我真的不明白接口存在的原因。据我所知,这是c#中不存在的多继承的一种工作(至少我是这么被告知的)。

我所看到的是,您预定义了一些成员和函数,然后必须在类中再次重新定义它们。从而使接口成为冗余。它只是感觉像句法……嗯,垃圾对我来说(请没有冒犯的意思。Junk是指无用的东西)。

在下面的例子中,我将创建一个名为Pizza的基类,而不是一个接口。

简单示例(取自不同的堆栈溢出贡献)

public interface IPizza
{
    public void Order();
}

public class PepperoniPizza : IPizza
{
    public void Order()
    {
        //Order Pepperoni pizza
    }
}

public class HawaiiPizza : IPizza
{
    public void Order()
    {
        //Order HawaiiPizza
    }
}

当前回答

接口的主要目的是在您和实现该接口的任何其他类之间建立契约,使您的代码解耦并允许可扩展性。

其他回答

我在这个页面上搜索了“合成”这个词,但一次也没看到。这个答案是对前面提到的答案的补充。

在面向对象项目中使用接口的一个绝对重要的原因是,它们允许你更倾向于组合而不是继承。通过实现接口,您可以将您的实现与应用于它们的各种算法解耦。

德里克·巴纳斯(Derek Banas)的“装饰图案”教程(有趣的是,它也以披萨为例)是一个有价值的插图:

https://www.youtube.com/watch?v=j40kRwSm4VE

考虑接口的最简单方法是认识继承的意义。如果类CC继承了类C,这意味着:

类CC可以使用类C的任何public或protected成员,就像它们是自己的一样,因此只需要实现父类中不存在的东西。 对CC的引用可以传递或分配给期望对C的引用的例程或变量。

遗传的这两个功能在某种意义上是相互独立的;虽然继承同时应用这两个,但也可以应用第二个而不应用第一个。这很有用,因为允许一个对象从两个或多个不相关的类继承成员要比允许一种类型可以替代多种类型复杂得多。

接口有点像抽象基类,但有一个关键的区别:继承基类的对象不能继承任何其他类。相反,一个对象可以实现一个接口,而不影响它继承任何所需类或实现任何其他接口的能力。

One nice feature of this (underutilized in the .net framework, IMHO) is that they make it possible to indicate declaratively the things an object can do. Some objects, for example, will want data-source object from which they can retrieve things by index (as is possible with a List), but they won't need to store anything there. Other routines will need a data-depository object where they can store things not by index (as with Collection.Add), but they won't need to read anything back. Some data types will allow access by index, but won't allow writing; others will allow writing, but won't allow access by index. Some, of course, will allow both.

If ReadableByIndex and Appendable were unrelated base classes, it would be impossible to define a type which could be passed both to things expecting a ReadableByIndex and things expecting an Appendable. One could try to mitigate this by having ReadableByIndex or Appendable derive from the other; the derived class would have to make available public members for both purposes, but warn that some public members might not actually work. Some of Microsoft's classes and interfaces do that, but that's rather icky. A cleaner approach is to have interfaces for the different purposes, and then have objects implement interfaces for the things they can actually do. If one had an interface IReadableByIndex and another interface IAppendable, classes which could do one or the other could implement the appropriate interfaces for the things they can do.

类比简单解释

无接口(例1):

无接口(例2):

有接口:

需要解决的问题:多态性的目的是什么?

比方说,我是一个建筑工地的领班。我不知道哪个商人会走进来。但我告诉他们该怎么做。

如果是木匠,我会说:搭建木质脚手架。 如果是管道工,我会说:安装管道 如果是人民党政府官员,我说,三袋现金,先生。

上述方法的问题在于:(1)我必须知道谁会走进那扇门,并且根据是谁,我必须告诉他们该做什么。这通常会使代码更难维护或更容易出错。

知道该做什么的含义:

This means if the carpenter's code changes from: BuildScaffolding() to BuildScaffold() (i.e. a slight name change) then I will have to also change the calling class (i.e. the Foreperson class) as well - you'll have to make two changes to the code instead of (basically) just one. With polymorphism you (basically) only need to make one change to achieve the same result. Secondly you won't have to constantly ask: who are you? ok do this...who are you? ok do that.....polymorphism - it DRYs that code, and is very effective in certain situations: with polymorphism you can easily add additional classes of tradespeople without changing any existing code. (i.e. the second of the SOLID design principles: Open-close principle).

解决方案

想象一下这样一个场景:无论谁走进来,我都可以说:“Work()”,他们做着自己擅长的工作:管道工处理管道,电工处理电线,官僚可能专门负责受贿,为其他人做双倍的工作。

这种方法的好处是:(i)我不需要确切地知道谁会走进那扇门——我只需要知道他们是一种手工工人,他们会工作,其次,(ii)我不需要知道任何关于特定行业的信息。手工工人会处理的。

所以不要这样:

if(electrician) then  electrician.FixCablesAndElectricity() 

if(plumber) then plumber.IncreaseWaterPressureAndFixLeaks() 

if(keralaCustoms) then keralaCustoms.askForBribes() 

我可以这样做:

ITradesman tradie = Tradesman.Factory(); // in reality i know it's a plumber, but in the real world you won't know who's on the other side of the tradie assignment.

tradie.Work(); // and then tradie will do the work of a plumber, or electrician etc. depending on what type of tradesman he is. The foreman doesn't need to know anything, apart from telling the anonymous tradie to get to Work()!!

有什么好处?

这样做的好处是,如果木匠等特定的工作要求发生了变化,那么领班就不需要改变他的代码——他不需要知道或关心。重要的是木匠知道Work()是什么意思。其次,如果一种新型的建筑工人来到工地上,那么工头不需要知道任何关于贸易的事情——工头所关心的是建筑工人(.e。焊工、上釉工、瓦工等)可以完成一些工作。

总结

界面允许您让人完成分配给他们的工作,而不需要您确切地知道他们是谁或他们可以做什么。这允许您轻松地添加新的(交易)类型,而无需更改现有的代码(从技术上讲,您确实更改了一点点),这是面向对象方法相对于更函数式编程方法的真正好处。

如果你不理解上面的任何内容,或者不清楚,请在评论中提问,我会尽量让答案更好。

接口定义了某种功能的提供者和相应的使用者之间的契约。它将实现与契约(接口)解耦。您应该了解一下面向对象的体系结构和设计。你可能想从维基百科开始:http://en.wikipedia.org/wiki/Interface_(computing)

接口实际上是实现类必须遵循的契约,它实际上是我所知道的几乎所有设计模式的基础。

在您的示例中,创建接口是因为这样就保证实现了比萨饼接口的is A Pizza

public void Order();

在你提到的代码之后,你可以这样写:

public void orderMyPizza(IPizza myPizza) {
//This will always work, because everyone MUST implement order
      myPizza.order();
}

这样您使用的是多态性,您所关心的只是对象对order()做出响应。