Events: many listeners, one broadcast

From one callback to many

A Func parameter holds exactly one method. The event pattern removes that limit:

class Thermometer
{
    public event Action<int>? Boiling;   // subscribers collect here

    public void Check(int celsius)
    {
        if (celsius >= 100)
            Boiling?.Invoke(celsius);    // broadcast: all subscribers run
    }
}

var t = new Thermometer();
t.Boiling += c => Console.WriteLine($"alarm: {c}");
t.Boiling += c => Stats.Record(c);
t.Check(101);   // both run

+= subscribes, -= unsubscribes. ?.Invoke broadcasts only when someone listens — null when the list is empty.

The contract

The publisher defines the event's delegate type; subscribers conform. Outside the class, an event is subscribe-only — you can't invoke or clear someone else's event. That's the difference from a plain delegate field: encapsulation on the invocation, which is what makes pub/sub safe at scale.

Unsubscribing matters too: a subscriber the publisher outlives leaks memory until -= runs.