c# - 多线程时循环索引超出范围 ArgumentOutOfRangeException

Question

我遇到了一些奇怪的行为...当我在 ThreadTest 方法中遍历 dummyText List 时，我得到了一个索引范围异常 (ArgumentOutOfRangeException)，但如果我删除线程并只打印出文本，那么一切正常。

这是我的主要方法:

public static Object sync = new Object();
static void Main(string[] args)
{
    ThreadTest();
    Console.WriteLine("Press any key to continue.");
    Console.ReadKey();
}

此方法抛出异常:

private static void ThreadTest()
{
    Console.WriteLine("Running ThreadTest");
    Console.WriteLine("Running ThreadTest");
    List<String> dummyText = new List<string>()
    { "One", "Two", "Three", "Four", "Five", 
      "Six", "Seven", "Eight", "Nine", "Ten"};

    for (int i = 0; i < dummyText.Count; i++)
    {
        Thread t = new Thread(() => PrintThreadName(dummyText[i])); // <-- Index out of range?!?
        t.Name = ("Thread " + (i));
        t.IsBackground = true;
        t.Start();
    }
}

private static void PrintThreadName(String text)
{
    Random rand = new Random(DateTime.Now.Millisecond);
    while (true)
    {
        lock (sync)
        {
            Console.WriteLine(Thread.CurrentThread.Name + " running " + text);
            Thread.Sleep(1000+rand.Next(0,2000));
        }
    }
}

这不会抛出异常:

private static void ThreadTest()
{
    Console.WriteLine("Running ThreadTest");
    List<String> dummyText = new List<string>()
    { "One", "Two", "Three", "Four", "Five", 
      "Six", "Seven", "Eight", "Nine", "Ten"};

    for (int i = 0; i < dummyText.Count; i++)
    {
        Console.WriteLine(dummyText[i]); // <-- No exception here
    }
}

有人知道为什么会这样吗？

Answer 1

当您通过闭包将局部变量传递给线程或ThreadPool 委托(delegate)时，您需要复制该变量。如:

for (int i = 0; i < dummyText.Count; i++)
{
    int index = i;
    Thread t = new Thread(() => PrintThreadName(dummyText[index]));
    // ...
}

如果你不这样做，那么变量基本上是通过引用传入的，索引将在 for 循环的最后超出数组的边界(这可能会发生早在执行闭包之前)。