为什么以下C#多线程代码虽然在调试器中没有输出零？

Question

class Program
{
    private static volatile int value;

    public static void Increment()
    {
        for (int i =0; i <100000; i++)
        {
            value++;
        }
    }

    public static void Decrement()
    {
        for (int j =0 ; j < 100000; j++)
        {
            value--;
        }
    }

    public static void ThreadTest()
    {
        value = 0;

        var incrementThread = new Thread(Increment);

        var decrementThread = new Thread(Decrement);

        incrementThread.Start();

        decrementThread.Start();

        incrementThread.Join();

        decrementThread.Join();

        Console.WriteLine("Value of value {0}", value);
    }

    static void Main(string[] args)
    {
        ThreadTest();
    }
}

Answer 1

因为它不应该...... ++和 - 不是原子操作(不像Interlocked.XXXX opreations - Interlocked.Increment).

如果你写下++的每一步,并且 - 看看两者如何通过不同的线程混合,你就会明白为什么:

增量

1: load value to temp
2: add temp, 1
3: store temp to value

减量

4: load value to temp2
5: substruct temp2, 1
6: store temp2 to value

因此,如果订单是1,2,3,4,5,6,则得到值= 0; 但如果订单是1,2,4,5,6,3,则得到值= 1.