C# Monitor類的使用

C#中， 通過System.Threading.Monitor類可以實現多線程中對某些代碼塊的同步訪問，以確保數據的安全性。

object obj=new object();

Monitor在鎖對象obj上會維持兩個線程隊列R和W以及一個引用T :

(1) T是對當前獲得了obj鎖的線程的引用（設此線程為CurrThread）; 

(2) R為就緒隊列， 其上的線程已經準備好獲取obj鎖。當obj鎖被CurrThread釋放后（CurrThread可通過Monitor.Exit(obj)或 Monitor.Wait(obj)來釋放其所獲的obj鎖）這些線程就會去競爭obj鎖，獲得obj鎖的線程將被T引用; 線程調用Monitor.Enter(obj)或Monitor.TryEnter(obj)將會使該線程直接進入R隊列。

(3) W為等待隊列，其上的線程是因為調用了Monitor.Wait(obj)而進入W隊列的；W上的線程不會被OS直接調度執行，也就是說它們沒有準備好獲取obj鎖，就是說在等待隊列上的線程不能去獲得obj鎖。當前獲得obj鎖的線程CurrThread調用Monitor.Pulse(obj)或Monitor.PulseAll(obj)后會使W隊列中的第一個等待線程或所有等待線程被移至R隊列，這時被移至R隊列的這些線程就有機會被OS直接調度執行，也就是有可以去競爭obj鎖。

Monitor類中的重要方法:

例子1：

private object lockObj = new object();
        private void Execute()
        {
            Monitor.Enter(lockObj);
            for (int i = 0; i < 5;i++ )
            {
                Console.WriteLine("Thread Name:" + Thread.CurrentThread.Name + ", Count:" + i);
                Thread.Sleep(new Random().Next(5000));
            }
            Monitor.Exit(lockObj);
        }
        public void Test()
        {
            Thread thread1 = new Thread(new ThreadStart(Execute));
            thread1.Name = "Thread1";
            thread1.Start();
            Thread thread2 = new Thread(new ThreadStart(Execute));
            thread2.Name = "Thread2";
            thread2.Start();
            
            thread1.Join();
            thread2.Join();
        }

輸出結果:

Thread Name:Thread1, Count:0
Thread Name:Thread1, Count:1
Thread Name:Thread1, Count:2
Thread Name:Thread1, Count:3
Thread Name:Thread1, Count:4
Thread Name:Thread2, Count:0
Thread Name:Thread2, Count:1
Thread Name:Thread2, Count:2
Thread Name:Thread2, Count:3
Thread Name:Thread2, Count:4

例子2：

         private void Execute1()
        {
            if (Monitor.TryEnter(lockObj,1000))
            {
                for (int i = 0; i < 5; i++)
                {
                    Console.WriteLine("Thread Name:" + Thread.CurrentThread.Name + ", Count:" + i);
                    Thread.Sleep(new Random().Next(5000));
                }
                Monitor.Exit(lockObj);
            }
        }
        public void Test1()
        {
            Thread thread1 = new Thread(new ThreadStart(Execute1));
            thread1.Name = "Thread1";
            thread1.Start();
            Thread thread2 = new Thread(new ThreadStart(Execute1));
            thread2.Name = "Thread2";
            thread2.Start();
           
            thread1.Join();
            thread2.Join();
        }

輸出結果:

Thread Name:Thread1, Count:0
Thread Name:Thread1, Count:1
Thread Name:Thread1, Count:2
Thread Name:Thread1, Count:3
Thread Name:Thread1, Count:4

因為Thread2嘗試獲取鎖失敗，所以沒有執行鎖內部的代碼塊。

例子3:

        private Queue queue = new Queue();
        public void SendThread()
        {
            int counter = 0;
            lock (queue)
            {
                while (counter < 5)
                {
                    //Wait, if the queue is busy.
                    Monitor.Wait(queue);
                    //Push one element.
                    queue.Enqueue(counter);
                    //Release the waiting thread.
                    Monitor.Pulse(queue);
                    counter++;
                }
            }
        }
        public void ConsumeThread()
        {
            lock (queue)
            {
                //Release the waiting thread.
                Monitor.Pulse(queue);
                //Wait in the loop, while the queue is busy.
                //Exit on the time-out when the first thread stops.
                while (Monitor.Wait(queue, 1000))
                {
                    //Pop the first element.
                    int counter = (int)queue.Dequeue();
                    //Print the first element.
                    Console.WriteLine("Queue Value:" + counter.ToString());
                    //Release the waiting thread.
                    Monitor.Pulse(queue);
                }
            }
        }
        public void Test2()
        {
            Thread thread1 = new Thread(new ThreadStart(SendThread));
            thread1.Name = "Thread1";
            thread1.Start();
            Thread thread2 = new Thread(new ThreadStart(ConsumeThread));
            thread2.Name = "Thread2";
            thread2.Start();
            thread1.Join();
            thread2.Join();
            Console.WriteLine("Queue Size:" + queue.Count);
        }

輸出結果:

Queue Value:0
Queue Value:1
Queue Value:2
Queue Value:3
Queue Value:4
Queue Size:0