为什么赋值运算符（=）在foreach循环无效？ [英] Why are assignment operators (=) invalid in a foreach loop?

问题描述

为什么赋值运算符（=）在的foreach 循环无效？我使用C＃，但我会假设参数是支持的foreach （如PHP）的其他语言的相同。例如，如果我做这样的事情：

Why are assignment operators (=) invalid in a foreach loop? I'm using C#, but I would assume that the argument is the same for other languages that support foreach (e.g. PHP). For example, if I do something like this:

string[] sArray = new string[5];

foreach (string item in sArray)
{
   item = "Some assignment.\r\n";
}

我得到一个错误，无法分配到项，因为它是一个'foreach迭代变量'

I get an error, "Cannot assign to 'item' because it is a 'foreach iteration variable'."

推荐答案

下面的代码：

foreach (string item in sArray)
{
   item = "Some assignment.\r\n";
}

下面的的粗略的估计与此编译器做什么：

Here's a rough approximation of what the compiler does with this:

using (var enumerator = sArray.GetEnumerator())
{
    string item;
    while (enumerator.MoveNext())
    {
        item = enumerator.Current;

        // Your code gets put here
    }
}

的

的IEnumerator< T> .Current 属性是只读的，但实际上不是与此有关，因为你正试图分配本地项目变量为一个新值。编译时检查阻止你这样做是在基本到位，以保护你这样做是行不通的像你期望的东西（比如改变一个局部变量，其对底层集合/序列无影响）。

The IEnumerator<T>.Current property is read-only, but that's not actually relevant here, as you are attempting to assign the local item variable to a new value. The compile-time check preventing you from doing so is in place basically to protect you from doing something that isn't going to work like you expect (i.e., changing a local variable and having no effect on the underlying collection/sequence).

如果你想修改索引的集合的内部，如的String [] 同时枚举，传统的方式是使用为循环，而不是一个的foreach ：

If you want to modify the internals of an indexed collection such as a string[] while enumerating, the traditional way is to use a for loop instead of a foreach:

for (int i = 0; i < sArray.Length; ++i)
{
    sArray[i] = "Some assignment.\r\n";
}

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