c# - 协方差是关于接受值吗？

Question

协方差是关于接受值吗？

我正在通过 学习 C# CLR 通过 C# 书。我遇到了以下摘录:

Since T is covariant, it is possible to have the following code compile and run successfully:

// This method accepts an IEnumerable of any reference type
Int32 Count(IEnumerable<Object> collection) { ... }

...
// The call below passes an IEnumerable<String> to Count
Int32 c = Count(new[] { "Grant" }); 

我在这里很困惑。因为协方差是关于具有所需类型的基本类型之一的类型。因此，协方差仅在返回类型的上下文中使用。而在上面的例子中，我们有一个 String (源自 Object ，因此是逆变的，但不是协变的)，它用于传递参数(但不返回值)的上下文中。

那么，我们应该在上面的例子中使用逆变而不是协变(意思是书中有错误)？

更新

在评论之后，我得到了另一个问题。以下定义是否正确？

Contravariant Meaning that the generic type parameter can change from a class to a class derived from it. In C#, you indicate contravariant generic type parameters with the in keyword. Contravariant generic type parameters can appear only in input positions such as a method’s argument.

Covariant Meaning that the generic type argument can change from a class to one of its base classes. In C#, you indicate covariant generic type parameters with the out keyword. Covariant

Answer 1

正如乔希指出的那样，这本书是正确的。
您可以查看 this link如果您想从其他来源确认。

IEnumerable<Cat> is a subtype of IEnumerable<Animal>. The subtyping is preserved because IEnumerable<T> is covariant on T.

C# 中用于这些概念的两个关键字是 out用于协变和 in为逆变。在 IEnumerable<T>案例这转化为 IEnumerable<out T> .
希望这可以帮助。
更新
您必须按如下方式反转您的定义。

Covariant Meaning that the generic type parameter can be a certain class and all the derived classes from it (IEnumerable<Object> can be a IEnumerable<String> since String is a subtype of Object). In C#, you indicate covariant generic type parameters with the out keyword.

Contravariant Meaning that the generic type argument can change from aclass to one of its base classes. In C#, you indicate contravariantgeneric type parameters with the in keyword.