Editing Observer pattern (section)

===C#===
C# provides the {{Code|IObservable}}.<ref>{{cite web |title=IObservable Interface (System) |url=https://learn.microsoft.com/en-us/dotnet/api/system.iobservable-1?view=net-8.0 |website=learn.microsoft.com |access-date=9 November 2024 |language=en-us}}</ref> and {{Code|IObserver}}<ref>{{cite web |title=IObserver Interface (System) |url=https://learn.microsoft.com/en-us/dotnet/api/system.iobserver-1?view=net-8.0 |website=learn.microsoft.com |access-date=9 November 2024 |language=en-us}}</ref> interfaces as well as documentation on how to implement the design pattern.<ref>{{cite web |title=Observer design pattern - .NET |url=https://learn.microsoft.com/en-us/dotnet/standard/events/observer-design-pattern |website=learn.microsoft.com |access-date=9 November 2024 |language=en-us |date=25 May 2023}}</ref>
<syntaxhighlight lang="csharp">
class Payload
{
    internal string Message { get; set; }
}

class Subject : IObservable<Payload>
{
    private readonly ICollection<IObserver<Payload>> _observers = new List<IObserver<Payload>>();

    IDisposable IObservable<Payload>.Subscribe(IObserver<Payload> observer)
    {         
        if (!_observers.Contains(observer))
        {
            _observers.Add(observer);
        }

        return new Unsubscriber(observer, _observers);
    }

    internal void SendMessage(string message)
    {
        foreach (var observer in _observers)
        {
            observer.OnNext(new Payload { Message = message });
        }
    }
}

internal class Unsubscriber : IDisposable
{
    private readonly IObserver<Payload> _observer;
    private readonly ICollection<IObserver<Payload>> _observers;

    internal Unsubscriber(
        IObserver<Payload> observer,
        ICollection<IObserver<Payload>> observers)
    {
        _observer = observer;
        _observers = observers;
    }

    void IDisposable.Dispose()
    {
        if (_observer != null && _observers.Contains(_observer))
        {
            _observers.Remove(_observer);
        }
    }
}

internal class Observer : IObserver<Payload>
{
    internal string Message { get; set; }

    public void OnCompleted()
    {
    }

    public void OnError(Exception error)
    {
    }

    public void OnNext(Payload value)
    {
        Message = value.Message;
    }

    internal IDisposable Register(IObservable<Payload> subject)
    {
        return subject.Subscribe(this);
    }
}
</syntaxhighlight>