同时避免读写数据

有时，你希望线程同时共享数据。当发生这种情况时，重要的是要了解代码并锁定可能出错的任何部分。两个线程计数的简单示例如下所示。

这是一些危险的（不正确的）代码：

using System.Threading;

class MainClass 
{    
    static int count { get; set; }

    static void Main() 
    {
        for (int i = 1; i <= 2; i++)
        {
            var thread = new Thread(ThreadMethod);
            thread.Start(i);
            Thread.Sleep(500);
        }
    }

    static void ThreadMethod(object threadNumber) 
    {
        while (true)
        {
            var temp = count;
            System.Console.WriteLine("Thread " + threadNumber + ": Reading the value of count.");
            Thread.Sleep(1000);
            count = temp + 1;
            System.Console.WriteLine("Thread " + threadNumber + ": Incrementing the value of count to:" + count);
            Thread.Sleep(1000);
        }
    }
}

你会注意到，而不是计算 1,2,3,4,5 …我们计算 1,1,2,2,3 ……

要解决这个问题，我们需要锁定 count 的值，以便多个不同的线程不能同时读取和写入它。通过添加锁和密钥，我们可以防止线程同时访问数据。

using System.Threading;

class MainClass
{

    static int count { get; set; } 
    static readonly object key = new object();

    static void Main()
    {
        for (int i = 1; i <= 2; i++)
        {
            var thread = new Thread(ThreadMethod);
            thread.Start(i);
            Thread.Sleep(500);
        }
    }

    static void ThreadMethod(object threadNumber)
    {
        while (true)
        {
            lock (key) 
            {
                var temp = count;
                System.Console.WriteLine("Thread " + threadNumber + ": Reading the value of count.");
                Thread.Sleep(1000);
                count = temp + 1;
                System.Console.WriteLine("Thread " + threadNumber + ": Incrementing the value of count to:" + count);
            }
            Thread.Sleep(1000);
        }
    }
}