As several people have probably already answered, interfaces can be used to enforce certain behaviors between classes that will not implement those behaviors the same way. So by implementing an interface you are saying that your class has the behavior of the interface. The IAnimal interface would not be a typical interface because Dog, Cat, Bird, etc. classes are types of animals, and should probably extend it, which is a case of inheritance. Instead, an interface would be more like animal behavior in this case, such as IRunnable, IFlyable, ITrainable, etc.

接口有很多好处，其中一个关键就是可插拔性。例如，声明一个具有List参数的方法将允许传入实现List接口的任何东西，允许开发人员在以后删除和插入不同的列表，而不必重写大量代码。

您可能永远不会使用接口，但如果您正在从头开始设计一个项目，特别是某种类型的框架，您可能会想要熟悉它们。

我建议大家阅读Coad、Mayfield和Kern撰写的《Java设计》中关于接口的章节。它们比一般的介绍性文本解释得好一点。如果你不使用Java，你可以只阅读本章的开头，主要是一些概念。

当您希望定义对象可以显示的行为时，通常会使用接口。

在. net世界中，一个很好的例子是IDisposable接口，它用于任何使用必须手动释放的系统资源的Microsoft类。它要求实现它的类具有Dispose()方法。

Dispose()方法也被VB的using语言构造调用。NET和c#，只适用于IDisposables)

请记住，您可以通过使用诸如TypeOf…之类的构造来检查对象是否实现了特定的接口。Is (VB.NET)， Is (c#)， instanceof (Java)，等等…

In my experience the driving force to create interfaces didn't occur until I start doing unit testing with a mocking framework. It became abundantly clear that using interfaces was going to make mocking much easier (since the framework depended on the methods being virtual). Once I started I saw the value of abstracting away the interface to my class from the implementation. Even if I don't create an actual interface, I try now to make my methods virtual (providing an implicit interface that can be overridden).

我发现还有许多其他原因可以加强重构到接口的良好实践，但是单元测试/模拟的事情提供了最初的“顿悟时刻”的实践经验。

EDIT: To clarify, with unit testing and mocking I always have two implementations -- the real, concrete implementation and an alternate mock implementation used in testing. Once you have two implementations, the value of the interface becomes obvious -- deal with it in terms of the interface so you can replace the implementation at any time. In this case I'm replacing it with a mock interface. I know that I can do this without an actual interface if my class is constructed properly, but using an actual interface reinforces this and makes it cleaner (clearer to the reader). Without this impetus, I don't think I would have appreciated the value of interfaces since most of my classes only, ever have a single concrete implementation.

扩展一下Larsenal所说的。接口是所有实现类都必须遵循的契约。因此，您可以使用一种称为契约编程的技术。这允许您的软件变得独立于实现。

一些非编程示例可能帮助您了解接口在编程中的适当使用。

There's an interface between electrical devices and the electricity network - it's the set of conventions about the shape of the plugs and sockets and the voltages/currents across them. If you want to implement a new electrical device, as long as your plug follows the rules it will be able to get services from the network. This makes extensibility very easy and removes or lowers the costs of coordination: you don't have to notify the electricity provider about how your new device works and come to a separate agreement about how to plug your new device into the network.

各国都有标准的铁路轨距。这使得铺设铁轨的工程公司和建造在这些铁轨上运行的列车的工程公司之间实现了分工，并使铁路公司有可能在不重新设计整个系统的情况下更换和升级列车。

The service a business presents to a client can be described as an interface: a well defined interface emphasises the service and hides the means. When you put a letter in a mailbox, you expect the postal system to deliver the letter within a given time but you have no expectations about how the letter is delivered: you don't need to know, and the postal service has the flexibility to choose the means of delivery that best meets the requirements and current circumstances. An exception to this is the ability of customers to choose airmail - that's not the kind of interface a modern computer programmer would have designed, since it reveals too much of the implementation.

来自自然的例子:我不太喜欢eats()， makesSound()， moves()等例子。它们确实描述了行为，这是正确的，但它们没有描述交互以及它们是如何被启用的。在自然界中，使相互作用成为可能的界面的一个明显的例子是与繁殖有关的，例如一朵花为蜜蜂提供了一个特定的界面，以便授粉能够发生。

它解决了一个具体的问题:

你有a b c d四种不同类型。在你的代码中，你可以这样写:

a.Process();
b.Process();
c.Process();
d.Process();

为什么不让他们实现IProcessable呢

List<IProcessable> list;

foreach(IProcessable p in list)
    p.Process();

当你添加50种类型的类，它们都做同样的事情时，这种伸缩性会更好。

另一个具体问题是:

你有没有看过System.Linq.Enumerable?它定义了大量的扩展方法，可以对实现IEnumerable的任何类型进行操作。因为任何实现IEnumerable的东西基本上都在说“我支持无序foreach类型模式中的迭代”，所以你可以为任何可枚举类型定义复杂的行为(Count、Max、Where、Select等)。