为什么在C＃foreach语句迭代变量只读？ [英] Why is The Iteration Variable in a C# foreach statement read-only?

问题描述

据我了解，C＃的foreach迭代变量是不可变的。

As I understand it, C#'s foreach iteration variable is immutable.

这意味着我不能修改这样的迭代器：

Which means I can't modify the iterator like this:

foreach (Position Location in Map)
{
     //We want to fudge the position to hide the exact coordinates
     Location = Location + Random();     //Compiler Error

     Plot(Location);
}

我不能直接修改迭代变量，而是，我必须用一个for循环

I can't modify the iterator variable directly and instead, I have to use a for loop

for (int i = 0; i < Map.Count; i++)
{
     Position Location = Map[i];
     Location = Location + Random();

     Plot(Location);        
     i = Location;
}

这是一个C ++背景的，我看到的foreach作为替代for循环。但与上述限制，我通常回退到使用的for循环。

Coming from a C++ background, I see foreach as an alternative to the for loop. But with the above restriction, I usually fallback to using the for loop.

我很好奇，是什么使得后面的迭代器不可变的理由？

I'm curious, what is the rationale behind making the iterator immutable?

---

编辑：

此问题更是一个好奇的问题，而不是作为编码问题。我AP preciated编码的答案，但我不能将它们标记为答案。

This question is more of a curiousity question and not as a coding question. I appreciated the coding answers but I can't mark them as answers.

另外，上面的例子是过于简单化。这里是什么，我想做一个C ++的例子：

Also, the example above was over-simplified. Here is a C++ example of what I want to do:

// The game's rules: 
//   - The "Laser Of Death (tm)" moves around the game board from the
//     start area (index 0) until the end area (index BoardSize)
//   - If the Laser hits a teleporter, destroy that teleporter on the
//     board and move the Laser to the square where the teleporter 
//     points to
//   - If the Laser hits a player, deal 15 damage and stop the laser.

for (int i = 0; i < BoardSize; i++)
{
    if (GetItem(Board[i]) == Teleporter)
    {
        TeleportSquare = GetTeleportSquare(Board[i]);
        SetItem(Board[i], FreeSpace);
        i = TeleportSquare;
    }

    if (GetItem(Board[i]) == Player)
    {
        Player.Life -= 15;
        break;
    }
}

我做不到上面的C＃的foreach，因为我的迭代器是不可变的。我认为（纠正我，如果我错了），这是特定的foreach的语言设计的。

I can't do the above in C#'s foreach because the iterator i is immutable. I think (correct me if I'm wrong), this is specific to the design of foreach in languages.

我感兴趣的是，为什么在foreach迭代器是不可变的。

I'm interested in why the foreach iterator is immutable.

推荐答案

让我们用愚蠢，但说明性的例子开始了：

Lets start out with a silly but illustrative example:

Object o = 15;
o = "apples";

在任何时候做我们得到的IM pression我们刚满15号到苹果的字符串。我们知道， 0 只是一个指针。现在，让我们在迭代器的形式做到这一点。

At no point do we get the impression that we just turned the number 15 into a string of apples. We know that o is simply a pointer. Now lets do this in iterator form.

int[] nums = { 15, 16, 17 };

foreach (Object o in nums) {
     o = "apples";
}

此外，这真的无所作为。或者至少它的将会的一事无成了编译它。这当然不会插入我们的字符串插入int数组 - ，这是不允许的，我们知道， 0 只是一个指针，反正

Again, this really accomplishes nothing. Or at least it would accomplish nothing were it to compile. It certainly wouldn't insert our string into the int array -- that's not allowed, and we know that o is just a pointer anyway.

让我们把你的例子：

foreach (Position Location in Map)
{
     //We want to fudge the position to hide the exact coordinates
     Location = Location + Random();     //Compiler Error

     Plot(Location);
}

被这个编译时，位置在您的示例明星出指的是值地图，但随后你改变它来指一个新的位置（由加法运算符隐式创建）。功能上它等同于本（它编译）：

Were this to compile, the Location in your example stars out referring to a value in Map, but then you change it to refer to a new Position (implicitly created by the addition operator). Functionally it's equivalent to this (which DOES compile):

foreach (Position Location in Map)
{
     //We want to fudge the position to hide the exact coordinates
     Position Location2 = Location + Random();     //No more Error

     Plot(Location2);
}

那么，为什么微软禁止您重新分配用于迭代指针？清晰度一件事 - 你不希望人们赋予它思想，他们已经改变了循环中的地位。易于实现的另一个：变量可能​​会隐藏一些内部逻辑表明正在进行循环的状态。

So, why does Microsoft prohibit you from re-assigning the pointer used for iteration? Clarity for one thing -- you don't want people assigning to it thinking they've changed your position within the loop. Ease of implementation for another: The variable might hide some internal logic indicating the state of the loop in progress.

但更重要的是，没有任何理由让你的需要的分配给它。它重新presents循环序列的当前元素。值分配给它打破了单一职责原则或卷毛定律如果你遵循编码恐怖。变量应该是说只有一件事。

But more importantly, there is no reason for you to want to assign to it. It represents the current element of the looping sequence. Assigning a value to it breaks the "Single Responsibility Principle" or Curly's Law if you follow Coding Horror. A variable should mean one thing only.

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