为什么using语句中捕获闭包一个可变结构变量改变其本地的行为？ [英] Why does capturing a mutable struct variable inside a closure within a using statement change its local behavior?

问题描述

更新：好吧，现在我已经走了，做到了：我提起与微软的错误报告有关这一点，因为我严重怀疑，这是正确的行为。话虽如此，我还没有100％的把握相信什么就这个问题;这样我就可以看到什么是正确是开放的部分的间pretation水平。

Update: Well, now I've gone and done it: I filed a bug report with Microsoft about this, as I seriously doubt that it is correct behavior. That said, I'm still not 100% sure what to believe regarding this question; so I can see that what is "correct" is open to some level of interpretation.

我的感觉是，无论是微软会接受，这是一个错误，或者响应一个可变的值类型的变量中的修改使用语句构成不确定的行为。

My feeling is that either Microsoft will accept that this is a bug, or else respond that the modification of a mutable value type variable within a using statement constitutes undefined behavior.

另外，对于它的价值，我至少有一个的猜到的以这里发生了什么。我怀疑是编译器生成的类为关闭，揭的局部变量的类的实例字段;并且由于它是一个使用块中，它使现场只读 。正如LukeH指出<一href="http://stackoverflow.com/questions/4662633/is-modifying-a-value-type-from-within-a-using-statement-undefined-behavior/4662740#4662740">a评论其他问题，这将prevent方法调用，如的MoveNext 修改该字段本身（他们会代替影响副本）。

Also, for what it's worth, I have at least a guess as to what is happening here. I suspect that the compiler is generating a class for the closure, "lifting" the local variable to an instance field of that class; and since it is within a using block, it's making the field readonly. As LukeH pointed out in a comment to the other question, this would prevent method calls such as MoveNext from modifying the field itself (they would instead affect a copy).

注：我已经缩短这个问题的可读性，但它仍然是不完全的短。对于全部原（长）的问题，请参见编辑历史。的

我已经经历了什么，我相信是ECMA-334的相关章节，似乎无法找到确凿的回答这个问题阅读。我会说明这个问题第一，然后提供一个链接到一些补充意见为那些有兴趣谁。

I have read through what I believe are the relevant sections of the ECMA-334 and cannot seem to find a conclusive answer to this question. I will state the question first, then provide a link to some additional comments for those who are interested.

如果我有一个实现的IDisposable ，我可以（1）调用，用于修改<$ C $中的局部变量的值的状态的方法的可变值类型C>使用语句和code行为与我期望的那样。有一次，我抓住有问题的变量封闭的内内的的使用语句，但是，（2）修改的值不再可见在当地的范围。

If I have a mutable value type that implements IDisposable, I can (1) call a method that modifies the state of the local variable's value within a using statement and the code behaves as I expect. Once I capture the variable in question inside a closure within the using statement, however, (2) modifications to the value are no longer visible in the local scope.

这行为是在情况下，变量被抓获使用语句中的关闭和里面唯一明显;这是看不出来的时候只有一个（使用）或其它条件（关闭）是present。

This behavior is only apparent in the case where the variable is captured inside the closure and within a using statement; it is not apparent when only one (using) or the other condition (closure) is present.

使用语句中为什么捕获可变的值类型的变量封闭内改变当地的行为？

Why does capturing a variable of a mutable value type inside a closure within a using statement change its local behavior?

下面是code举例说明项目1和2两个例子都将采用下面的演示可变值类型：

Below are code examples illustrating items 1 and 2. Both examples will utilize the following demonstration Mutable value type:

struct Mutable : IDisposable
{
    int _value;
    public int Increment()
    {
        return _value++;
    }

    public void Dispose() { }
}

1。一个使用块内变异值类型变量

1. Mutating a value type variable within a using block

using (var x = new Mutable())
{
    Console.WriteLine(x.Increment());
    Console.WriteLine(x.Increment());
}

输出code输出：

The output code outputs:

0
1

2。一个使用块中捕获的值类型变量闭包

2. Capturing a value type variable inside a closure within a using block

using (var x = new Mutable())
{
    // x is captured inside a closure.
    Func<int> closure = () => x.Increment();

    // Now the Increment method does not appear to affect the value
    // of local variable x.
    Console.WriteLine(x.Increment());
    Console.WriteLine(x.Increment());
}

以上code输出：

The above code outputs:

0
0

更多评论

据指出，单声道编译器提供我期望的行为（改变局部变量的值仍然在使用 +封闭的情况可见）。无论这种行为的正确与否我不清楚。

Further Comments

It has been noted that the Mono compiler provides the behavior I expect (changes to the value of the local variable are still visible in the using + closure case). Whether this behavior is correct or not is unclear to me.

对于一些在这个问题上我更多的想法，看到<一href="http://philosopherdeveloper.word$p$pss.com/2011/01/11/further-comments-on-the-mutable-structclosureusing-issue/">here.

For some more of my thoughts on this issue, see here.

推荐答案

这是一个已知的bug;我们几年前发现了它。此修复程序的话会有潜在破坏，问题是pretty的晦涩难懂;这些都是反对将其固定点。因此，它从来没有被优先足够高，真正解决这个问题。

It's a known bug; we discovered it a couple years ago. The fix would be potentially breaking, and the problem is pretty obscure; these are points against fixing it. Therefore it has never been prioritized high enough to actually fix it.

这一直是我的潜力博客题目为一对夫妇现在的年队列;也许我应该把它写了起来。

This has been in my queue of potential blog topics for a couple years now; perhaps I ought to write it up.

和顺便说一句，你的猜想，以解释这个错误是完全正确的机制;好的心理调试那里。

And incidentally, your conjecture as to the mechanism that explains the bug is completely accurate; nice psychic debugging there.

所以，是的，已知的bug，但由于该报告无论！

So, yes, known bug, but thanks for the report regardless!

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