Question

这是Cuts在Nutshell中提供的一个示例程序的略微修改版本：

using System;
using System.Threading;

namespace SemaphorTest
{
    class Program
    {
        static Semaphore gate = new Semaphore(3, 3);
        static void Main(string[] args)
        {
            for (int i = 0; i < 5; i++)
            {
                Thread t = new Thread(Enter);
                t.Start(i);
            }
        }

        public static void Enter(Object id)
        {
            Console.WriteLine("Thread " + id + " wants to enter.");
            gate.WaitOne();
            Console.WriteLine("Thread " + id + " is in.");
            Thread.Sleep(500 * (Int32)id);
            Console.WriteLine("Thread " + id + " leaving.");
            gate.Release();
        }
    }
}

这将打印以下输出（看似以随机顺序）：

Thread 0 wants to enter.
Thread 1 wants to enter.
Thread 1 is in.
Thread 0 is in.
Thread 2 wants to enter.
Thread 3 wants to enter.
Thread 0 leaving.
Thread 3 is in.
Thread 4 wants to enter.
Thread 2 is in.
Thread 1 leaving.
Thread 4 is in.
Thread 2 leaving.
Thread 3 leaving.
Thread 4 leaving.

但是，如下添加Thread.Join（）会极大地改变输出。我：

for (int i = 0; i < 5; i++)
            {
                Thread t = new Thread(Enter);
                t.Start(i);
                t.Join();
            }

将输出更改为：

Thread 0 wants to enter.
Thread 0 is in.
Thread 0 leaving.
Thread 1 wants to enter.
Thread 1 is in.
Thread 1 leaving.
Thread 2 wants to enter.
Thread 2 is in.
Thread 2 leaving.
Thread 3 wants to enter.
Thread 3 is in.
Thread 3 leaving.
Thread 4 wants to enter.
Thread 4 is in.
Thread 4 leaving.

为什么会这样？我知道这些线程默认是前台线程，并且主线程不需要等待它们完成（它们将在Main完成后运行，因此它们不需要thread.Join（））。但我不明白是什么让它们按顺序运行，因为它们是在更改后创建的。有什么想法吗？

Thanks
Dileep Balakrishnan

Answer 1

为什么会这样？

因为你问过它！

你已经启动了一个线程，然后等待它完成，然后启动下一个线程等。这正是Thread.Join所做的：它阻塞当前正在执行的线程，直到你调用它的线程终止

我感兴趣的是你期望要执行的代码...如果你只是想等到所有线程都完成之后才让主线程完成，你需要启动所有线程，在你去的时候记住它们，然后依次在每个线程上调用Join。例如：

List<Thread> threads = new List<Thread>();
// First start all the threads
for (int i = 0; i < 5; i++)
{
    Thread t = new Thread(Enter);
    t.Start(i);
    threads.Add(t);
}

// Then wait for them to finish
foreach (var thread in threads)
{
    thread.Join();
}

Answer 2

您正在使用调用线程加入每个线程。这将基本上阻止调用线程并等待每个线程依次结束。

在documentation中声明：

阻止调用线程，直到线程终止。

Answer 3

Thread.join（）不会使事情同时运行。它实际上通过阻止当前线程直到它完成来强制执行您所看到的行为。文档中明确提到了这一点：http://msdn.microsoft.com/en-us/library/95hbf2ta.aspx

Thread.Join（）在C＃中更改程序输出

3 个答案: