试图投装箱的int到byte [英] Trying to cast a boxed int to byte

问题描述

code来说明：

        int i = 5;
        object obj = i;
        byte b = (byte)obj; // X

在运行，这会产生一个System.InvalidCastException（指定的转换无效）于行X。做一个双投作品：

When run, this generates a System.InvalidCastException ("Specified cast is not valid") at line "X". Doing a double cast works :

        byte b = (byte)(int)obj;

我本来以为你应该能够施展装箱的int（如果它在范围0..255的值），以一个字节。任何人都可以阐明这个任何光线？

I would have thought that you ought to be able to cast a boxed int (if it has a value in the range 0..255) to a byte. Can anyone shed any light on this ?

（这是.NET 2.0，如果该事项）。

(This is in .net 2.0, in case that matters).

推荐答案

在你所看到的行为偏差在<一的区别href="http://blogs.msdn.com/b/ericlippert/archive/2009/03/19/re$p$psentation-and-identity.aspx">identity并重新presentation 。

The difference in behaviour you're seeing is the difference between identity and representation.

拆箱是的的身份的演员阵容，和再presentation- preserving 的操作。铸造一个 INT 到字节，但是，的再presentation变化的（因为是precision潜在损失）

Unboxing is an identity cast, and a representation-preserving operation. Casting an int to a byte, however, is representation-changing (since there is a potential loss of precision).

您获得 InvalidCastException的当您尝试拆箱 INT 为字节因为装箱值的的身份不是字节，它是一个 INT 。当你写字节B =（字节）目标文件，你说的是运行时，的我知道这里面有什么了是 字节 的，但你真正的意思是说，我认为这里面有什么了可转换到字节 的。

You get an InvalidCastException when you try to unbox the int as a byte because the identity of the boxed value is not a byte, it is an int. When you write byte b = (byte)obj, you are telling the runtime, I know that what's in there is a byte, but what you really mean to say is, I think that what's in there can be converted to a byte.

为了使后者的说法，你首先要声明的的身份的对象，这是一个 INT 。那时，也只有这样你才能做出重新presentation变化转化为字节。

In order to make the latter statement, you first have to declare the identity of the object, which is an int. Then and only then can you make a representation-changing conversion to byte.

请注意，这适用即使目标类型为大 - 即一个的Int64 。 所有的显式转换为其目标类型不在源类型的继承树被认为是重新presentation变化。由于所有类型从 System.Object的导出，顾名思义拆箱不能改变重presentation。

Note that this applies even if the target type is "larger" - i.e. an Int64. All explicit conversions for which the destination type is not in the inheritance tree of the source type are considered to be representation-changing. And since all types derive from System.Object, unboxing by definition cannot change the representation.

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