NET多线程同步方法详解(三)：读写锁（ReadWriteLock)

读写锁的出现主要是在很多情况下，我们读资源的操作要多于写资源的操作。但是如果每次只对资源赋予一个线程的访问权限显然是低效的，读写锁的优势是同时可以有多个线程对同一资源进行读操作。因此在读操作比写操作多很多，并且写操作的时间很短的情况下使用读写锁是比较有效率的。读写锁是一个非静态类所以你在使用前需要先声明一个读写锁对象： 

static private ReaderWriterLock _rwlock = new ReaderWriterLock(); 

    读写锁是通过调用AcquireReaderLock，ReleaseReaderLock，AcquireWriterLock，ReleaseWriterLock来完成读锁和写锁控制的 

        static public void ReaderThread(int thrdId) 
        { 
            try 
            { // 请求读锁，如果100ms超时退出 
                _rwlock.AcquireReaderLock(10); 
                try 
                { 
                    int inx = _rand.Next(_list.Count); 
                    if (inx < _list.Count) 
                        Console.WriteLine("{0}thread {1}", thrdId, _list[inx]); 
                } 
                finally 
                {
                    _rwlock.ReleaseReaderLock(); 
                } 
            } 
            catch (ApplicationException) // 如果请求读锁失败 
            { 
                Console.WriteLine("{0}thread get reader lock out time!", thrdId); 
            } 
        } 
        static public void WriterThread() 
        { 
            try 
            {
                // 请求写锁 
                _rwlock.AcquireWriterLock(100); 
                try 
                { 
                    string val = _rand.Next(200).ToString(); 
                    _list.Add(val); // 写入资源 
                    Console.WriteLine("writer thread has written {0}", val); 
                } 
                finally 
                { // 释放写锁 
                    _rwlock.ReleaseWriterLock(); 
                } 
            } 
            catch (ApplicationException) 
            { 
                Console.WriteLine("Get writer thread lock out time!"); 
            } 
        } 

    如果你想在读的时候插入写操作请使用UpgradeToWriterLock和DowngradeFromWriterLock来进行操作，而不是释放读锁。

        static private void UpgradeAndDowngrade(int thrdId) 
        { 
            try 
            { 
                _rwlock.AcquireReaderLock(10); 
                try 
                { 
                    try 
                    {
                        // 提升读锁到写锁 
                        LockCookie lc = _rwlock.UpgradeToWriterLock(100);
                        try
                        {
                            string val = _rand.Next(500).ToString();
                            _list.Add(val); Console.WriteLine("Upgrade Thread{0} add {1}", thrdId, val); 
                        } 
                        finally
                        { // 下降写锁 
                            _rwlock.DowngradeFromWriterLock(ref lc); 
                        } 
                    } 
                    catch (ApplicationException)
                    { 
                        Console.WriteLine("{0}thread upgrade reader lock failed!", thrdId); 
                    } 
                } 
                finally 
                {
                    // 释放原来的读锁 
                    _rwlock.ReleaseReaderLock();
                }
            } 
            catch (ApplicationException) 
            { 
                Console.WriteLine("{0}thread get reader lock out time!", thrdId);
            }
        }

     这里有一点要注意的就是读锁和写锁的超时等待时间间隔的设置。通常情况下设置写锁的等待超时要比读锁的长，否则会经常发生写锁等待失败的情况。