C#多线程编程实例实战(2)

释放阅读锁的函数原型为：public void ReleaseReaderLock()。函数定义如下：

public void ReleaseReaderLock()

{   

System.LocalDataStoreSlot slot = Thread.GetNamedDataSlot(m_strThreadSlotName );

LockFlags flag = (LockFlags)Thread.GetData( slot );

if( flag == LockFlags.None )

{   return;

}
bool bReader = true;  switch( flag )

{   

case LockFlags.None:   

break;

case LockFlags.Writer:

bReader = false;

break;

}

if( !bReader )

return;

Thread.SetData( slot, LockFlags.None );

m_mutex.WaitOne();

AutoResetEvent autoresetevent = null;

this.m_nActive --;

if( this.m_nActive == 0 )

{    if( this.m_nWaitingReaders > 0 )

{   

m_nActive ++ ;   

m_nWaitingReaders --;   

autoresetevent = this.m_aeReaders;

}   

else if( this.m_nWaitingWriters > 0)   

{  

m_nWaitingWriters--;

m_nActive --;

autoresetevent = this.m_aeWriters ;

}    }

m_mutex.ReleaseMutex();   

if( autoresetevent != null )   

autoresetevent.Set();   

}

释放阅读锁时，首先判断当前线程是否拥有阅读锁（通过线程局部存储的标志），然后判断是否有等待的阅读线程，如果有，先将当前活动线程加1，等待阅读线程数目减1，然后置事件为有信号。如果没有等待的阅读线程，判断是否有等待的写入线程，如果有则活动线程数目减1，等待的写入线程数目减1。释放写入锁与释放阅读锁的过程基本一致，可以参看源代码。

注意在程序中，释放锁时，只会唤醒一个阅读程序，这是因为使用AutoResetEvent的原历，读者可自行将其改成ManualResetEvent，同时唤醒多个阅读程序，此时应令m_nActive等于整个等待的阅读线程数目。

测试

测试程序取自.Net FrameSDK中的一个例子，只是稍做修改。测试程序如下，

using System;   

using System.Threading;  

using MyThreading;   

class Resource {   

myReaderWriterLock rwl = new myReaderWriterLock();   

public void Read(Int32 threadNum) {   

rwl.AcquireReaderLock(Timeout.Infinite);

try {   Console.WriteLine("Start Resource reading (Thread={0})", threadNum);

Thread.Sleep(250);

Console.WriteLine("Stop Resource reading (Thread={0})", threadNum);

}

finally {  rwl.ReleaseReaderLock();

}   }

public void Write(Int32 threadNum) {   

rwl.AcquireWriterLock(Timeout.Infinite);   

try {   

Console.WriteLine("Start Resource writing (Thread={0})", threadNum);

Thread.Sleep(750);

Console.WriteLine("Stop Resource writing (Thread={0})", threadNum);

}

finally {  rwl.ReleaseWriterLock();   

}   }   

}   

class App {

static Int32 numAsyncOps = 20;   

static AutoResetEvent asyncOpsAreDone = new AutoResetEvent(false);   

static Resource res = new Resource();      

public static void Main() {   

for (Int32 threadNum = 0; threadNum < 20; threadNum++) {   

ThreadPool.QueueUserWorkItem(new WaitCallback(UpdateResource), threadNum);   

}

asyncOpsAreDone.WaitOne();

Console.WriteLine("All operations have completed.");   

Console.ReadLine();   

}

// The callback method''s signature MUST match that of a System.Threading.TimerCallback   

// delegate (it takes an Object parameter and returns void)  

static void UpdateResource(Object state) {   

Int32 threadNum = (Int32) state;   

if ((threadNum % 2) != 0) res.Read(threadNum);   

else res.Write(threadNum);   

if (Interlocked.Decrement(ref numAsyncOps) == 0)   

asyncOpsAreDone.Set();   

}   

}