Program.cs

public class EventStream<T>
{
    private event Action<T> _eventStream;
    
    public IDisposable Subscribe(Action<T> observer)
    {
        _eventStream += observer;
        return new Disposable(() => _eventStream -= observer);
    }
    
    public void Publish(T data)
    {
        _eventStream?.Invoke(data);
    }
    
    private class Disposable : IDisposable
    {
        private readonly Action _disposeAction;
        
        public Disposable(Action disposeAction)
        {
            _disposeAction = disposeAction;
        }
        
        public void Dispose()
        {
            _disposeAction?.Invoke();
        }
    }
}

// Usage
var stream = new EventStream<string>();
var subscription = stream.Subscribe(msg => Console.WriteLine($"Received: {msg}"));
stream.Publish("Hello, Reactive World!");
subscription.Dispose();