C＃ThreadStatic + volatile成员不按预期方式工作

Question

我正在阅读tips and tricks文章，我想我会尝试一些以前从未做过的C＃内容。因此，下面的代码没有实际用途，但仅仅是一个'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; 

正如你所看到的，我测试ThreadStaticAttribute和挥发性关键字。

无论如何，我有一个测试方法，看起来像这样：

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; 
} 

我希望看到从该函数返回的是一个像这样的输出：

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 

然而，是什么我得到的是这样的：

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”的输出是预期的（我想这意味着th在ThreadStatic领域工作就像我想的那样），但是似乎有些初始输出已经从第一个'half'中“跳过”了。

此外，第一个'half'中的线程出现故障，但我知道一旦您调用Start（），线程就不会立即运行;而是内部操作系统控制器会根据它认为合适的方式启动线程。 编辑：没有他们没有，其实，我只是以为他们是因为我的大脑错过了连续数

---

所以，我的问题是：什么错误导致我失去的几行第一个'一半'的输出？例如，线'线程3设置threadInt到84'在哪里？

Answer 1

线程正在同时执行。什么概念发生的情况是这样的：

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追加的东西，并写入staticVolatileTestString回
5. 线程2追加东西并写入staticVolatileTestString返回
6. 线程3追加东西并写入staticVolatileTestString返回

这会导致您的线路丢失。挥发性对此没有帮助;连接字符串的整个过程不是原子的。您需要使用锁围绕这些操作：

private static object sync = new object(); 

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

Answer 2

的MSDN介绍了这些关键字volatile确实here：

volatile关键字指示一个字段可能由 多个并发执行多个线程进行修改。声明为 易失性的字段不受编译器优化的限制，假设单个线程访问 。这确保了最新的数值始终在现场出现在 。

这意味着，在你的榜样，会发生什么情况是约这个（这可以改变不时;取决于调度）：

• 线程0被读取串出的staticVolatileTestString
• 线程0被追加 '线程0设置到threadInt 88'
• 线程0被写入字符串回staticVolatileTestString

这就像预期到目前为止，但随后：

• 螺纹1-4正在阅读的串出staticVolatileTestString
• 线程1的被附加 '线程1设置threadInt至97'
• 螺纹2正在添加 '线程2设置threadInt 11'
• 线2写入字符串回staticVolatileTestString
• ...线程1，2，3读取和追加，写
• 线程4是写入字符串回staticVolatileTestString
• 等等...

看到这里发生了什么？线程4读取字符串'线程0设置threadInt为88'，附加它的'线程4 ...'并写回，覆盖已写入字符串的所有线程1,2和3。