C# ThreadStatic + volatile 成员未按预期工作

Question

我正在阅读 tips and tricks发布，我想我会尝试一些我以前从未做过的 C# 东西。因此，下面的代码没有任何实际用途，只是一个“测试函数”，看看会发生什么。

无论如何，我有两个静态私有(private)字段:

private static volatile string staticVolatileTestString = "";
[ThreadStatic]
private static int threadInt = 0;

如您所见，我正在测试 ThreadStaticAttribute 和 volatile 关键字。

无论如何，我有一个看起来像这样的测试方法:

private static string TestThreadStatic() {
    // Firstly I'm creating 10 threads (DEFAULT_TEST_SIZE is 10) and starting them all with an anonymous method
    List<Thread> startedThreads = new List<Thread>();
    for (int i = 0; i < DEFAULT_TEST_SIZE; ++i) {
        Thread t = new Thread(delegate(object o) {
            // The anon method sets a newValue for threadInt and prints the new value to the volatile test string, then waits between 1 and 10 seconds, then prints the value for threadInt to the volatile test string again to confirm that no other thread has changed it
            int newVal = randomNumberGenerator.Next(10, 100);
            staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;
            threadInt = newVal;
            Thread.Sleep(randomNumberGenerator.Next(1000, 10000));
            staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " finished: " + threadInt;
        });
        t.Start(i);
        startedThreads.Add(t);
    }

    foreach (Thread th in startedThreads) th.Join();

    return staticVolatileTestString;
}

我希望看到从这个函数返回的是这样的输出:

thread 0 setting threadInt to 88
thread 1 setting threadInt to 97
thread 2 setting threadInt to 11
thread 3 setting threadInt to 84
thread 4 setting threadInt to 67
thread 5 setting threadInt to 46
thread 6 setting threadInt to 94
thread 7 setting threadInt to 60
thread 8 setting threadInt to 11
thread 9 setting threadInt to 81
thread 5 finished: 46
thread 2 finished: 11
thread 4 finished: 67
thread 3 finished: 84
thread 9 finished: 81
thread 6 finished: 94
thread 7 finished: 60
thread 1 finished: 97
thread 8 finished: 11
thread 0 finished: 88

但是，我得到的是:

thread 0 setting threadInt to 88
thread 4 setting threadInt to 67
thread 6 setting threadInt to 94
thread 7 setting threadInt to 60
thread 8 setting threadInt to 11
thread 9 setting threadInt to 81
thread 5 finished: 46
thread 2 finished: 11
thread 4 finished: 67
thread 3 finished: 84
thread 9 finished: 81
thread 6 finished: 94
thread 7 finished: 60
thread 1 finished: 97
thread 8 finished: 11
thread 0 finished: 88

第二个“一半”的输出符合预期(我想这意味着 ThreadStatic 字段的工作方式与我想象的一样)，但似乎一些初始输出已从第一个“一半”中“跳过” .

此外，前“半”中的线程是乱序的，但我知道线程不会在您调用 Start() 后立即运行；但是内部操作系统控件将在它认为合适的时候启动线程。 编辑:不，他们不是，实际上，我只是认为他们是因为我的大脑错过了连续的数字

---

所以，我的问题是:导致我在输出的前“一半”中丢失几行的问题是什么？例如，行“thread 3 setting threadInt to 84”在哪里？

Answer 1

多个线程同时执行。概念上发生的是这样的:

staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;

1. 线程 1 读取 staticVolatileTestString
2. 线程 2 读取 staticVolatileTestString
3. 线程 3 读取 staticVolatileTestString
4. 线程 1 追加内容并写回 staticVolatileTestString
5. 线程 2 追加内容并写回 staticVolatileTestString
6. 线程 3 追加内容并写回 staticVolatileTestString

这会导致您的台词丢失。 Volatile 在这里没有帮助；连接字符串的整个过程不是原子的。您需要对这些操作使用锁:

private static object sync = new object();

lock (sync) {
    staticVolatileTestString += Environment.NewLine + "\tthread " + ((int) o) + " setting threadInt to " + newVal;
}