Visual Studio EventHandler Delegate 和 EventArgs

EventHandler代理 用来表示处理一个没有事件数据（event data）的事件（event）的 方法。

无论何时事件发生时，事件代理就被调用来触发以前事件驱动的其他事件（监听当前事件TCurrentEvent += TListenerEvent）。

public delegate void EventHandler(
	Object sender,    //事件发起者
	EventArgs e       //处理事件数据的对象
)

EventArgs 类是事件变量的基类，提供事件数据类型给某个事件。

System.Object 
  System.EventArgs
    More...

public class EventArgs  

The following example shows an event named ThresholdReached that is associated with an EventHandler delegate. The method assigned to the EventHandler delegate is called in the OnThresholdReached method.

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

namespace delegrate
{
    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter(new Random().Next(10));
            c.ThresholdReached += c_ThresholdReached;  //声明一个事件监听

            Console.WriteLine("press 'a' key to increase total");
            while (Console.ReadKey(true).KeyChar == 'a')
            {
                Console.WriteLine("adding one");  
                c.Add(1);    
            }
        }

        static void c_ThresholdReached(object sender, ThresholdReachedEventArgs e)
        {
            Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached);
            Environment.Exit(0);
        }
    }

    class Counter
    {
        private int threshold;
        private int total;

        public Counter(int passedThreshold)
        {
            threshold = passedThreshold;
        }

        public void Add(int x)
        {
            total += x;
            if (total >= threshold)
            {
                ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
                args.Threshold = threshold;
                args.TimeReached = DateTime.Now;
                OnThresholdReached(args);          //传输一个 TEventArgs-->ThresholdReachedEventArgs
            }
        }

        protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
        {
            EventHandler<ThresholdReachedEventArgs> handler = ThresholdReached;  //handle an Event
            if (handler != null)
            {
                handler(this, e);   //触发 c_ThresholdReached
            }
        }

        public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;
    }
       
    public class ThresholdReachedEventArgs : EventArgs
    {
        public int Threshold { get; set; }
        public DateTime TimeReached { get; set; }
    }
}