永久投派生类基地 [英] Permanently cast derived class to base

问题描述

Class A { }
Class B : A { }

B ItemB = new B();
A ItemA = (A)B;

Console.WriteLine(ItemA.GetType().FullName);

是否有可能做这样的事情上面，并让编译器打印出A型，而不是B型基本上是有可能永久对象转换，因此失去的所有导出数据？

Is it possible to do something like above and have the compiler print out type A instead of type B. Basically, is it possible to permanently cast an object so it "loses" all the derived data?

推荐答案

你问什么是不可能的，原因有二：

What you ask for is impossible for two reasons:

1. ItemA.GetType（）不返回变量的编译时类型意达 - 它返回运行时类型的对象的在意达简称的。
2. 有没有办法可以让（一）乙结果在重新presentation变化的转化率（即一个新的 A ）对象，因为用户定义的转换操作符（在这里）你唯一的希望不能来源于立足类转换。你只是要得到一个正常的，安全的，参考的转换。

1. ItemA.GetType() does not return the compile-time type of the variable ItemA - it returns the run-time type of the object referred to by ItemA.
2. There's no way you could make (A)B result in a representation-changing conversion (i.e. a new A object) because user-defined conversion operators (your only hope here) cannot convert from derived to base-classes. You're just going to get a normal, safe, reference-conversion.

这一边，你问什么是很奇怪的;人们会认为你想真的很难违反里氏的substiution原则。目前几乎可以肯定一个严重的设计缺陷在这里，你应该解决的问题。

That aside, what you ask for is very strange; one would think you're trying really hard to violate Liskov's substiution principle. There's almost certainly a serious design-flaw here that you should address.

如果您仍然想做到这一点;可以编写手动构造方法的 A 从 B 通过newing了一个 A 键，然后在复制数据。这可能存在一个的ToA（）  在实例方法 B 。

If you still want to do this; you could write a method that manually constructs an A from a B by newing up an A and then copying data over. This might exist as a ToA() instance-method on B.

如果你将这个问题，我如何构建一个从现有的 A ？，这让很多更有意义：创建 A拷贝构造函数，其声明如下公开发行A（A一）{...} ，这是不可知的子类的具体细节。这给你一个通用方法来创建一个 A 从 A 或它的一个子类的现有实例。

If you characterized this problem as "How do I construct an A from an existing A?", it makes a lot more sense: create a copy-constructor on A, whose declaration looks like public A(A a){...}, which is agnostic to subclass-specific details. This gives you a general means to create an A from an existing instance of A or one of it's subclasses.

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