协方差和向上转换之间的区别 [英] Difference between covariance and upcasting

问题描述

协方差和向上转换之间有什么区别，或者更具体地说，为什么它们的名称不同?

What is the difference between covariance and upcasting, or, more specifically, why are they given different names?

我见过以下称为向上转换"的示例:

I've seen the following example referred to as 'upcasting':

string s = "hello";
object o = s;  //upcast to 'string' to 'object'

然而，我看到以下称为协方差":

Whereas, the following I have seen called 'covariance':

string[] s = new string[100];
object[] o = s;

IEnumerable<string> ies = new List<string>();
IEnumerable<object> ieo = ies;

现在，在我未经训练的眼睛看来，协方差似乎与向上转换相同，只是它指的是集合的转换.(对于逆变和向下转换，可以做出类似的声明).

Now, to my untrained eye, covariance seems to be the same as upcasting, except that it refers the casting of collections. (And of a similar statement can be made regarding contravariance and downcasting).

真的那么简单吗?

推荐答案

现在，在我未经训练的眼睛看来，协方差似乎与向上转换相同，只是它指的是集合的转换.(对于逆变和向下转换，可以做出类似的声明).

Now, to my untrained eye, covariance seems to be the same as upcasting, except that it refers the casting of collections. (And of a similar statement can be made regarding contravariance and downcasting).

真的那么简单吗?

协方差与向上转换无关，尽管我明白你为什么认为它是相关的.

Covariance isn't about upcasting, although I can see why you think it's related.

协方差是关于以下非常简单的想法.假设您有一个 IEnumerable 类型的变量 derivedSequence.假设您有一个 IEnumerable 类型的变量 baseSequence.这里，Derived 派生自 Base.然后，通过协方差，以下是合法赋值，并发生隐式引用转换:

Covariance is about the following very simple idea. Let's say you have a variable derivedSequence of type IEnumerable<Derived>. Let's say you have a variable baseSequence of type IEnumerable<Base>. Here, Derived derives from Base. Then, with covariance, the following is a legal assignment, and an implicit reference conversion occurs:

baseSequence = derivedSequence;

请注意，这不是向上转换.IEnumerable 不是从 IEnumerable 派生的.相反，协方差允许您将变量 derivedSequence 的值分配给变量 baseSequence.这个想法是可以从 Derived 类型的对象分配 Base 类型的变量，并且由于 IEnumerable 在其参数中是协变的，IEnumerable 类型的对象可以分配给 IEnumerable 类型的变量.

Note that this is not upcasting. It is not the case that IEnumerable<Derived> derives from IEnumerable<Base>. Rather, it is covariance that allows you to assign the value of the variable derivedSequence to the variable baseSequence. The idea is that variables of type Base can be assigned from objects of type Derived, and since IEnumerable<T> is covariant in its parameter, objects of type IEnumerable<Derived> can be assigned to variables of type IEnumerable<Base>.

当然，我还没有真正解释什么是协方差.一般来说，协方差是关于以下简单的想法.假设您有一个从类型到类型的映射 F(我将用 F 表示这个映射；给定类型 T映射 F 下的图像是 F.)假设这个映射具有以下非常特殊的属性:

Of course, I haven't yet really explained what covariance is. In general, covariance is about the following simple idea. Let's say you have a mapping F from types to types (I'll denote this mapping by F<T>; given a type T its image under the mapping F is F<T>.) Let's say that this mapping has the following very special property:

如果X与Y赋值兼容，那么F与F赋值兼容代码>以及.

if X is assignment compatible with Y, then F<X> is assignment compatible with F<Y> as well.

在这种情况下，我们说 F 在其参数 T 中是协变的.(这里，要说A 是与 B 兼容的赋值"，其中 A 和 B 是引用类型意味着 B 的实例可以存储在 A 类型的变量中.)

In this case, we say that F is covariant in its parameter T. (Here, to say that "A is assignment compatible with B" where A and B are reference types means that instances of B can be stored in variables of type A.)

在我们的例子中，IEnumerable 在 C# 4.0 中，从 IEnumerable 的实例到 IEnumerable 的隐式引用转换code> 如果 Derived 是从 Base 派生的.保留了赋值兼容性的方向，这就是为什么我们说 IEnumerable 在其类型参数中是协变的.

In our case, IEnumerable<T> in C# 4.0, an implicit reference conversion from instances of IEnumerable<Derived> to IEnumerable<Base> if Derived is derived from Base. The direction of assignment compatibility is preserved, and this is why we say that IEnumerable<T> is covariant in its type parameter.

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