为了使锁定机制允许锁定多个读取器(而不是一个写入器)访问某个资源，可以使用 ReaderWriterLockSlim 类。这个类提供了一个锁定功能，如果没有写入器锁定资源，就允许多个读取器访问资源，但只能有一个写入器锁定该资源。

ReaderWriterLockSlim 类有阻塞或不阻塞的方法来获取读取锁，如阻塞的EnterReadLock() 和不阻塞的 TryEnterReadLock() 方法，还可以使用阻塞的EnterWriteLock() 和不阻塞的 TryEnterWriteLock() 方法获得写入锁定。如果任务先读取资源,之后写入资源,它就可以使用 EnterUpgradableReadLock() 或 TryEnterUpgradableReadLock() 方法获得可升级的读取锁定。有了这个锁定，就可以获得写入锁定，而不需要释放读取锁定。

这个类的几个属性提供了当前锁定的相关信息。如CurrentReadCount、WaitingReadCount、WaitingUpgradableReadCount 和 WaitingWriteCount。

下面的示例程序创建了一个包含 6 项的集合和一个 ReaderWriterLockSlim() 对象。ReaderMethod() 方法获得一个读取锁定，读取列表中的所有项，并把它们写到控制台中。WriterMethod(0 方法试图获得一个写入锁定，以改变集合的所有值。在Main()方法中，启动6个任务，以调用 ReaderMethod()或WriterMethod()方法。

class Program
{private static List<int> _items = new List<int>() { 0，1，2，3，4，5}; private static ReaderWriterLockSlim _rwl =new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);public static void ReaderMethod(object reader)
{try{_rwl.EnterReadLock();For (int i = 0; i < _items.Count; i++){Console.WriteLine($"reader {reader}, loop: {i}, item: {_items[i]}"); Task.Delay(40).Wait();}}finally{_rwl.ExitReadLock();}}public static void WriterMethod(object writer)
{try{while (!_rwl.TryEnterWriteLock(50)){Console.WriteLine($"Writer {writer} waiting for the write lock"); Console.WriteLine($"current reader count:(_rwl.CurrentReadCount}");}Console.WriteLine($"Writer {writer) acquired the lock");for (int i = 0; i < _items.Count; i++){_items[i]++;Task.Delay(50).Wait();}Console.WriteLine($"Writer {writer} finished");}finally{_rwl.ExitWriteLock();}}static void Main()
{var taskFactory = new TaskFactory(TaskCreationOptions.LongRunning,TaskContinuationOptions.None); var tasks = new Task[6];tasks[0] = taskFactory.StartNew(WriterMethod, 1); tasks[1] = taskFactory.StartNew(ReaderMethod, 1);tasks[2] = taskFactory.StartNew(ReaderMethod, 2);tasks[3] = taskFactory.StartNew(WriterMethod, 2);tasks[4] = taskFactory.StartNew(ReaderMethod, 3); tasks[5] = taskFactory.StartNew(ReaderMethod, 4); Task.WaitAll(tasks);}}

运行这个应用程序，可以看到第一个写入器先获得锁定。第二个写入器和所有的读取器需要等待。接着，读取器可以同时工作，而第二个写入器仍在等待资源。

Writer 1 acquired the lock
Writer 2 waiting for the write lock
current reader count: 0
Writer 2 waiting for the write lock
current reader count: 0
Writer 2 waiting for the write lock
current reader count: 0
Writer 2 waiting for the write lock
current reader count: 0
Writer 1 finished
reader 4, loop: 0，item: 1
reader 1, loop: 0, item: 1
Writer 2 waiting for the write lock
current reader count: 4
reader 2, loop: 0，item: 1
reader 3, loop: 0, item: 1
reader 4，loop: 1, item: 2
reader 1, loop: 1, item: 2
reader 3, loop: 1，item: 2
reader 2, loop: 1, item: 2
Writer 2 waiting for the write lock
current reader count: 4
reader 4, loop: 2, item: 3
reader l, loop: 2，item: 3
reader 2, loop: 2，item: 3
reader 3, loop: 2, item: 3
Writer 2 waiting for the write lock
current reader count: 4
reader 4, loop: 3, item: 4
reader 1, loop: 3，item: 4
reader 2, loop: 3, item: 4
reader 3, loop: 3, item: 4
reader 4, loop: 4, item: 5
reader 1, loop: 4, item: 5
Writer 2 waiting for the write lock
current reader count: 4
reader 2, loop: 4, item: 5
reader 3, loop: 4, item: 5
reader 4, loop: 5, item: 6
reader 1，loop: 5，item: 6
reader 2, loop: 5, item: 6
reader 3, loop: 5, item: 6
Writer 2 waiting for the write lock
current reader count: 4
Writer 2 acquired the lock
Writer 2 finished

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