并与QUOT为什么，INT []是UINT [] ==真＆QUOT;在C＃中 [英] Why does "int[] is uint[] == true" in C#

问题描述

有人可以澄清C＃是关键字请。特别是，这些2个问题：

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

Q1）5号线;为什么这回是真的吗？

Q1) line 5; Why does this return true?

Q2）7号线;为什么没有强制转换异常？

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的描述不明确的情况。它指出，是将返回true，如果这些条件得到满足。 （http://msdn.microsoft.com/en-us/library/scekt9xw(VS.71).aspx>MDSN条）

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INT []的有效铸入UINT []。这是因为：

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

A）这code不能编译：

A) This code does not compile:

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

B）做在调试器中投给出了一个错误：

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

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

果然，如果第3行是为int []，而不是对象，然后它永远不会编译。这使我与Q2最后一个问题。

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＃提高铸件/转换错误调试器，编译器，但是在运行时不会？

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有稍有不同的转换规则。

C# and the CLR have somewhat different conversion rules.

您不能的直接的投之间的 INT [] 和 UINT [] 在C＃中，因为语言的不相信任何转换是可用的。然而，如果你通过去对象，结果是到CLI。从CLI规范第8.7（我希望 - 我引述<一个href=\"http://groups.google.com/group/microsoft.public.dotnet.languages.csharp/browse%5Fthread/thread/2d21bf036a23918e#5a5c351206ebd999\">email交流我对这个话题埃里克利珀的前一段时间）：

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):

符号和无符号整数主
  类型可以被分配给彼此;
  例如，INT8：= UINT8是有效的。为了这
  目的，布尔应被视为
  与 UINT8 ，反之亦然兼容，
  这使得 BOOL：= UINT8 有效，
  反之亦然。这也是真正的
  阵列的符号和无符号整数
  原始类型相同的大小;
  例如， INT32 []：= UINT32 [] 是有效的

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，但我认为这样的引用类型转换为有效的是什么使是回也是如此。）

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

这有点不幸的是，有语言和底层的执行引擎之间的脱节，但它是pretty从长远来看多是不可避免的，我怀疑。还有一些其他的情况下，像这样，但好消息是，他们似乎很少造成显著伤害。

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：正如马克删除他的回答，我已经联系到埃里克邮件全部内容，作为发布到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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