接口继承的声明问题 [C#, BCL]

接口继承的声明问题

Written by Allen Lee

某天，小新问我这样一个问题：

类System.Collections.CollectionBase是从IList、ICollection继承而来，IList是从ICollection和IEnumerable继承而来，那CollectionBase为什么还要从ICollection继承呢？

我们先来看看这些类和接口在MSDN文档中的声明：

public interface IEnumerable

public interface ICollection : IEnumerable

public interface IList : ICollection, IEnumerable

public abstract class CollectionBase : IList, ICollection, IEnumerable

根据接口继承的规则，我们知道CollectionBase只需要声明实现IList，就必须同时实现ICollection，也就必须实现IEnumerable，那么，我们为什么还要明确地把所有的这些接口都写下来呢？

换句话说，下面两种声明没有实质的区别：

// Code #1

public interface IEnumerable

public interface ICollection : IEnumerable

public interface IList : ICollection, IEnumerable

public class ArrayList : IList, ICollection, IEnumerable, ICloneable

// Code #2

public interface IEnumerable

public interface ICollection : IEnumerable

public interface IList : ICollection

public class ArrayList : IList, ICloneable

那为何MSDN要使用上面那种呢？我和小新讨论后，一致认为这样做仅仅为了提高代码的可读性。为了验证我们的想法，我分别发邮件给Eric Gunnerson（Eric是C# Compiler Team的成员）和Kit George（Kit是BCL Team的成员）询问这个问题，他们的回信如下：

Allen,
I think that readability would be the primary reason people would do this.

Eric

Allen, what you’re seeing is simply a doc thing. ArrayList actually only implements IList directly, but we decided in V1.0 of the docs, to highlight the interface hierarchy so you didn’t have to wonder ‘what does IList implement’, you get to see it there on the type.

There is no benefit to ACTUALLY doing this. Try this code, and you’ll see it compiles:

using System;
using System.IO;

public class Test : IBob2
{
void IBob.M() {}
}

interface IBob
{
void M();
}

interface IBob2 : IBob {}

Regards,

Kit