C＃4.0：int对象的真正子类型？协方差，即数和值类型

问题描述

我想知道为什么不能将 IEnumerable< int> 分配给 IEnumerable< object> 。毕竟 IEnumerable 是少数支持协方差的接口之一...

I wonder why IEnumerable<int> can't be assigned to a IEnumerable<object>. After all IEnumerable is one of the few interfaces that supports covariance...

• 子类型关系和协方差材料适用于引用类型


• int 似乎是 object



• The subtype relation and covariance stuff works with reference types
• int seems to be a proper subtype of object

但这两个功能的组合不起作用...

The combination of both features doesn't work however...

class A
{
}

class B : A
{
}

class Program
{
    static void Main(string[] args)
    {
        bool b;
        b = typeof(IEnumerable<A>).IsAssignableFrom(typeof(List<B>));
        Console.WriteLine("ienumerable of ref types is covariant: " + b); //true

        b = typeof(IEnumerable<object>).IsAssignableFrom(typeof(List<int>));
        Console.WriteLine("ienumerable of value tpyes is covariant: " + b); //false

        b = typeof(object).IsAssignableFrom(typeof(int));
        Console.WriteLine("int is a subtype of object: " + b); //true
    }
}

感谢您的帮助！
sebastian

thanks for your help!sebastian

推荐答案

在将值类型装箱之前，它们不是对象的LSP子类型。

Value types aren't LSP-subtypes of object until they're boxed.

方差不适用于值类型。

Variance doesn't work with value types. At all.

说明 int 不是 object 的> proper子类型（在LSP中是子类型）：

Demonstration that int is not a proper subtype (subtype in the LSP sense) of object:

作品：

object x = new object();
lock (x) { ... }

不起作用（违反了可替代性）：

Does not work (substitutability violated):

int y = new int();
lock (y) { ... }

返回true：

object x = new object();
object a = x;
object b = x;
return ReferenceEquals(a, b);

返回假（违反可替代性）：

Returns false (substitutability violated):

int y = new int();
object a = y;
object b = y;
return ReferenceEquals(a, b);

当然，问题的主题（界面变化）是第三次演示。

Of course, the topic of the question (interface variance) is a third demonstration.

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