如何在C＃中重写子类的重写方法中的子类型？ [英] How to return subtype in overridden method of subclass in C#?

问题描述

我有一个带有over-ridden方法的子类，我知道它总是返回基类中声明的返回类型的特定子类型。如果我以这种方式编写代码，它将无法编译。由于这可能没有意义，让我给出一个代码示例：

I have a subclass with an over-ridden method that I know always returns a particular subtype of the return type declared in the base class. If I write the code this way, it won't compile. Since that probably doesn't make sense, let me give a code example:

class BaseReturnType { }
class DerivedReturnType : BaseReturnType { }

abstract class BaseClass {
    public abstract BaseReturnType PolymorphicMethod();
}

class DerivedClass : BaseClass {
    // Compile Error: return type must be 'BaseReturnType' to match 
    // overridden member 'BaseClass.PolymorphicMethod()'
    public override DerivedReturnType PolymorphicMethod() { 
        return new DerivedReturnType(); 
    }
}

有没有办法在C＃中实现这一目标？如果没有，那么实现类似目标的最佳方法是什么？为什么不允许这样做？它似乎不允许任何逻辑上的不一致，因为从over-ridden方法返回的任何对象仍然是是BaseReturnType 。也许有些东西我没考虑过。或者原因可能是技术或历史。

Is there any way to accomplish this in C#? If not, what's the best way to achieve something similar? And why isn't it allowed? It doesn't seem to allow any logical inconsistency, since any object returned from the over-ridden method still is BaseReturnType. Maybe there is something I hadn't considered though. Or maybe the reason is technological or historical.

推荐答案

不幸的是，C＃中不支持协变返回类型以进行方法覆盖。 （Ditto contravariant参数类型。）

Unfortunately no, covariant return types aren't supported in C# for method overriding. (Ditto contravariant parameter types.)

如果您正在实现一个接口，您可以使用弱版本明确实现它，并提供具有更强合同的公共版本。对于父类的简单覆盖，你恐怕没有这种奢侈:(

If you're implementing an interface you can implement it explicitly with the "weak" version and also provide a public version with the stronger contract. For simple overriding of a parent class, you don't have this luxury I'm afraid :(

（编辑：Marc有合理的解决方案 - 虽然它非常丑陋，方法隐藏是通常是可读性的坏事。没有冒犯意味着，Marc;）

( Marc has a reasonable solution - although it's pretty ugly, and method hiding is generally a bad thing for readability. No offence meant, Marc ;)

我相信这实际上是一个CLR限制，而不仅仅是一种语言 - 但我可能错了。

I believe this is actually a CLR restriction, not just a language one - but I could well be wrong.

（作为历史问题，Java（该语言）在1.5之前具有相同的限制 - 但它同时获得了协方差作为泛型。）

(As a matter of history, Java (the language) had the same restriction until 1.5 - but it gained covariance at the same time as generics.)

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