c#多线程之线程基础

一、简介

1.为了防止一个应用程序控制CPU而导致其他应用程序和操作系统本身永远被挂起这一可能情况，操作系统不得不使用某种方式将物理计算分割为一些虚拟的进程，并给予每个执行程序一定量的计算能力。此外操作系统必须始终能够优先访问CPU，并能调整不同程序访问CPU的优先级。线程正式这一慨念的实现。

2.多线程优缺点：
多线程优点：可以同时执行多个计算任务，有可能提高计算机的处理能力，使得计算机每秒能执行越来越多的命令
多线程缺点：消耗大量的操作系统资源。多个线程共享一个处理器将导致操作系统忙于管理这些线程，而无法运行程序。

二、创建线程

class Program
    {
        static void Main(string[] args)
        {
            Thread t1 = new Thread(new ThreadStart(PrintNumbers));//无参数的委托，把方法的引用当做参数
            t1.Start();

            Thread t2 = new Thread(new ParameterizedThreadStart(PrintNumbers));//有参数的委托，把方法的引用当做参数
            t2.Start(10);
            Console.ReadLine();
        }

        static void PrintNumbers()
        {
            Console.WriteLine("1.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine(i);
            }
        }

        //注意：要使用ParameterizedThreadStart，定义的参数必须为object
        static void PrintNumbers(object count)
        {
            Console.WriteLine("2.Starting...");
            for (int i = 0; i < Convert.ToInt32(count); i++)
            {
                Console.WriteLine(i);
            }
        }
    }
}

注释：

1.我们只需指定在不同线程运行的方法名，而C#编译器会在后台创建这些对象。

2.要使用ParameterizedThreadStart，定义的参数必须为object类型。

三、暂停线程

class Program
    {
        static void Main(string[] args)
        {
            Thread t1 = new Thread(PrintNumbersWithDelay);
            t1.Start();
            PrintNumbers();
            Console.ReadLine();
        }

        static void PrintNumbers()
        {
            Console.WriteLine("1.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("In 1.Starting: " + i);
            }
        }

        static void PrintNumbersWithDelay()
        {
            Console.WriteLine("2.Starting...");
            for (int i = 0; i < 10; i++)
            {
                //var a = TimeSpan.FromSeconds(2);
                Thread.Sleep(TimeSpan.FromSeconds(2));//暂停两秒
                Console.WriteLine("In 2.Starting: " + i);
            }
        }
    }

注释：使用Thread.Sleep(TimeSpan.FromSeconds(2));暂停线程一段时间

四、线程等待

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Starting...");
            Thread t = new Thread(PrintNumbersWithDelay);
            t.Start();
            t.Join();   //使用Join等待t完成后，再向下执行PrintNumbers，如果注释掉输出明显不同
            PrintNumbers();
            Console.WriteLine("Thread Complete");
            Console.ReadLine();
        }

        static void PrintNumbers()
        {
            Console.WriteLine("1.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("In 1.Starting:" + i);
            }
        }

        static void PrintNumbersWithDelay()
        {
            Console.WriteLine("2.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(2));
                Console.WriteLine("In 2.Starting:" + i);
            }
        }
    }

注释：使用t.Join(); 等待t完成。

五、终止线程

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Starting Program...");
            Thread t1 = new Thread(PrintNumbersWithDelay);
            t1.Start();
            Thread.Sleep(TimeSpan.FromSeconds(7));//此时t1线程会执行7秒
            t1.Abort();    //使用Abort()终止线程
            Console.WriteLine("Thread t1 has been aborted");
            Thread t2 = new Thread(PrintNumbers);
            t2.Start();
            Console.ReadLine();
        }

        static void PrintNumbers()
        {
            Console.WriteLine("1.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Console.WriteLine("In 1.Starting:" + i);
            }
        }
        static void PrintNumbersWithDelay()
        {
            Console.WriteLine("2.Starting...");
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(2));
                Console.WriteLine("In 2.Starting:" + i);
            }
        }
    }

注释：使用Thread实例的Abort方法终止线程。

六、检测线程状态

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Start Program...");
            Thread t1 = new Thread(PrintNumbersWithStatus);
            Thread t2 = new Thread(DoNothing);
            Console.WriteLine("t1 status:" + t1.ThreadState.ToString());//获取实例线程状态
            t2.Start();
            t1.Start();
            for (int i = 0; i < 30; i++)
            {
                Console.WriteLine("t1 status:" + t1.ThreadState.ToString() + "\t" + "t2 status:" + t2.ThreadState.ToString());
            }
            Thread.Sleep(TimeSpan.FromSeconds(7));
            t1.Abort();
            Console.WriteLine("thread t1 has been aborted");
            Console.WriteLine("t1 status:" + t1.ThreadState.ToString());
            Console.WriteLine("t2 status:" + t2.ThreadState.ToString());
            Console.ReadLine();
        }

        private static void PrintNumbersWithStatus()
        {
            Console.WriteLine("1.Starting...");
            Console.WriteLine("In 1.Starting t1 status:" + Thread.CurrentThread.ThreadState.ToString());//获取当前线程状态
            for (int i = 0; i < 10; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(2));
                Console.WriteLine("In 1.Starting:" + i);
            }
        }

        private static void DoNothing()
        {
            Thread.Sleep(TimeSpan.FromSeconds(2));
            Console.WriteLine("t2 Console...");
        }
    }

注释：使用Thread.ThreadState获取线程的运行状态。ThreadState是一个C#枚举。谨记：不要在程序中使用线程终止，否则可能会出现意想不到的结果

七、线程优先级

class Program
    {
        static void Main(string[] args)
        {
            //让操作系统的所有线程运行在多个CPU核心上
            Console.WriteLine($"Current thread priority: {Thread.CurrentThread.Priority}");
            Console.WriteLine("Running on all cores available");//获取实例线程状态
            RunThreads();

            Thread.Sleep(TimeSpan.FromSeconds(2));
            Console.WriteLine("Running on a single Core");
            //让操作系统的所有线程运行在单个CPU核心上
            Process.GetCurrentProcess().ProcessorAffinity = new IntPtr(1);
            RunThreads();
            Console.ReadLine();
        }

        private static void RunThreads()
        {
            var sample = new ThreadSample();
            var t1 = new Thread(sample.CountNumbers);
            t1.Name = "Thread One";
            var t2 = new Thread(sample.CountNumbers);
            t2.Name = "Thread Two";

            t1.Priority = ThreadPriority.Highest;//使用Priority设置线程的优先级
            t2.Priority = ThreadPriority.Lowest;
            t1.Start();
            t2.Start();//此处t2优先级低于t1,t2等待t1释放资源。

            Thread.Sleep(TimeSpan.FromSeconds(2));
            sample.Stop();
        }
    }

    class ThreadSample
    {
        private bool _isStopped = false;
        public void Stop()
        {
            _isStopped = true;
        }
        public void CountNumbers()
        {
            long counter = 0;
            while (!_isStopped)
            {
                counter++;
            }
            Console.WriteLine($"{Thread.CurrentThread.Name} with {Thread.CurrentThread.Priority} priority has a count={counter.ToString("N0")}");
        }
    }

注释：单核执行多线程耗费的时间比多核的多很多。

八、前台线程和后台线程

class Program
    {
        static void Main(string[] args)
        {
            var sampleForground = new ThreadSample(10);
            var sampleBackground = new ThreadSample(20);
            var t1 = new Thread(sampleForground.CountNumbers);//方法的引用
            t1.Name = "ForegroundThread"; //没有明确声明的均为前台线程

            var t2 = new Thread(sampleBackground.CountNumbers);
            t2.Name = "BackgroundThread";
            t2.IsBackground = true;   //设置为后台线程

            t1.Start();
            t2.Start();
            Console.ReadLine();

        }
    }
    class ThreadSample
    {
        private readonly int _iteration;

        public ThreadSample(int iteration)
        {
            _iteration = iteration;
        }

        public void CountNumbers()
        {
            for (int i = 0; i < _iteration; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine($"{Thread.CurrentThread.Name} prints {i}");
            }
        }
    }

注释：进程会等待所有的前台线程完成后再结束工作，但是如果只剩下后台线程，则会直接结束工作。

九、向线程传递参数

class Program
    {
        static void Main(string[] args)
        {
            ThreadSample sample = new ThreadSample(5);
            Thread t1 = new Thread(sample.CountNumbers);
            t1.Name = "ThreadOne";
            t1.Start();
            t1.Join();
            Console.WriteLine("--------------------------");

            Thread t2 = new Thread(Count);
            t2.Name = "ThreadTwo";
            t2.Start(3);
            t2.Join();
            Console.WriteLine("--------------------------");

            //使用lambda表达式引用另一个C#对方的方式被称为闭包。当在lambda表达式中使用任何局部变量时，C#会生成一个类，并将该变量作为该类的一个属性，但是我们无须定义该类，C#编译器会自动帮我们实现
            Thread t3 = new Thread(() => CountNumbers(5));
            t3.Name = "ThreadThree";
            t3.Start();
            t3.Join();
            Console.WriteLine("--------------------------");

            int i = 10;
            Thread t4 = new Thread(() => PrintNumber(i));

            i = 20;
            Thread t5 = new Thread(() => PrintNumber(i));
            t4.Start();
            t5.Start();
            //t4, t5都会输出20， 因为t4，t5没有Start之前i已经变成20了
            Console.ReadKey();
        }

        static void Count(object iterations)
        {
            CountNumbers((int)iterations);
        }

        static void CountNumbers(int iterations)
        {
            for (int i = 1; i <= iterations; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine($"{Thread.CurrentThread.Name} prints {i}");
            }
        }

        static void PrintNumber(int number)
        {
            Console.WriteLine(number);
        }
    }
    class ThreadSample
    {
        private readonly int _iteration;

        public ThreadSample(int iteration)
        {
            _iteration = iteration;
        }

        public void CountNumbers()
        {
            for (int i = 1; i <= _iteration; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine($"{Thread.CurrentThread.Name} prints {i}");
            }
        }
    }

十、使用C# Lock 关键字

class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Incorrect Counter");
            Counter c1 = new Counter();
            var t1 = new Thread(() => TestCounter(c1));
            var t2 = new Thread(() => TestCounter(c1));
            var t3 = new Thread(() => TestCounter(c1));
            t1.Start();
            t2.Start();
            t3.Start();
            t1.Join();
            t2.Join();
            t3.Join();
            Console.WriteLine($"Total Count: {c1.Count}");
            Console.WriteLine("------------------------");

            //使用LOCK关键字，Count同一时刻只允许一个线程访问
            Console.WriteLine("Correct counter");
            CounterWithLock c2 = new CounterWithLock();
            t1 = new Thread(() => TestCounter(c2));
            t2 = new Thread(() => TestCounter(c2));
            t3 = new Thread(() => TestCounter(c2));
            t1.Start();
            t2.Start();
            t3.Start();
            t1.Join();
            t2.Join();
            t3.Join();
            Console.WriteLine($"Total count:{c2.Count}");
            Console.ReadLine();
        }

        static void TestCounter(CounterBase c)
        {
            for (int i = 0; i < 100000; i++)
            {
                c.Increment();
                c.Decrement();
            }
        }

        //子类
        class Counter : CounterBase
        {
            public int Count { get; private set; }
            //重写基类方法
            public override void Decrement()
            {
                Count--;
            }

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

        //子类
        class CounterWithLock : CounterBase
        {
            private readonly object _asyncRoot = new object();
            public int Count { get; private set; }
            //重写基类方法
            public override void Decrement()
            {
                lock (_asyncRoot)
                {
                    Count--;
                }
            }

            public override void Increment()
            {
                lock (_asyncRoot)
                {
                    Count++;
                }
            }
        }

        //基类
        abstract class CounterBase
        {
            public abstract void Increment();

            public abstract void Decrement();
        }
    }
    class ThreadSample
    {
        private readonly int _iteration;

        public ThreadSample(int iteration)//构造函数
        {
            _iteration = iteration;
        }

        public void CountNumbers()
        {
            for (int i = 1; i <= _iteration; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(0.5));
                Console.WriteLine($"{Thread.CurrentThread.Name} prints {i}");
            }
        }
    }

注释：不加锁，得出的结果不确定，竞争条件下很容易出错。加锁得出的结果是正确的，但是性能受到了影响

十一、使用Monitor类锁定资源

class Program
    {
        static void Main(string[] args)
        {
            object lock1 = new object();
            object lock2 = new object();
            new Thread(() => LockTooMuch(lock1, lock2)).Start();
            lock (lock2)
            {
                Thread.Sleep(1000);
                Console.WriteLine("Monitor.TryEnter allows not to get stuck, returning false after a specified timeout is elapsed");
               
                //直接使用Monitor.TryEnter, 如果在第二个参数之前还未获取到lock保护的资源会返回false
                if (Monitor.TryEnter(lock1, TimeSpan.FromSeconds(5)))
                {
                    Console.WriteLine("Acquired a protected resource successfully");
                }
                else
                {
                    Console.WriteLine("Timeout acquiring a resource");
                }
            }
            new Thread(() => LockTooMuch(lock1, lock2)).Start();
            Console.WriteLine("-----------------------------");
         
            /* 下面代码会造成死锁， 所以注释掉
            lock (lock2)
            {
                Console.WriteLine("This will be a deadlock!");
                Thread.Sleep(1000);
                lock (lock1)
                {
                    Console.WriteLine("Acquired a protected resource successfully");
                }
            }
            */
        }

        static void LockTooMuch(object lock1, object lock2)
        {
            lock (lock1)
            {
                Thread.Sleep(1000);
                lock (lock2);
            }
        }
    }

注释：Monitor.TryEnter在指定的时间内尝试获取指定对象上的排他锁

十二、处理异常

class Program
    {
        static void Main(string[] args)
        {
            Thread t = new Thread(FaultyThread);
            t.Start();
            t.Join();
            try
            {
                t = new Thread(BadFaultyThread);
                t.Start();
            }
            catch (Exception ex)
            {
                Console.WriteLine("We won't get here");
            }
        }
        static void BadFaultyThread()
        {
            Console.WriteLine("Starting a bad faulty thread.....");
            Thread.Sleep(TimeSpan.FromSeconds(2));
            //这个异常主线程无法捕捉到，因为是在子线程抛出的异常。需要在子线程中加入try...catch捕获异常
            throw new Exception("Boom!");
        }
        static void FaultyThread()
        {
            try
            {
                Console.WriteLine("Starting a faulty thread...");
                Thread.Sleep(TimeSpan.FromSeconds(1));
                throw new Exception("Boom!");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"Exception handled: {ex.Message}");
            }
        }
    }

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