为什么“int []是uint [] == true”在C＃ [英] Why does "int[] is uint[] == true" in C#

问题描述

有人可以澄清C＃是关键字。特别是这两个问题：

Can somebody clarify the C# is keyword please. In particular these 2 questions:

Q1）第5行;为什么这会返回true？

Q1) line 5; Why does this return true?

Q2）line 7;为什么没有转义异常？

Q2) line 7; Why no cast exception?

public void Test()
{
    object intArray = new int[] { -100, -200 };            

    if (intArray is uint[]) //why does this return true?
    {
        uint[] uintArray = (uint[])intArray; //why no class cast exception?

        for (int x = 0; x < uintArray.Length; x++)
        {
            Console.Out.WriteLine(uintArray[x]);
        }
    }
}

MSDN的说明情况。它表示是将返回true，如果满足这些条件之一。 （http://msdn.microsoft.com/en-us/library/scekt9xw(VS.71).aspx>MDSN文章）

MSDN's description does not clarify the situation. It states that is will return true if either of these conditions are met. (http://msdn.microsoft.com/en-us/library/scekt9xw(VS.71).aspx>MDSN Article)

expression is not null.
expression can be cast to type.

我不相信你可以做一个有效的int []到uint []。因为：

I don't believe that you can do a valid cast of int[] into uint[]. Because:

A）此代码不编译：

int[] signed = new int[] { -100 };
uint[] unsigned = (uint[])signed; 

B）在调试器中执行转换会产生错误：

B) Doing the cast in the debugger gives an error:

(uint[])signed
"Cannot convert type 'int[]' to 'uint[]'"

没错，如果第3行是int []而不是object，那么它永远不会编译。

Sure enough, if line 3 was int[] instead of object then it would never compile. Which brings me to a final question related to Q2.

Q3）为什么C＃在调试器和编译器中引发了转换/转换错误，而在运行时却没有？

Q3) Why does C# raise a cast/conversion error in the debugger and compiler but not at runtime?

推荐答案

C＃和CLR有一些不同的转换规则。

C# and the CLR have somewhat different conversion rules.

在C＃中的 int [] 和 uint [] 之间直接 >语言不相信有任何转换可用。但是，如果你通过 object ，结果是由CLI。从CLI规范第8.7节（我希望 - 我引用

You can't directly cast between int[] and uint[] in C# because the language doesn't believe any conversion is available. However, if you go via object the result is up to the CLI. From the CLI spec section 8.7 (I hope - I'm quoting an email exchange I had on this topic with Eric Lippert a while ago):

签署和无符号整数原语
类型可以彼此分配;
例如，int8：= uint8有效。对于此
目的，bool应被视为
兼容 uint8 ，反之亦然，
使 bool： = uint8 有效，
反之亦然。这同样适用于
相同大小的有符号和无符号整数
原始类型的数组;
eg， int32 []：= uint32 [] 有效。

Signed and unsigned integral primitive types can be assigned to each other; e.g., int8 := uint8 is valid. For this purpose, bool shall be considered compatible with uint8 and vice versa, which makes bool := uint8 valid, and vice versa. This is also true for arrays of signed and unsigned integral primitive types of the same size; e.g., int32[] := uint32[] is valid.

（我没有检查，但我认为这种类型的引用类型转换是有效的是是也会返回true。）

(I haven't checked, but I assume that this sort of reference type conversion being valid is what makes is return true as well.)

有些不幸的是，语言和底层执行引擎之间存在断开连接，但是从长远来看，这是非常不可避免的，我怀疑。还有一些其他的情况下这样，但好消息是，他们很少似乎造成重大损害。

It's somewhat unfortunate that there are disconnects between the language and the underlying execution engine, but it's pretty much unavoidable in the long run, I suspect. There are a few other cases like this, but the good news is that they rarely seem to cause significant harm.

编辑：因为马克删除了他的答案，我链接来自Eric的完整邮件，发布到C＃新闻组。

As Marc deleted his answer, I've linked to the full mail from Eric, as posted to the C# newsgroup.

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