问题描述

我正在测试一些同步结构，我注意到一些让我感到困惑的东西.当我枚举一个集合同时写入它时，它抛出了一个异常(这是预期的)，但是当我使用 for 循环遍历集合时，它没有.有人可以解释一下吗?我认为 List 不允许读取器和写入器同时操作.我本来希望循环遍历集合会表现出与使用枚举器相同的行为.

I was testing out some synchronization constructs and I noticed something that confused me. When I was enumerating through a collection while writing to it at the same time, it threw an exception (this was expected), but when I looped through the collection using a for loop, it did not. Can someone explain this? I thought that a List does not allow a reader and writer to operate at the same time. I would have expected looping through the collection to exhibit the same behavior as using an enumerator.

更新:这是一个纯粹的学术练习.我知道如果同时写入列表，枚举列表是不好的.我也明白我需要一个同步构造.我的问题再次是关于为什么一个操作按预期抛出异常，而另一个却没有.

代码如下:

   class Program
   {
    private static List<string> _collection = new List<string>();
    static void Main(string[] args)
    {
        ThreadPool.QueueUserWorkItem(new WaitCallback(AddItems), null);
        System.Threading.Thread.Sleep(5000);
        ThreadPool.QueueUserWorkItem(new WaitCallback(DisplayItems), null);
        Console.ReadLine();
    }

    public static void AddItems(object state_)
    {
        for (int i = 1; i <= 50; i++)
        {
            _collection.Add(i.ToString());
            Console.WriteLine("Adding " + i);
            System.Threading.Thread.Sleep(150);
        }
    }

    public static void DisplayItems(object state_)
    {
        // This will not throw an exception
        //for (int i = 0; i < _collection.Count; i++)
        //{
        //    Console.WriteLine("Reading " + _collection[i]);
        //    System.Threading.Thread.Sleep(150);
        /