为什么我不能写从基类的隐式操作员在C＃中派生类？ [英] Why can't I write an implicit operator from a Base class to a Derived class in C#?

问题描述

public class Derived : BaseClass
{
    public Derived(string name) : base(name) {}

    public static implicit operator BaseClass(Derived derived)
    {
        return new BaseClass(derived.ColorHex);
    }

    public static implicit operator Derived(BaseClass baseclass)
    {
        return new Derived(baseclass.name);
    }
}

这是行不通的。为什么是不是允许的？结果
我的可以的潜在写逻辑必要为它是有意义的，特别是从碱转化为派生之一，当。

This won't work. why isn't it allowed?
I can potentially write the logic necessary for it to make sense, especially when converting from the base to the derived one.

编辑：改变了问题。

推荐答案

由于已经存在的隐式转换的标题从导出到的BaseClass ，以及反之则没有任何意义。

Because there is already an implicit conversion from Derived to BaseClass, and the converse does not make any sense.

关于后者：如果你的基础的对象，目的是要隐式转换为派生 - 为什么是他们不派生放在首位对象

Regarding the latter: if your Base objects are meant to be implicitly convertible to Derived -- why aren't they Derived objects in the first place?

从的standard ：

隐式引用转换为：

• [。 ..]


• 从任意类类型S到任意类类型T，只要S是在T衍生
  

这表示有一个隐式转换派生 => 基础，因为大家都知道。

This says there's an implicit conversion Derived => Base, as we all know.

显式引用转换为：

• [...]


• 从任何类类型S到任意类类型T，只要S是一个基类的T。


• [...]

这表示已经有一个明确的转换基本 => 导出（这是什么让你尝试在运行时向下转换）。

This says there's already an explicit conversion Base => Derived (which is what allows you to try downcasting at runtime).

C＃只允许某些用户定义的转换。 <强>在
特别地，它是不可能重新定义一个已经存在的
隐式或显式转换。

6.4.1 Permitted user-defined conversions

C# permits only certain user-defined conversions to be declared. In particular, it is not possible to redefine an already existing implicit or explicit conversion.

和本说，由于感兴趣的两次转换已经由语言定义的，你不能重新定义它们。

And this says that since the two conversions of interest are already defined by the language, you can't redefine them.

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