Question

There is code sample in the chapter about events in the book about c#:

class CountDown
{
    private uint _seconds;
    public CountDown(uint seconds)
    {
        _seconds = seconds;
    }
    public void Start()
    {
        new Thread(() =>
        {
            uint n = _seconds;
            while (n > 0u)
            {
                var tick = Tick;              ///??????
                if (tick != null)
                    tick(n);

                Thread.Sleep(1000);
                n--;
            }
            var finished = Finished;          ///??????
            if (finished != null)
                finished();
        }).Start();
    }
}
    public event Action<uint> Tick;
    public event Action Finished;

What is the reason for creating local copy of event (tick and finished) and raising event via it? Is it common practice and have some sense? I tried but couldn’t get it from the book.

Answer 1

What is the reason for creating local copy of event (tick and finished) and raising event via it?

It avoids the possibility of Tick becoming null after the nullity check but before the call. If you had:

if (Tick != null)
{
    Tick(n);
}

… and the final listener was unsubscribed when you’d already got into “if” body, you’d get a NullReferenceException.