代码1：

 class Program
    {
        static void Main(string[] args)
        {
            MessageProducer messageProducer = new MessageProducer();

            Subscriber zhangsan = new Subscriber("张三");
            Subscriber lisi = new Subscriber("李四");
            Subscriber wangwu = new Subscriber("王五");

            messageProducer.Subscribe(zhangsan);
            messageProducer.Subscribe(lisi);
            messageProducer.Subscribe(wangwu);

            messageProducer.Produce("hello");

            messageProducer.Unsubscribe(lisi);

            messageProducer.Produce("hello again");
        }
    }

    public delegate void MessageProducerHandler(string message);
    public class MessageProducer
    {
        public event MessageProducerHandler MessageHandle;

        /// <summary>
        /// 订阅
        /// </summary>
        /// <param name="subscriber"></param>
        public void Subscribe(Subscriber subscriber)
        {
            Console.WriteLine($"【{subscriber.Name}】订阅了消息");
            MessageHandle += subscriber.OnMessage;
        }
        /// <summary>
        /// 取消订阅
        /// </summary>
        /// <param name="subscriber"></param>
        public void Unsubscribe(Subscriber subscriber)
        {
            Console.WriteLine($"【{subscriber.Name}】取消了消息订阅");
            MessageHandle -= subscriber.OnMessage;
        }

        /// <summary>
        /// 发布一条消息
        /// </summary>
        /// <param name="message"></param>
        public void Produce(string message)
        {
            Console.WriteLine($"发布消息【{message}】");
            MessageHandle?.Invoke(message);
        }
    }

    public class Subscriber
    {
        public Subscriber(string name)
        {
            Name = name;
        }

        /// <summary>
        /// 订阅者名称
        /// </summary>
        public string Name { get; }

        /// <summary>
        /// 消息处理方法
        /// </summary>
        /// <param name="message"></param>
        public void OnMessage(string message)
        {
            Console.WriteLine($"【{Name}】收到消息【{message}】");
        }
    }

代码2:

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

namespace EventDemo
{
    public delegate void MsgHandler(string msg);
    public class PinDuoDuo
    {
        public event MsgHandler msgHandler;
        public void Order(User user)
        {
            Console.WriteLine("{0}已拼单", user.Name);
            msgHandler += user.OnMessage;
        }
        public void UnOrder(User user)
        {
            Console.WriteLine("{0}取消拼单", user.Name);
            msgHandler -= user.OnMessage;
        }
        public void ShipGoods(string msg)
        {
            Console.WriteLine("发货");
            msgHandler?.Invoke(msg);
        }
    }
    public class User
    {
        public User(string name)
        {
            Name = name;
        }
        public string Name { get; set; }
        public void OnMessage(string msg)
        {
            Console.WriteLine("{0}收到消息:{1}", Name, msg);
        }
    }
    public class Program
    { 
        public static void Main()
        {
            PinDuoDuo pdd = new PinDuoDuo();
            User zs = new User("张三");
            User lisi = new User("李四");
            User wmz = new User("王麻子");
            pdd.Order(zs);
            pdd.Order(lisi);
            pdd.Order(wmz);
            pdd.UnOrder(lisi);
            pdd.ShipGoods("货已发出");
        }
    }
}