指定类本身的通用类的基类约束 [英] Base class constraint on generic class specifying the class itself

问题描述

昨天，我在向朋友们解释C#的通用约束.在演示where T : CLASSNAME约束时，我鞭打了这样的内容:

Yesterday, I was explaining C#'s generic constraints to my friends. When demonstrating the where T : CLASSNAME constraint, I whipped up something like this:

public class UnusableClass<T> where T : UnusableClass<T>
{
    public static int method(T input){
        return 0;
    }
}

看到它可以编译真的很惊讶.然而，经过一番思考，从编译器的角度来看，我认为这是完全合法的-UnusableClass<T>与该约束中可以使用的任何其他类一样多.

And was really surprised to see it compile. After a bit of thinking, however, I figured it was perfectly legal from the point of view of the compiler - UnusableClass<T> is as much of a class as any other that can be used in this constraint.

但是，这留下了两个问题:如何使用此类?有可能

However, that leaves a couple of questions: how can this class ever be used? Is it possible to

1. 实例化吗?
2. 要继承吗?
3. 调用其静态方法int method?

如果可以，怎么办?

如果这些中的任何一个是可能的，T的类型是什么?

If any of these is possible, what would the type of T be?

推荐答案

好吧.

public class Implementation : UnusableClass<Implementation>
{
}

完全有效，正因如此

var unusable = new UnusableClass<Implementation>();

和

UnusableClass<Implementation>.method(new Implementation());

有效.

因此，可以的，可以通过提供一个继承类型作为type参数来实例化，并且与对static方法的调用类似.例如，它对于树状结构很有用，您需要在该树状结构中通常指定节点具有的子代类型，而节点本身的类型相同.

So, yes, it can be instantiated by supplying an inheriting type as the type parameter, and similarly with the call to the static method. It's for instance useful for tree-like structures where you want to generically specify the type of children the node has, while it being the same type itself.

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