C# Thread Lock 筆記

多線程， 當使用共有變量時， 保持計算結果正確很重要：

看下面的例子:

private static int count = 10;
        //private static object objlock = new object();

        public static void Main(string[] args)
        {
            try
            {
                for (int i = 0; i < 20; i++)
                {
                    Thread t1 = new Thread(new ThreadStart(WriteNumber));
                    t1.Name = "Name-" + i.ToString();
                    t1.Start();
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }

            Console.ReadLine();
        }

        public static void WriteNumber()
        {
            //lock (objlock)
            //{
                count--;
                Thread.Sleep(1000);
                Console.WriteLine("Name:" + Thread.CurrentThread.Name + " count:" + count.ToString());
            //}
        }

結果這樣，全錯了。

 

加上lock以後， 代碼修改成這樣

private static int count = 10;
        private static object objlock = new object();

        public static void Main(string[] args)
        {
            try
            {
                for (int i = 0; i < 20; i++)
                {
                    Thread t1 = new Thread(new ThreadStart(WriteNumber));
                    t1.Name = "Name-" + i.ToString();
                    t1.Start();
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }

            Console.ReadLine();
        }

        public static void WriteNumber()
        {
            lock (objlock)
            {
                count--;
                Thread.Sleep(1000);
                Console.WriteLine("Name:" + Thread.CurrentThread.Name + " count:" + count.ToString());
            }
        }

運行結果是這樣：

 

注意：private static object objlock = new object();  //定義這個變量的位置，

各線程運行前後， 由CPU隨機執行，　每次運行均可能看到不一樣的前後順序。