为什么 C# 接口不能包含字段? [英] Why can't C# interfaces contain fields?

问题描述

例如，假设我想要一个 ICar 接口，并且所有实现都将包含字段 Year.这是否意味着每个实现都必须单独声明Year?简单地在接口中定义这个不是更好吗?

For example, suppose I want an ICar interface and that all implementations will contain the field Year. Does this mean that every implementation has to separately declare Year? Wouldn't it be nicer to simply define this in the interface?

推荐答案

虽然许多其他答案在语义层面是正确的，但我发现从实现细节层面解决这些问题也很有趣.

Though many of the other answers are correct at the semantic level, I find it interesting to also approach these sorts of questions from the implementation details level.

一个接口可以被认为是一个槽的集合，其中包含方法.当一个类实现一个接口时，该类需要告诉运行时如何填充所有必需的槽.当你说

An interface can be thought of as a collection of slots, which contain methods. When a class implements an interface, the class is required to tell the runtime how to fill in all the required slots. When you say

interface IFoo { void M(); } 
class Foo : IFoo { public void M() { ... } }

类说当你创建我的一个实例时，在 IFoo.M 的插槽中填充对 Foo.M 的引用.

the class says "when you create an instance of me, stuff a reference to Foo.M in the slot for IFoo.M.

然后当你打电话时:

IFoo ifoo = new Foo();
ifoo.M();

编译器生成的代码表示向对象询问 IFoo.M 的插槽中的方法是什么，然后调用该方法.

the compiler generates code that says "ask the object what method is in the slot for IFoo.M, and call that method.

如果接口是包含方法的槽的集合，那么其中一些槽还可以包含属性的 get 和 set 方法、索引器的 get 和 set 方法以及事件的 add 和 remove 方法.但是字段不是方法.没有与字段关联的槽"，然后您可以使用对字段位置的引用来填充".因此，接口可以定义方法、属性、索引器和事件，但不能定义字段.

If an interface is a collection of slots that contain methods, then some of those slots can also contain the get and set methods of a property, the get and set methods of an indexer, and the add and remove methods of an event. But a field is not a method. There's no "slot" associated with a field that you can then "fill in" with a reference to the field location. And therefore, interfaces can define methods, properties, indexers and events, but not fields.

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