อีเวนต์

using System;

class Counter
{
    public event EventHandler? ThresholdReached;
    public void ReachThreshold() => ThresholdReached?.Invoke(this, EventArgs.Empty);
}

class Program
{
    static void Main()
    {
        var counter = new Counter();
        counter.ThresholdReached += (_, _) => Console.WriteLine("Threshold reached");
        counter.ReachThreshold();
    }
}