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falseusing (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：

转载于:https://www.cnblogs.com/wangyulong/p/7759788.html