如何使用C#读写锁ReaderWriterLockSlim

读写锁的概念很简单，允许多个线程同时获取读锁，但同一时间只允许一个线程获得写锁，因此也称作共享-独占锁。在C#中，推荐使用ReaderWriterLockSlim类来完成读写锁的功能。
某些场合下，对一个对象的读取次数远远大于修改次数，如果只是简单的用lock方式加锁，则会影响读取的效率。而如果采用读写锁，则多个线程可以同时读取该对象，只有等到对象被写入锁占用的时候，才会阻塞。
简单的说，当某个线程进入读取模式时，此时其他线程依然能进入读取模式，假设此时一个线程要进入写入模式，那么他不得不被阻塞。直到读取模式退出为止。
同样的，如果某个线程进入了写入模式，那么其他线程无论是要写入还是读取，都是会被阻塞的。
进入写入/读取模式有2种方法：
EnterReadLock尝试进入写入模式锁定状态。
TryEnterReadLock(Int32) 尝试进入读取模式锁定状态，可以选择整数超时时间。
EnterWriteLock 尝试进入写入模式锁定状态。
TryEnterWriteLock(Int32) 尝试进入写入模式锁定状态，可以选择超时时间。
退出写入/读取模式有2种方法：
ExitReadLock 减少读取模式的递归计数，并在生成的计数为 0（零）时退出读取模式。
ExitWriteLock 减少写入模式的递归计数，并在生成的计数为 0（零）时退出写入模式。
下面演示一下用法：

public class Program
  {
    static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
    static void Main(string[] args)
    {
      Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
      t_read1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
      Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
      t_read2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
      Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
      t_write1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
    }
    static public void ReadSomething()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterReadLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(5000);//模拟读取信息
        Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitReadLock();
        Console.WriteLine("{0} Thread ID {1} ExitReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
    static public void WriteSomething()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterWriteLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(10000);//模拟写入信息
        Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitWriteLock();
        Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
  }

可以看到3号线程和4号线程能够同时进入读模式，而5号线程过了5秒钟后（即3,4号线程退出读锁后），才能进入写模式。
把上述代码修改一下，先开启2个写模式的线程，然后在开启读模式线程，代码如下：

 static void Main(string[] args)
    {
      Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
      t_write1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
      Thread t_write2 = new Thread(new ThreadStart(WriteSomething));
      t_write2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write2.GetHashCode());
      Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
      t_read1.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
      Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
      t_read2.Start();
      Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
    }

结果如下：

可以看到，3号线程和4号线程都要进入写模式，但是3号线程先占用写入锁，因此4号线程不得不等了10s后才进入。5号线程和6号线程需要占用读取锁，因此等4号线程退出写入锁后才能继续下去。
TryEnterReadLock和TryEnterWriteLock可以设置一个超时时间，运行到这句话的时候，线程会阻塞在此，如果此时能占用锁，那么返回true，如果到超时时间还未占用锁，那么返回false，放弃锁的占用，直接继续执行下面的代码。
EnterUpgradeableReadLock
ReaderWriterLockSlim类提供了可升级读模式，这种方式和读模式的区别在于它还有通过调用 EnterWriteLock 或 TryEnterWriteLock 方法升级为写入模式。 因为每次只能有一个线程处于可升级模式。进入可升级模式的线程，不会影响读取模式的线程，即当一个线程进入可升级模式，任意数量线程可以同时进入读取模式，不会阻塞。如果有多个线程已经在等待获取写入锁，那么运行EnterUpgradeableReadLock将会阻塞，直到那些线程超时或者退出写入锁。
下面代码演示了如何在可升级读模式下，升级到写入锁。

static public void UpgradeableRead()
    {
      Console.WriteLine("{0} Thread ID {1} Begin EnterUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      rwl.EnterUpgradeableReadLock();
      try
      {
        Console.WriteLine("{0} Thread ID {1} doing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        rwl.EnterWriteLock();
        try
        {
          Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
          Thread.Sleep(10000);//模拟写入信息
          Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        }
        finally
        {
          rwl.ExitWriteLock();
          Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        }
        Thread.Sleep(10000);//模拟读取信息
        Console.WriteLine("{0} Thread ID {1} doing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitUpgradeableReadLock();
        Console.WriteLine("{0} Thread ID {1} ExitUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }

读写锁对于性能的影响是明显的。
下面测试代码：

public class Program
  {
    static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
    static void Main(string[] args)
    {
      Stopwatch sw = new Stopwatch();
      sw.Start();
      List<Task> lstTask = new List<Task>();
      for (int i = 0; i < 500; i++)
      {
        if (i % 25 != 0)
        {
          var t = Task.Factory.StartNew(ReadSomething);
          lstTask.Add(t);
        }
        else
        {
          var t = Task.Factory.StartNew(WriteSomething);
          lstTask.Add(t);
        }
      }
      Task.WaitAll(lstTask.ToArray());
      sw.Stop();
      Console.WriteLine("使用ReaderWriterLockSlim方式，耗时：" + sw.Elapsed);
      sw.Restart();
      lstTask = new List<Task>();
      for (int i = 0; i < 500; i++)
      {
        if (i % 25 != 0)
        {
          var t = Task.Factory.StartNew(ReadSomething_lock);
          lstTask.Add(t);
        }
        else
        {
          var t = Task.Factory.StartNew(WriteSomething_lock);
          lstTask.Add(t);
        }
      }
      Task.WaitAll(lstTask.ToArray());
      sw.Stop();
      Console.WriteLine("使用lock方式，耗时：" + sw.Elapsed);
    }
    static private object _lock1 = new object();
    static public void ReadSomething_lock()
    {
      lock (_lock1)
      {
        //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(10);//模拟读取信息
        //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
    static public void WriteSomething_lock()
    {
      lock (_lock1)
      {
        //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(100);//模拟写入信息
        //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
    }
    static public void ReadSomething()
    {
      rwl.EnterReadLock();
      try
      {
        //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(10);//模拟读取信息
        //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitReadLock();
      }
    }
    static public void WriteSomething()
    {
      rwl.EnterWriteLock();
      try
      {
        //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
        Thread.Sleep(100);//模拟写入信息
        //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
      }
      finally
      {
        rwl.ExitWriteLock();
      }
    }
  }

上述代码，就500个Task，每个Task占用一个线程池线程，其中20个写入线程和480个读取线程，模拟操作。其中读取数据花10ms，写入操作花100ms，分别测试了对于lock方式和ReaderWriterLockSlim方式。可以做一个估算，对于ReaderWriterLockSlim，假设480个线程同时读取，那么消耗10ms，20个写入操作占用2000ms，因此所消耗时间2010ms，而对于普通的lock方式，由于都是独占性的，因此480个读取操作占时间4800ms+20个写入操作2000ms=6800ms。运行结果显示了性能提升明显。

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