I like Jimmy's answer a lot, but I feel I need to add something to it. The key to the whole thing is the "able" in IProcessable . It indicates a capability (or property, but meaning "intrinsic quality", not in the sense of C# properties) of the object that implements the interface. IAnimal is probably not a good example for an interface, but IWalkable might be a good interface to have if your system has many things that can walk. You might have classes derived from Animal such as Dog, Cow, Fish, Snake. The first two would probably implement IWalkable, the latter two don't walk, so they wouldn't. Now you ask "why not just have another superclass, WalkingAnimal, that Dog and Cow derive from?". The answer is when you have something completely outside the inheritance tree that also can walk, such as a robot. Robot would implement IWalkable, but probably wouldn't derive from Animal. If you want a list of things that can walk, you type it as IWalkable and you can put all walking animals plus robots in the list.

现在，将IWalkable替换为IPersistable这样更像软件的东西，这样的类比就更接近你在实际程序中看到的情况了。

当相同功能的实现不同时使用接口。

当你需要共享一个公共的具体实现时，使用一个抽象/基类。

当您希望能够对多个类型使用单个变量，但所有这些类型通过接口声明实现相同的方法时，Jimmy是正确的。然后你可以在接口类型变量上调用它们的main方法。

然而，使用接口还有第二个原因。当项目架构师和实现编码员是不同的人时，或者有几个实现编码员和一个项目经理。负责人可以编写一大堆接口，并查看系统的互操作，然后让开发人员用实现类填充接口。这是确保多人编写兼容类的最佳方法，而且他们可以并行地完成。

使用接口有很多目的。

Use in polymorphic behavior. Where you want to call specific methods of a child class with an inteface having a reference to the child class. Having a contract with classes to implement all of the methods where it is necessary, like most common use is with COM objects , where a wrapper class is generated on a DLL which inherits the interface; these methods are called behind the scenes, and you just need to implement them but with the same structure as defined in the COM DLL which you can only know through the interface that they expose. To reduce memory usage by loading specific methods in a class. Like if you have three business objects and they are implemented in a single class, you can use three interfaces.

例如IUser, IOrder, IOrderItem

public interface IUser()
{

void AddUser(string name ,string fname);

}

// Same for IOrder and IOrderItem
//


public class  BusinessLayer: IUser, IOrder, IOrderItem

{    
    public void AddUser(string name ,string fname)
    {
        // Do stuffs here.
    }

    // All methods from all interfaces must be implemented.

}

如果你只想添加一个用户，可以这样做:

IUser user = new (IUser)BusinessLayer();

// It will load  all methods into memory which are declared in the IUser interface.

user.AddUser();

I like Jimmy's answer a lot, but I feel I need to add something to it. The key to the whole thing is the "able" in IProcessable . It indicates a capability (or property, but meaning "intrinsic quality", not in the sense of C# properties) of the object that implements the interface. IAnimal is probably not a good example for an interface, but IWalkable might be a good interface to have if your system has many things that can walk. You might have classes derived from Animal such as Dog, Cow, Fish, Snake. The first two would probably implement IWalkable, the latter two don't walk, so they wouldn't. Now you ask "why not just have another superclass, WalkingAnimal, that Dog and Cow derive from?". The answer is when you have something completely outside the inheritance tree that also can walk, such as a robot. Robot would implement IWalkable, but probably wouldn't derive from Animal. If you want a list of things that can walk, you type it as IWalkable and you can put all walking animals plus robots in the list.

现在，将IWalkable替换为IPersistable这样更像软件的东西，这样的类比就更接近你在实际程序中看到的情况了。

假设你想要模拟当你试图睡觉时可能发生的烦恼。

接口前的模型

class Mosquito {
    void flyAroundYourHead(){}
}

class Neighbour{
    void startScreaming(){}
}

class LampJustOutsideYourWindow(){
    void shineJustThroughYourWindow() {}
}

正如你清楚地看到的，当你试图睡觉时，许多“事情”都可能令人讨厌。

使用没有接口的类

但是在使用这些类时，我们遇到了一个问题。他们毫无共同之处。您必须分别调用每个方法。

class TestAnnoyingThings{
    void testAnnoyingThinks(Mosquito mosquito, Neighbour neighbour, LampJustOutsideYourWindow lamp){
         if(mosquito != null){
             mosquito.flyAroundYourHead();
         }
         if(neighbour!= null){
             neighbour.startScreaming();
         }
         if(lamp!= null){
             lamp.shineJustThroughYourWindow();
         }
    }
}

带有接口的模型

为了克服这个问题，我们可以引入一个iterface

interface Annoying{
   public void annoy();

}

并在类中实现它

class Mosquito implements Annoying {
    void flyAroundYourHead(){}

    void annoy(){
        flyAroundYourHead();
    }
}

class Neighbour implements Annoying{
    void startScreaming(){}

    void annoy(){
        startScreaming();
    }
}

class LampJustOutsideYourWindow implements Annoying{
    void shineJustThroughYourWindow() {}

    void annoy(){
        shineJustThroughYourWindow();
    }
}

接口使用

这将使这些类的使用更容易

class TestAnnoyingThings{
    void testAnnoyingThinks(Annoying annoying){
        annoying.annoy();
    }
}