.NET 線程同步方法

1 使用Monitor來同步

Monitor類提供兩個靜態方法Monitor.Enter()和Monitor.Exit()來保證代碼的同步。有一點要保證的是調用了Monitor.Enter()後必須有一個相對應的Monitor.Exit()。所以，一個很好的做法是將這兩個方法用一個Try/Finally塊包圍起來。

示例：

 class Program
    {
        public const int _total = 100000;
        //聲明爲readonly防止在其他地方被修改
        readonly static object _sync = new object();
        static long _count = 0;
        static void Main(string[] args)
        {
            ThreadStart threadStart =new ThreadStart(DoWork);
            Thread thread = new Thread(DoWork);
            thread.Start();
           
            Thread t = Thread.CurrentThread;
            t.Name = "Main thread";
            for (int i = 0; i < _total; i++)
            {
                try
                {
                Monitor.Enter(_sync);

               //Console.Write("+");
                    _count++;
                }
                finally
                {
                    Monitor.Exit(_sync);
                }
            }
            thread.Join();
            Console.WriteLine(t.Name+"has finish");
            Console.WriteLine("Count:" + _count);
            Console.Read();
        }

        public static void DoWork()
        {
            Thread.CurrentThread.Name = " NewThread ";
            for (int i = 0; i < _total; i++)
            {
                try
                {
                    Monitor.Enter(_sync);
                    _count--;
                    //Console.Write("-");
                }
                finally
                {
                    Monitor.Exit(_sync);
                }
            }
            Console.WriteLine(Thread.CurrentThread.Name + "has finish");
            Console.WriteLine("Count:" + _count);
        }
    }

輸出：

Count爲0.

若把Monitor.Enter()和Monitor.Exit()註釋掉的話，Count就不一定爲0，特別是循環次數_total 特別大的時候。

Monitor類有一個缺點就是對性能影響很大。方法中有使用Monitor.Enter()和沒有使用執行時間相差很大。

2 使用Lock關鍵字

如果你覺得Monitor要既要寫Enter()又要調用Exit()很麻煩，那你可以使用Lock()關鍵字。並且使用Lock()的性能會比Monitor好一點點。

class Program
    {
        public const int _total = 100000;
        //聲明爲readonly防止在其他地方被修改
        readonly static object _sync = new object();
        static long _count = 0;
        static void Main(string[] args)
        {
            ThreadStart threadStart =new ThreadStart(DoWork);
            Thread thread = new Thread(DoWork);
            thread.Start();
           
            Thread t = Thread.CurrentThread;
            t.Name = "Main thread";
            for (int i = 0; i < _total; i++)
            {
                lock (_sync)
                {
                    _count++;
                }
            }
            thread.Join();
            Console.WriteLine(t.Name+"has finish");
            Console.WriteLine("Count:" + _count);

            Console.Read();
        }

        public static void DoWork()
        {
            Thread.CurrentThread.Name = " NewThread ";
            for (int i = 0; i < _total; i++)
                lock (_sync)
                {
                    _count--;
                }
            }
            Console.WriteLine(Thread.CurrentThread.Name + "has finish");
            Console.WriteLine("Count:" + _count);
        }
    }

3 MethodImplAttribute類

MethodImplAttribute會告訴CLR方法是如何實現的，其屬性設置爲MethodImplOptions.Synchronized時，它指定在任一時刻只允許一個線程訪問這個方法。

 class MethodImpi
    {
        [MethodImpl(MethodImplOptions.Synchronized)]
        public static void DoSomeWorkSync()
        {
            Thread.CurrentThread.Name="Sync"+DateTime.Now.Ticks;
            Console.WriteLine("DoSomeWorkSync() --Lock  by Thread" + Thread.CurrentThread.GetHashCode());
            Thread.Sleep(5 * 1000);
            Console.WriteLine("DoSomeWorkSync() --Released by Thread" + Thread.CurrentThread.GetHashCode());
        }

        public static void DoSomeWorkNoSync()
        {
            Thread.CurrentThread.Name = "Sync" + DateTime.Now.Ticks;
            Console.WriteLine("DoSomeWorkNoSync() --Lock  by Thread" + Thread.CurrentThread.GetHashCode());
            Thread.Sleep(5 * 1000);
            Console.WriteLine("DoSomeWorkNoSync() --Released by Thread" + Thread.CurrentThread.GetHashCode());
        }
        static void Main(string[] args)
        {
            Thread th1 = new Thread(DoSomeWorkSync);
            Thread th2 = new Thread(DoSomeWorkSync);
            th1.Start();
            th2.Start();

            Thread th3 = new Thread(DoSomeWorkNoSync);
            Thread th4 = new Thread(DoSomeWorkNoSync);
            th3.Start();
            th4.Start();
        }
    }

結果：