写在前面

以下是微软官方对屏障类的介绍，System.Threading.Barrier 可用来作为实现并发同步操作的基本单元，让多个线程(参与者)分阶段并行处理目标算法。在达到代码中的屏障点之前，每个参与者将继续执行，屏障表示工作阶段的末尾；单个参与者到达屏障后将被阻止，直至所有参与者都已达到同一障碍。 所有参与者都已达到屏障后，你可以选择调用阶段后操作。 此阶段后操作可由单线程用于执行操作，而所有其他线程仍被阻止。执行此操作后，所有参与者将不受阻止，继续执行直到满足退出条件。

下面的程序用于统计两个线程使用随机算法重新随机选择字词，分别在同一阶段查找一半解决方案时所需的迭代次数（或阶段数）。在每个线程随机选择字词后，屏障后阶段操作会比较两个结果，以确定整个句子是否按正确的字词顺序呈现。

关键代码如下：

barrier.SignalAndWait()

 设置了代码屏障点，代码运行到这里会等待所有参与的线程都执行完之前的代码。

代码实现

//#define TRACE
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace BarrierSimple
{
    class Program
    {
        static string[] words1 = new string[] { "brown", "jumps", "the", "fox", "quick" };
        static string[] words2 = new string[] { "dog", "lazy", "the", "over" };
        static string solution = "the quick brown fox jumps over the lazy dog.";

        static bool success = false;
        static Barrier barrier = new Barrier(2, (b) =>
        {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < words1.Length; i++)
            {
                sb.Append(words1[i]);
                sb.Append(" ");
            }
            for (int i = 0; i < words2.Length; i++)
            {
                sb.Append(words2[i]);

                if (i < words2.Length - 1)
                    sb.Append(" ");
            }
            sb.Append(".");
#if TRACE
            System.Diagnostics.Trace.WriteLine(sb.ToString());
#endif
            Console.CursorLeft = 0;
            Console.Write("Current phase: {0}", barrier.CurrentPhaseNumber);
            if (String.CompareOrdinal(solution, sb.ToString()) == 0)
            {
                success = true;
                Console.WriteLine("\r\nThe solution was found in {0} attempts", barrier.CurrentPhaseNumber);
            }
        });

        static void Main(string[] args)
        {

            Thread t1 = new Thread(() => Solve(words1));
            Thread t2 = new Thread(() => Solve(words2));
            t1.Start();
            t2.Start();

            // Keep the console window open.
            Console.ReadLine();
        }

        // Use Knuth-Fisher-Yates shuffle to randomly reorder each array.
        // For simplicity, we require that both wordArrays be solved in the same phase.
        // Success of right or left side only is not stored and does not count.
        static void Solve(string[] wordArray)
        {
            while (success == false)
            {
                Random random = new Random();
                for (int i = wordArray.Length - 1; i > 0; i--)
                {
                    int swapIndex = random.Next(i + 1);
                    string temp = wordArray[i];
                    wordArray[i] = wordArray[swapIndex];
                    wordArray[swapIndex] = temp;
                }

                // We need to stop here to examine results
                // of all thread activity. This is done in the post-phase
                // delegate that is defined in the Barrier constructor.
                barrier.SignalAndWait();
            }
        }
    }
}

调用示例