How does delegate work? Explanation via C#. Example based on abstract user account in bank.

delegate_1.cs

using System;

namespace MyApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create a new account instance with initial amount.
            Account acc = new Account(500);
            // Register delegate to obtain internal operation messages.
            acc.RegisterStatService(MessageHandler.Show);

            // Now amount equals 900.
            acc.Put(400);
            // Now amount equals 900.
            acc.Withdraw(700);
            // Now amount equals 200.
            acc.Withdraw(200);
            // Will be refused.
            acc.Withdraw(100);

            Console.Read();

        }

    }

    /// <summary>
    /// Main class to work with user account in bank.
    /// </summary>
    public class Account
    {
        private int _sum;
        public Account(int sum = 200)
        {
            _sum = sum;
        }
        public delegate void AccountStats(string message);

        private AccountStats del;

        public void RegisterStatService(AccountStats _del)
        {
            del = _del;
        }

        public void Withdraw(int num)
        {
            if (num <= _sum)
            {
                del("Amount " + num + "$ was withdrawn.");
                _sum -= num;
            }
            else
            {
                del("An attempt to withdraw "+ num +" $ was refused. Reason: Insufficient funds.");
            }
        }

        public void Put(int num)
        {
            _sum += num;
            del("You have put " + num + "$ into your account.");
        }
    }

    /// <summary>
    /// A special class to manage messages.
    /// </summary>
    public static class MessageHandler
    {
        public static void Show(string message)
        {
            Console.WriteLine(message);
        }
    }

}