类型铸造 [英] Type Casting

问题描述

您好，


有没有人知道如何做以下事情？


我有A级和B级，100％继承自A类（这意味着

我在B类中没有其他代码，而不是继承声明）。


现在我想在类型A的对象上进行类型转换，这样对象

就变成了B类型。


我试过了，但是得到错误''System.InvalidCastException：指定

强制转换无效。''，所以想要我失踪。


请给出提示。

$ b $bRené

解决方案

" ;RenéNordby <重** @ bluedot.dk>在消息中写道

新闻：％2 **************** @ TK2MSFTNGP09.phx.gbl ...

我有一个A类和一个B类，100％继承自A类（这个
意味着我在B类中没有其他代码，而不是继承语句。）

现在我想在类型A的对象上进行类型转换，这样
对象就变成了B类型。

我已经尝试过它，但得到了错误'' System.InvalidCastException：
指定的演员表是无效的。''，所以想要我错过。

你错过了它可以'不行。你可以将派生类

转换为它的基类，但你不能将基类强制转换为派生类。

如果我是谁，有人会纠正我错误。

你好Jeff，


感谢您的快速回答。


当你这样说的时候，我就会发现String和Integer的数据类型，你可以将
从String转换为Integer，反之亦然，那么

是如何完成的？


我知道String和Integer都是从Object派生的，所以我也试过让我的两个类继承自基类，但是它给我

相同的结果。


Jeff Johnson [MVP：VB]" <我*** @ enough.spam>在留言中写道

news：％2 **************** @ TK2MSFTNGP12.phx.gbl ...

RenéNordby <重** @ bluedot.dk>在留言中写道
新闻：％2 **************** @ TK2MSFTNGP09.phx.gbl ...

我有A类和B类，100％继承自A类（这意味着

我在B类中没有其他代码，而不是继承语句。）

现在我想在类型A的对象上进行类型转换，以便

对象

成为B类型。
我已经试过了，但得到了错误''System.InvalidCastException：

指定的

cast无效。''，所以想要我错过。

你错过了无法完成的事实。你可以将派生的

类强制转换为它的基类，但你不能将基类强制转换为派生类。
有人纠正我，如果我是错误。

你好Jeff，

你错过了它可以的事实不行。你可以将派生的
类强制转换为它的基类，但你不能将基类强制转换为派生类。
如果我错了，有人会纠正我。

您确实可以将基类引用转换为子类引用，并且

操作将导致缩小转换。但是，如果你想要投射的基类参考是

最初是从早期从子类扩展转换到
$，那么转换将会成功。 b $ b base-class。

Hi there,

Is there anyone that knows how to do the following?

I have a class A and a class B, that 100% inherits from class A (this means
that I don''t have other code in class B, than the Inherit statement).

Now I want to make a Type Casting on an object of the type A, so that object
become a B type.

I have tryied it, but gets the error ''System.InvalidCastException: Specified
cast is not valid.'', so want am I missing.

Please give a hint.

René

解决方案

"René Nordby" <re**@bluedot.dk> wrote in message
news:%2****************@TK2MSFTNGP09.phx.gbl...

I have a class A and a class B, that 100% inherits from class A (this means that I don''t have other code in class B, than the Inherit statement).

Now I want to make a Type Casting on an object of the type A, so that object become a B type.

I have tryied it, but gets the error ''System.InvalidCastException: Specified cast is not valid.'', so want am I missing.

You''re missing the fact that it can''t be done. You can cast a derived class
into its base class, but you can''t cast a base class to a derived class.
Someone correct me if I''m wrong.

Hi Jeff,

Thanks for your quick answer.

When you say so, then I thnink of String and Integer datatypes, where you
can cast from String to Integer and the other way around, so how is that
done?

I know that both String and Integer is derived from Object, so I have also
tryied to let my two classes inherit from a base class, but it give me the
same result.

"Jeff Johnson [MVP: VB]" <i.***@enough.spam> wrote in message
news:%2****************@TK2MSFTNGP12.phx.gbl...

"René Nordby" <re**@bluedot.dk> wrote in message
news:%2****************@TK2MSFTNGP09.phx.gbl...

I have a class A and a class B, that 100% inherits from class A (this means

that I don''t have other code in class B, than the Inherit statement).

Now I want to make a Type Casting on an object of the type A, so that

object

become a B type.

I have tryied it, but gets the error ''System.InvalidCastException:

Specified

cast is not valid.'', so want am I missing.

You''re missing the fact that it can''t be done. You can cast a derived

class into its base class, but you can''t cast a base class to a derived class.
Someone correct me if I''m wrong.

Hi Jeff,

You''re missing the fact that it can''t be done. You can cast a derived class into its base class, but you can''t cast a base class to a derived class.
Someone correct me if I''m wrong.

You can indeed cast a base class reference to a sub-class reference and the
operation would result in a ''narrowing'' conversion. BUT the conversion will
succeed ONLY if the base class reference you are trying to cast was
originally obtained from an earlier widening conversion from sub-class to
base-class.

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