(精华)2021年02月19日 .NET Core 多线程底层详解(原子操作)

一：原则操作的概念

所谓原子操作是指不会被线程调度机制打断的操作；这种操作一旦开始，就一直运行到结束，中间不会有任何 context switch
（切换到另一个线程）。

原子操作案例

//Interlocked为原子操作相关类，a值为最终值，b可能因为同时操作返回不同的值
public static class AtomicSaple
{
    public static int a = 0;

    public static void Sample()
    {
        var b = 0;
        b = Interlocked.Increment(ref a);
        Console.WriteLine(b);

        b = Interlocked.Decrement(ref a);
        Console.WriteLine(b);

        b = Interlocked.Add(ref a, 2);
        Console.WriteLine(b);
		//a的值改变为1
        Interlocked.Exchange(ref a, 1);
		//如果a=0返回1
        Interlocked.CompareExchange(ref a,1,0);
    }
}

原子操作实现无锁算法

public class Counter
{
    public int TimesA { get; set; }
    public int TimesB { get; set; }
    public Counter(int timesA, int timesB)
    {
        TimesA = timesA;
        TimesB = timesB;
    }
}

public static class CounterSample
{
    public static Counter Counter1 = new Counter(0, 0);
    /// <summary>
    /// 递增计数器，返回递增后的值
    /// </summary>
    /// <param name="counter"></param>
    /// <returns></returns>
    public static Counter Increment(ref Counter counter)
    {
        Counter oldCounter, newCounter;
        do
        {
            oldCounter = counter;
            newCounter = new Counter(oldCounter.TimesA + 1, oldCounter.TimesB + 1);

            // 使用循环确保成功
        } while (Interlocked.CompareExchange(ref counter, newCounter,oldCounter) != oldCounter);

        return newCounter; 
    }

    public static void IncrementCounter()
    {
        var result = Increment(ref Counter1);
        Console.WriteLine($"TimesA={result.TimesA},TimesB={result.TimesB}");
    }

    public static void ThreadIncrement()
    {
        var thread1 = new Thread(IncrementCounter);
        var thread2 = new Thread(IncrementCounter);

        thread1.Start();
        thread2.Start();
    }

    public static void PrintCounter()
    {
        var c = Counter1;
        Console.WriteLine($"Counter={c.TimesA},{c.TimesB}");
    }

    public static void Run()
    {
        ThreadIncrement();
        Thread.Sleep(1000);
        PrintCounter();
    }
}