C# 使用屏障来使多线程并发操作保持同步

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 TRACESystem.Diagnostics.Trace.WriteLine(sb.ToString());
#endifConsole.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();}}}
}