同時避免讀寫資料

有時，你希望執行緒同時共享資料。當發生這種情況時，重要的是要了解程式碼並鎖定可能出錯的任何部分。兩個執行緒計數的簡單示例如下所示。

這是一些危險的（不正確的）程式碼：

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);
        }
    }
}

你會注意到，而不是計算 1,2,3,4,5 …我們計算 1,1,2,2,3 ……

要解決這個問題，我們需要鎖定 count 的值，以便多個不同的執行緒不能同時讀取和寫入它。通過新增鎖和金鑰，我們可以防止執行緒同時訪問資料。

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);
        }
    }
}