Head First C# Code: Chapter 11
Baseball with Callback
Fan.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace _1_Baseball___with_callback
{
    public class Fan
    {
        public Fan(Ball ball)
        {
            ball.BallInPlay += new EventHandler(ball_BallInPlay);
        }

        void ball_BallInPlay(object sender, EventArgs e)
        {
            if (e is BallEventArgs)
            {
                BallEventArgs ballEventArgs = e as BallEventArgs;
                if (ballEventArgs.Distance > 400 && ballEventArgs.Trajectory > 30)
                    Console.WriteLine("Fan: Home run! I’m going for the ball!");
                else
                    Console.WriteLine("Fan: Woo-hoo! Yeah!");

            }
        }
    }
}