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(10);//模拟写信息
//Console.WriteLine(\”{0} Thread ID {1} writingend.\”, DateTime.Now.ToString(\”hh:mm:ss fff\”), Thread.CurrentThread.GetHashCode());
}
}
