Challenge 3: A Safely-Guarded Alarm Event — Possible Solution
====================================================================

Alarm.cs:

class Alarm
{
    public event Action Triggered;

    public void Sound()
    {
        Triggered?.Invoke();   // safe even if nobody has subscribed
    }
}

Program.cs:

var alarm1 = new Alarm();
alarm1.Sound();   // no subscriber attached -- should NOT throw

var alarm2 = new Alarm();
alarm2.Triggered += () => Console.WriteLine("Alarm triggered!");
alarm2.Sound();   // subscriber attached -- runs it

Console.WriteLine("Both calls completed without throwing.");

Output:

    Alarm triggered!
    Both calls completed without throwing.

Explanation:
alarm1.Sound() calls Triggered?.Invoke() while Triggered is still
null (nobody has subscribed yet). The ?. operator short-circuits the
call entirely instead of invoking a null delegate, so nothing happens
and no exception is thrown. alarm2 has a real subscriber attached via
+=, so Triggered is no longer null, and Triggered?.Invoke() runs it
normally, printing the message. Without the ?. guard, alarm1.Sound()
would have thrown NullReferenceException instead.

WHY THIS WORKS AS AN ANSWER
------------------------------
This demonstrates the exact null-event gotcha the chapter's warn-box
describes, using ?.Invoke() to safely handle both the no-subscriber
and has-subscriber cases without ever throwing.