Question

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

无论如何，我有两个静态私有字段：

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;
}

我期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

输出的第二个'一半'是预期的（我想这意味着ThreadStatic字段的工作方式与我想的一样），但似乎有一些初始输出已经从第一个'半'跳过'

~~此外，第一个“half”中的线程出现故障，但我知道一旦调用Start（），线程就不会立即运行;但相反，内部OS控件将在它认为合适时启动线程。~~ 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数字

所以，我的问题是：导致我在输出的前半部分丢失几行会出现什么问题？例如，'线程3将threadInt设置为84 '？

Answer 1

线程正在同时执行。概念上发生的是：

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

线程1读取staticVolatileTestString
线程2读取staticVolatileTestString
线程3读取staticVolatileTestString
线程1附加内容并将staticVolatileTestString写回
线程2附加内容并将staticVolatileTestString写回
线程3附加内容并将staticVolatileTestString写回

这导致你的线路丢失。挥发无助于此;连接字符串的整个过程不是原子的。你需要围绕这些操作使用锁定：

private static object sync = new object();

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

Answer 2

MSDN描述了关键字volatile的作用here：

volatile关键字表示字段可能被修改多个并发执行的线程。声明的字段 volatile不受假定访问的编译器优化的影响通过一个线程。这可确保获得最新的价值在任何时候都在现场。

这意味着，在您的示例中，会发生什么 about this（这可能会不时变化;取决于调度程序）：

线程0正在读取staticVolatileTestString
线程0正在附加＆＃39;线程0将threadInt设置为88＆＃39;
线程0将字符串写回staticVolatileTestString

这是预期到目前为止，但随后：

主题1-4正在从staticVolatileTestString
主题1正在附加＆＃39;主题1将threadInt设置为97＆＃39;
主题2正在附加＆＃39;主题2将threadInt设置为11＆＃39;
主题2正在将字符串写回staticVolatileTestString
...主题1,2,3正在阅读和追加，以及写作
主题4正在将字符串写回staticVolatileTestString
等......

看看这里发生了什么？线程4读取字符串＆＃39;线程0设置threadInt到88＆＃39;，附加其线程4 ...＆＃39;并写回来，覆盖线程1,2和3已经写入字符串的所有内容。

C＃ThreadStatic + volatile成员未按预期工作

2 个答案: