C#多线程编程实战（二）：线程同步

2.1 简介

竞争条件：多个线程同时使用共享对象。需要同步这些线程使得共享对象的操作能够以正确的顺序执行

线程同步问题：多线程的执行并没有正确的同步，当一个线程执行递增和递减操作时，其他线程需要依次等待

线程同步解决方案：

无须共享对象：大部分时候可以通过重新设计来移除共享对象，去掉复杂的同步构造，避免多线程使用单一对象

必须共享对象：只使用原子操作，一个操作只占用一个量子的时间，无须实现其他线程等待当前操作完成

内核模式：将等待的线程置于阻塞状态，消耗少量的CPU资源，但会引入至少一次上下文切换，适用于线程等待较长时间

用户模式：只是简单的等待，线程等待会浪费CPU时间但是可以节省上下文切换消耗的CPU时间，适用于线程等待较短时间

混合模式：先尝试用户模式，如果等待时间较长，则会切换到内核模式

2.2 执行基本的原子操作

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Incorrect counter");
            //没有限定，会遇到竞争条件，得出的结果大部分不是正确的
            var c = new Counter();
            var t1 = new Thread(() => TestCounter(c));
            var t2 = new Thread(() => TestCounter(c));
            var t3 = new Thread(() => TestCounter(c));
            t1.Start();
            t2.Start();
            t3.Start();
            t1.Join();
            t2.Join();
            t3.Join();
            Console.WriteLine($"Total count:{c.Count}");
            Console.WriteLine("--------------------------");
            Console.WriteLine("Correct counter");
            //使用Interlocked类提供的原子操作方法，无需锁定任何对象可得出正确结果
            var c1 = new CounterNoLock();
            t1 = new Thread(() => TestCounter(c1));
            t2 = new Thread(() => TestCounter(c1));
            t3 = new Thread(() => TestCounter(c1));
            t1.Start();
            t2.Start();
            t3.Start();
            t1.Join();
            t2.Join();
            t3.Join();
            Console.WriteLine($"Total count:{c1.Count}");
            Console.ReadLine();
        }
        static void TestCounter(CounterBase c)
        {
            for (int i = 0; i < 100000; i++)
            {
                c.Increment();
                c.Decrement();
            }
        }
        class Counter : CounterBase
        {
            private int _count;
            public int Count { get { return _count; } }

            public override void Decrement()
            {
                _count--;
            }

            public override void Increment()
            {
                _count++;
            }
        }

        class CounterNoLock : CounterBase
        {
            private int _count;
            public int Count { get { return _count; } }

            public override void Decrement()
            {
                //Interlocked提供了Increment()、Decrement()和Add等基本数学操作的原子方法
                Interlocked.Decrement(ref _count);
            }

            public override void Increment()
            {
                Interlocked.Increment(ref _count);
            }
        }

        abstract class CounterBase
        {
            public abstract void Increment();
            public abstract void Decrement();
        }
    }
}

注释：Interlocked提供了Increment()、Decrement()和Add等基本数学操作的原子方法，不用锁也可以得出正确结果

2.3 使用Mutex类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
        {
            const string MutexName = "CSharpThreadingCookbook";
            //Mutex是一种原始的同步方式，只对一个线程授予对共享资源的独占访问
            //定义一个指定名称的互斥量，设置initialOwner标志为false
            using (var m = new Mutex(false, MutexName))
            {
                //如果互斥量已经被创建，获取互斥量，否则就执行else语句
                if (!m.WaitOne(TimeSpan.FromSeconds(5), false))
                {
                    Console.WriteLine("Second instance is running!");
                }
                else
                {
                    Console.WriteLine("Running!");
                    Console.ReadLine();
                    m.ReleaseMutex();
                }
            }
            //如果再运行同样的程序，则会在5秒内尝试获取互斥量，如果第一个程序按下了任何键，第二个程序开始执行。
            //如果等待5秒钟，第二个程序将无法获取该互斥量
        }
    }
}

注释：互斥量是全局操作对象，必须正确关闭，最好用using

2.4 使用SemaphoreSlim类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            //启动6个线程，启动的顺序不一样
            for (int i = 0; i <= 6; i++)
            {
                string threadName = "Thread " + i;
                int secondsToWait = 2 + 2 * i;
                var t = new Thread(() => AccessDatabase(threadName, secondsToWait));
                t.Start();
            }

            Console.ReadLine();
        }
        //SemaphoreSlim的构造函数参数为允许的并发线程数目
        static SemaphoreSlim semaphore = new SemaphoreSlim(4);

        static void AccessDatabase(string name, int seconds)
        {
            Console.WriteLine($"{name} waits to access a database");
            semaphore.Wait();
            Console.WriteLine($"{name} was granted an access to a database");
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine($"{name} is Completed");
            //调用Release方法说明线程已经完成，可以开启一个新的线程了
            semaphore.Release();
        }
    }
}

注释：这里使用了混合模式，允许我们在等待时间很短的情况下无需上下文切换。SemaphoreSlim并不使用Windows内核信号量，而且也不支持进程间同步

2.5 使用AutoResetEvent类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            Thread t = new Thread(() => Process(10));
            t.Start();
            Console.WriteLine("Waiting for another thread to complete work");
            //开启一个线程后workEvent等待，直到收到Set信号
            workEvent.WaitOne();
            Console.WriteLine("First operation is complete");
            Console.WriteLine("Performing an operation on a main thread");
            Thread.Sleep(TimeSpan.FromSeconds(5));
            mainEvent.Set();
            Console.WriteLine("Now running the second operation on a second thread");
            workEvent.WaitOne();
            Console.WriteLine("Second operation is complete");
            Console.ReadLine();
        }
        //初始状态为unsignaled，子线程向主线程发信号
        private static AutoResetEvent workEvent = new AutoResetEvent(false);
        //初始状态为unsignaled，主线程向子线程发信号
        private static AutoResetEvent mainEvent = new AutoResetEvent(false);

        static void Process(int seconds)
        {
            Console.WriteLine("Starting a long running work...");
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine("Work is done!");
            workEvent.Set();//将事件设为终止状态允许一个或多个线程继续
            Console.WriteLine("Waiting for a main thread to complete its work");
            
            mainEvent.WaitOne();//阻止当前线程，直到mainEvent收到信号
            Console.WriteLine("Starting second operation...");
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine("Work is done");
            workEvent.Set();
        }
    }
}

注释：AutoResetEvent采用的是内核模式，所以等待时间不能太长

2.6 使用ManualResetEventSlim类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            Thread t1 = new Thread(() => TravelThroughGates("Thread 1", 5));
            Thread t2 = new Thread(() => TravelThroughGates("Thread 2", 6));
            Thread t3 = new Thread(() => TravelThroughGates("Thread 3", 12));
            t1.Start();
            t2.Start();
            t3.Start();
            Thread.Sleep(TimeSpan.FromSeconds(6));
            Console.WriteLine("The gates are now open");
            mainEvent.Set();//将时间设置为有信号，从而让一个或多个等待该事件的线程继续
            Thread.Sleep(TimeSpan.FromSeconds(2));
            mainEvent.Reset();//将事件设置为非终止，从而导致线程受阻
            Console.WriteLine("The gates have been closed!");
            Thread.Sleep(TimeSpan.FromSeconds(10));
            Console.WriteLine("The gates are now open for the second time");
            mainEvent.Set();
            Thread.Sleep(TimeSpan.FromSeconds(2));
            Console.WriteLine("The gates have been closed!");
            mainEvent.Reset();
            Console.ReadLine();
        }
        static ManualResetEventSlim mainEvent = new ManualResetEventSlim(false);

        static void TravelThroughGates(string threadName, int seconds)
        {
            Console.WriteLine($"{threadName} falls to sleep");
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine($"{threadName} waits for the gates to open!");
            mainEvent.Wait();//阻止当前线程
            Console.WriteLine($"{threadName} enter the gates!");
        }
    }
}

注释：ManualResetEventSlim工作方式像人群通过的大门，一直保持大门敞开直到调用reset，set相当于打开大门，reset相当于关闭大门

2.7 使用CountDownEvent类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            Console.WriteLine("Starting two operations");
            Thread t1 = new Thread(() => PerformOperation("Operation 1 is completed", 4));
            Thread t2 = new Thread(() => PerformOperation("Operation 2 is completed", 8));
            t1.Start();
            t2.Start();
            //开启了两个线程，调用Wait方法阻止当前线程，知道所有线程都完成
            countdown.Wait();
            Console.WriteLine("Both operations have been completed");
            countdown.Dispose();
            Console.ReadLine();
        }
        //计数器初始化CountdownEvent实例，计数器表示：当计数器个数完成操作发出信号
        static CountdownEvent countdown = new CountdownEvent(2);

        static void PerformOperation(string message, int seconds)
        {
            Thread.Sleep(TimeSpan.FromSeconds(seconds));
            Console.WriteLine(message);
            //向CountdownEvent注册信息，并减少当前计数器数值
            countdown.Signal();
        }
    }
}

注释：如果Signal方法没有达到指定的次数，那么countdown.wait()会一直等待，所以请确保所有线程完成后都要调用Signal方法

2.8 使用Barrier类

using System;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            Thread t1 = new Thread(() => PlayMusic("the gutarist", "play an amazing solo", 5));
            Thread t2 = new Thread(() => PlayMusic("the signer", "sing his song", 2));
            t1.Start();
            t2.Start();
            Console.ReadLine();
        }
        //后面的Lamda表达式是回调函数。执行完SignalAndWait后执行
        static Barrier barrier = new Barrier(2, b=>Console.WriteLine($"End of phase {b.CurrentPhaseNumber + 1}"));

        static void PlayMusic(string name, string message, int seconds)
        {
            for (int i = 0; i < 3; i++)
            {
                Console.WriteLine("===========================");
                Thread.Sleep(TimeSpan.FromSeconds(seconds));
                Console.WriteLine($"{name} starts to {message}");
                Thread.Sleep(TimeSpan.FromSeconds(seconds));
                Console.WriteLine($"{name} finishes to {message}");
                //等所有调用线程都结束
                barrier.SignalAndWait();
            }
        }
    }
}

注释：

2.9 使用ReaderWriterlockSlim类

using System;
using System.Collections.Generic;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            new Thread(Read) { IsBackground = true }.Start();
            new Thread(Read) { IsBackground = true }.Start();
            new Thread(Read) { IsBackground = true }.Start();
            new Thread(() => Write("Thread 1")) { IsBackground = true }.Start();
            new Thread(() => Write("Thread 2")) { IsBackground = true }.Start();
            Thread.Sleep(TimeSpan.FromSeconds(30));
            Console.ReadLine();
        }
        //实现线程安全
        static ReaderWriterLockSlim _rw = new ReaderWriterLockSlim();
        static Dictionary<int, int> items = new Dictionary<int, int>();

        static void Read()
        {
            Console.WriteLine("Reading contents of a dictionary");
            while (true)
            {
                try
                {
                    //读锁
                    _rw.EnterReadLock();
                    foreach (var key in items.Keys)
                    {
                        Thread.Sleep(TimeSpan.FromSeconds(0.1));
                    }
                }
                finally
                {
                    //计数为0时退出读取模式
                    _rw.ExitReadLock();
                }
            }
        }

        static void Write(string threadName)
        {
            while (true)
            {
                try
                {
                    int newKey = new Random().Next(250);
                    _rw.EnterUpgradeableReadLock();
                    if (!items.ContainsKey(newKey))
                    {
                        try
                        {
                            //写锁
                            _rw.EnterWriteLock();
                            items[newKey] = 1;
                            Console.WriteLine($"New key {newKey} is added to a dictionary by a {threadName}");
                        }
                        finally
                        {
                            //计数为0时退出写入模式
                            _rw.ExitWriteLock();
                        }
                    }
                    Thread.Sleep(TimeSpan.FromSeconds(0.1));
                }
                finally
                {
                    //计数为0时退出可升级模式
                    _rw.ExitUpgradeableReadLock();
                }
            }
        }
    }
}

注释：从集合读取数据时，根据当前数据决定是否获取一个写锁并修改该集合。获取写锁后集合会处于阻塞状态。

2.10 使用SpinWait类

using System;
using System.Collections.Generic;
using System.Threading;

namespace MulityThreadNote
{
    class Program
    {
        static void Main(string[] args)
         {
            Thread t1 = new Thread(UserModeWait);
            Thread t2 = new Thread(HybridSpinWait);
            Console.WriteLine("Running user mode waiting");
            t1.Start();
            Thread.Sleep(20);
            _isComplete = true;
            Thread.Sleep(TimeSpan.FromSeconds(1));
            _isComplete = false;
            Console.WriteLine("Running hybrid SpinWait construct waiting");
            t2.Start();
            Thread.Sleep(5);
            _isComplete = true;
            Console.ReadLine();
        }
        //volatile 一个字段可能会被多个线程同时修改，不会被编译器和处理器优化为只能被单个线程访问
        static volatile bool _isComplete = false;

        static void UserModeWait()
        {
            while (!_isComplete)
            {
                Console.WriteLine(".");
            }
            Console.WriteLine();
            Console.WriteLine("Waiting is complete");
        }

        static void HybridSpinWait()
        {
            var w = new SpinWait();
            while (!_isComplete)
            {
                //执行单一自旋
                w.SpinOnce();
                //NextSpinWillYield:获取对SpinOnce的下一次调用是否将产生处理，同时触发强制上下文切换
                //显示线程是否切换为阻塞状态
                Console.WriteLine(w.NextSpinWillYield);
            }
            Console.WriteLine("Waiting is complete");
        }
    }
}

注释：