delegate vs event
What are the differences between delegate and an event?
An event declaration adds a layer of abstraction and protection on the delegate instance. This protection prevents clients of the delegate from resetting the delegate and its invocation list and only allows adding or removing targets from the invocation list.
To understand the differences you can look at 2 examples below:
Example with Delegate
First let's try to implement an "event trigger" by using "delegate" instead of "event".
in this case, the example delegate Run is an Action - that is a kind of delegate that doesn't return a value.
publicclassAnimal
{
publicActionRun{get;set;}
publicvoidRaiseEvent()
{
if(Run!=null)
{
Run();
}
}
}
To use the delegate, you should do something like this:
Animal animal=newAnimal();
animal.Run+=()=>Console.WriteLine("I'm running");
animal.Run+=()=>Console.WriteLine("I'm still running");
animal.RaiseEvent();
This code works well but you could have some weak spots:
For example, if I write this:
animal.Run+=()=>Console.WriteLine("I'm running");
animal.Run+=()=>Console.WriteLine("I'm still running");
animal.Run=()=>Console.WriteLine("I'm sleeping");
with the last line of code, I have overridden the previous behaviors just with one missing + (I have used = instead of +=)
Another weak spot is that every class which uses your Animal class can raise the Run event without calling the public RaiseEvent function, but with code snippet like:
if(animal.Run!=null)
{
animal.Run();
}
To avoid these weak spots you can use events in c#.
Example with Event
Your "event version" of the Animal class will looks like:
publicclassArgsSpecial:EventArgs
{
publicArgsSpecial(string val)
{
Operation=val;
}
publicstringOperation{get;set;}
}
publicclassAnimal
{
// Empty delegate. In this way you are sure that value is always != null
// because no one outside of the class can change it.
publiceventEventHandler<ArgsSpecial>Run=delegate{};
publicvoidRaiseEvent()
{
Run(this,newArgsSpecial("Run faster"));
}
}
to call events
Animal animal=newAnimal();
animal.Run+=(sender, e)=>Console.WriteLine("I'm running. My value is {0}", e.Operation);
animal.RaiseEvent();
Differences:
1. You aren't using a public property but a public field.
Using events, the compiler protects your fields from unwanted access
2. Event can't be assigned directly.
In this case, it is impossible to override the previous behaviors by using = instead of +=.
3. No one outside of your class can raise the event.
Even the Run event is public, a compiler error will occur if someone tries to raise the event with code snippet below:
// Error: the event 'delegateEvent.Animal.Run' can only appear on the left hand side of += or -=
// (except when used from within the type 'delegateEvent.Animal')
animal.Run(animal,newArgsSpecial("Run slower"));
4. Event can be included in an interface declaration, whereas a delegate field cannot.
delegate vs event的更多相关文章
- 浅谈c#中的delegate和event了
一.开篇忏悔 对自己最拿手的编程语言C#,我想对你说声对不起,因为我到现在为止才明白c#中的delegate和event是怎么用的,惭愧那.好了,那就趁着阳光明媚的早晨简单来谈谈delegate和ev ...
- 第一章、C#委托和事件(Delegate、Event、EventHandler、EventArgs)
第一章.C#委托和事件(Delegate.Event.EventHandler.EventArgs) 分类: 学习笔记-C#网络编程2012-12-08 14:10 7417人阅读 评论(3) 收藏 ...
- delegate和event
经过查阅资料和自己的理解整理出来的,欢迎大家指教. delegate和event 何时使用: 异步的时候,比如加载完成通知. 需要回调的时候,比如按钮点击.动画播放结束等. 发送事件通知的时候. 比如 ...
- .NET 中易混淆的概念(Delegate vs Event)
事件(event)是一个非常重要的概念,我们的程序时刻都在触发和接收着各种事件:鼠标点击事件,键盘事件,以及处理操作系统的各种事件.所谓事件就是 由某个对象发出的消息.比如用户按下了某个按钮,某个文件 ...
- 观察者模式与.NET的delegate、event机制
1.引言 最近在写一些程序玩的时候,接触到了delegate(委托)和event(事件),网上查找了很多的资料,有些博文说可以把delegate近似当做C++当中的函数指针来看,由于自己本身对C++的 ...
- [转] C#中的delegate 和 event
转至:here 终于会用c#中的delegate(委托) 作者:qq826364410 引言 Delegate是Dotnet1.0的时候已经存在的特性了,但由于在实际工作中一直没有机会使用Delega ...
- C#: Delegate and Event
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.T ...
- delegate 和 event
delegate 和 event 观察者模式 这里面综合了几本书的资料. 需求 有这么个项目: 需求是这样的: 一个气象站, 有三个传感器(温度, 湿度, 气压), 有一个WeatherData对象, ...
- [转载]C#委托和事件(Delegate、Event、EventHandler、EventArgs)
原文链接:http://blog.csdn.net/zwj7612356/article/details/8272520 14.1.委托 当要把方法作为实参传送给其他方法的形参时,形参需要使用委托.委 ...
- C#中Delegate和Event以及它们的区别(转载)
一.Delegate委托可以理解为一个方法签名. 可以将方法作为另外一个方法的参数带入其中进行运算.在C#中我们有三种方式去创建委托,分别如下: public delegate void Print( ...
随机推荐
- java web面试
1. session和cookie的区别和联系,session的生命周期,多个服务部署时session管理. 原博http://blog.csdn.net/shuaishenkkk/article/d ...
- javascript AOP
Function.prototype.bind = function(b) { var a = this; return function() { a.apply(b, arguments) } }; ...
- java 开发之linux 下 tomcat
tomcat作为java开发的服务器,可以部署在各种系统中. 官方下载地址:http://tomcat.apache.org/download-80.cgi 1. 装tomcat 之前,先要装jdk ...
- 读【10问PHP程序员】 有感
http://bbs.phpchina.com/thread-174331-1-1.html 看到前人的文章,总结自己的学习心得,颇有感悟,下面是自己的总结,平时就拿出来多问问自己.1.上了十几年的学 ...
- js模块化开发——AMD规范
这个系列的第一部分介绍了Javascript模块的基本写法,今天介绍如何规范地使用模块. 七.模块的规范 先想一想,为什么模块很重要? 因为有了模块,我们就可以更方便地使用别人的代码,想要什么功能,就 ...
- UltraISO制作linux启动盘(包含写入不完整解决方法)
网上教程挺多,主要是自己记录一下.
- ACE首页更改
@{ Layout = null; } <!DOCTYPE html> <html lang="zh-cn"> <head> <meta ...
- There is no getter for property named 'userId' in 'class java.lang.String'
[ERROR] 2017-01-18 04:37:06:231 cn.dataenergy.common.CenterHandlerExceptionResolver (CenterHandlerEx ...
- 微信扫码下载APP
前段时间开发过程中,要实现一个扫描二维码下载APP的功能,但是苹果系统中,微信不可以直接跳转苹果商店,需要先下载应用宝,显然太麻烦... 这样我们可以做个中间页,用中间页面生成二维码链接,在中间页代码 ...
- 对lua中_ENV表的理解(lua5.2版本以后)
当我拿到_ENV表的时候,会去想这个_ENV表是干什么用的? 首先看如下代码: print(_ENV) --0x1d005f0 print(_G) --0x1d005f0 ViewCode 看了上面的 ...