为什么这个线程示例根本不可预测（每次输出不同的结果）？

Question

我正在尝试了解线程。所以我在一本官方书籍中看到了这篇文章：

public static class Program
{
    public static void ThreadMethod()
    {
        for (int i = 0; i < 10; i++)
        {
            Console.WriteLine("Thread Step: {0}", i); // A - see comment!!
            Thread.Sleep(0); // B 
        }
    }
    public static void Main()
    {
        Thread t = new Thread(new ThreadStart(ThreadMethod));
        t.Start();
        for (int i = 0; i < 4; i++)
        {
            Console.WriteLine("This is supposedly the main thread.");
            Thread.Sleep(0);
        }
        t.Join();

        Console.ReadLine();
    }
}

现在，书中说预期的行为应该是这样的（暗示输出是可预测的，因为如果您知道规则以及如何配置线程，线程是可预测的）：

// Displays
//This is supposedly the main thread.
//Thread step: 0
//This is supposedly the main thread.
//Thread step: 1
//This is supposedly the main thread.
//Thread step: 2
//This is supposedly the main thread.
//Thread step: 3
//Thread step: 4
//Thread step: 5
//Thread step: 6
//Thread step: 7
//Thread step: 8
//Thread step: 9
//Thread step: 10

但是，每次运行控制台应用程序的实际结果都不同，为什么？ 其次，我希望首先在屏幕上看到“Thread Step：0”，因为 A）第一个线程将其输出到控制台，并且仅在此之后 B）运行 Thread.Sleep(0)，这意味着当前线程被放置休眠以等待另一个同等优先级的线程来接管（如果可用）。

This is supposedly the main thread.
This is supposedly the main thread.
Thread Step: 0
This is supposedly the main thread.
Thread Step: 1
This is supposedly the main thread.
Thread Step: 2
Thread Step: 3
Thread Step: 4
Thread Step: 5
Thread Step: 6
Thread Step: 7
Thread Step: 8
Thread Step: 9

再次运行，得到另一个结果：

This is supposedly the main thread.
This is supposedly the main thread.
Thread Step: 0
Thread Step: 1
Thread Step: 2
Thread Step: 3
Thread Step: 4
Thread Step: 5
Thread Step: 6
Thread Step: 7
Thread Step: 8
Thread Step: 9
This is supposedly the main thread.
This is supposedly the main thread.

还有一个：

This is supposedly the main thread.
Thread step: 0
This is supposedly the main thread.
Thread step: 1
Thread step: 2
Thread step: 3
Thread step: 4
Thread step: 5
Thread step: 6
Thread step: 7
Thread step: 8
Thread step: 9
This is supposedly the main thread.
This is supposedly the main thread.

还有一个：

This is supposedly the main thread.
Thread step: 0
This is supposedly the main thread.
Thread step: 1
This is supposedly the main thread.
Thread step: 2
This is supposedly the main thread.
Thread step: 3
Thread step: 4
Thread step: 5
Thread step: 6
Thread step: 7
Thread step: 8
Thread step: 9

还有一个：

Thread step: 0
Thread step: 1
This is supposedly the main thread.
Thread step: 2
This is supposedly the main thread.
This is supposedly the main thread.
This is supposedly the main thread.
Thread step: 3
Thread step: 4
Thread step: 5
Thread step: 6
Thread step: 7
Thread step: 8
Thread step: 9

Answer 1

这是因为你的程序中有2个线程，一个是主线程，正在执行main方法，另一个是在这个main方法内部创建的线程，正在执行ThreadMethod方法。这两个线程正在访问一个公共输出，在这种情况下是您的控制台，并且操作系统可以选择以任何顺序安排它们的访问。这就是为什么如果你多次运行这个程序，每次你都会得到不同的顺序。日程安排不在您的程序的控制范围内。

Answer 2

我认为要学习线程，您需要了解进程、CPU 在计算机中的工作方式。他们如何分配/安排任务。

线程永远无法预测，不确定这本书在什么意义上说，但是 CPU 正在调度这些任务，并且永远不知道哪个先开始。您可以分配优先级、监控令牌等技术来控制多线程中的某些行为。

您在控制台上看到的内容可能令人困惑，但是，这样看，线程异步运行，这意味着如果两个进程同时执行Console.WriteLine，cpu 仍然需要决定首先显示哪个输出关于 CPU 的调度方式。