为什么列表< T> .ForEach允许修改其列表？ [英] Why does List<T>.ForEach allow its list to be modified?

问题描述

如果我使用：

var strings = new List<string> { "sample" };
foreach (string s in strings)
{
  Console.WriteLine(s);
  strings.Add(s + "!");
}

的

添加中在的foreach 引发InvalidOperationException（集合已修改;枚举操作可能不执行），我认为合乎逻辑的，因为我们从我们脚下拉地毯

the Add in the foreach throws an InvalidOperationException (Collection was modified; enumeration operation may not execute), which I consider logical, since we are pulling the rug from under our feet.

不过，如果我使用：

var strings = new List<string> { "sample" };
strings.ForEach(s =>
  {
    Console.WriteLine(s);
    strings.Add(s + "!");
  });

它及时通过循环，直到它抛出一个OutOfMemoryException拍摄自己的脚。

it promptly shoots itself in the foot by looping until it throws an OutOfMemoryException.

这是作为一个惊喜给我，因为我一直认为List.ForEach要么只是为的foreach 或<$ C的包装$ C>为。结果
没有人有该怎么解释和此行为的，为什么？

This comes as a suprise to me, as I always thought that List.ForEach was either just a wrapper for foreach or for for.
Does anyone have an explanation for the how and the why of this behavior?

（通过反复 ForEach循环的泛型列表Inpired不休的）

推荐答案

这是因为的ForEach 方法不使用枚举，它通过与为循环的项目循环：

It's because the ForEach method doesn't use the enumerator, it loops through the items with a for loop:

public void ForEach(Action<T> action)
{
    if (action == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.match);
    }
    for (int i = 0; i < this._size; i++)
    {
        action(this._items[i]);
    }
}

（与JustDecompile获得的代码）

(code obtained with JustDecompile)

由于不使用枚举，它从来没有检查，如果该列表已经改变，永远达不到为循环的结束条件因为 _size 在每次迭代增加。

Since the enumerator is not used, it never checks if the list has changed, and the end condition of the for loop is never reached because _size is increased at every iteration.

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