问题描述
IEquatable可能在 T 中被声明为逆变，因为它只在输入位置使用 T(或者，等效地，U 作为 T 的子类型应该意味着 IEquatable 是 [一个] IEquatable 的子类型).
IEquatable<T> could have been declared to be contravariant in T, since it only uses T in an input position (or, equivalently, U being a subtype of T should imply that IEquatable<T> is [a subtype of] IEquatable<U>).
那么，为什么 BCL 团队没有像对许多其他通用接口(例如完全类似的 IComparable)那样使用 'in' 关键字对其进行注释(对于 C# 4.0)?
So, why did the BCL team not annotate it (for C# 4.0) with the 'in' keyword, as they did with many other generic interfaces (like the entirely analogous IComparable)?
推荐答案
我认为这主要是出于哲学原因而不是技术限制——因为完全可以简单地对界面进行注释.IEquatable 用于比较相同类型的对象是否完全相等.超类的实例通常不被认为等同于子类的实例.这种意义上的平等也意味着类型平等.这与 IComparable 有点不同.定义跨不同类型的相对排序顺序是明智的.
I think this is mainly for a philosophical reason rather than a technical limitation–as it's perfectly possible to simply annotate the interface. IEquatable<T> is meant to compare objects of the same type for exact equality. An instance of a superclass is not usually considered equal to an instance of a subclass. Equality in this sense implies type equality too. This is a bit different from IComparable<in T>. It can be sensible to define a relative sort order across different types.
To quote MSDN page on IEquatable<T>:
给实施者的注意事项:
将IEquatable接口的类型参数替换为实现该接口的类型.
Replace the type parameter of the IEquatable<T> interface with the type that is implementing this interface.
这句话进一步证明了 IEquatable 旨在在单个具体类型的实例之间工作.
This sentence further demonstrates the fact that IEquatable<T> is meant to work between instances of a single concrete type.
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