsPublisher class is the concrete implementation of the subject. It maintains a list of observers and provides methods to subscribe, unsubscribe, and notify them when a new news article is available. public class NewsPublisher : INewsPublisher

Example code

{
private readonly List<IObserver> _observers = new
List<IObserver>();
public void Subscribe(IObserver observer) => _observers.Add(observer); public void Unsubscribe(IObserver observer) => _observers.Remove(observer); public void Notify(string news)
{
foreach (var observer in _observers)
{ observer.Update(news); }
}
}

Real-world example (ShopNest)

When order status changes, observers update email, SMS, and analytics without the Order class knowing each one.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.