如何简化进入和退出ReaderWriterLock的过程？

Question

这对我来说似乎很嘈杂。五条线路的开销太大了。

m_Lock.EnterReadLock()
Try
    Return m_List.Count
Finally
    m_Lock.ExitReadLock()
End Try

那么，您如何简化这一过程呢？

Answer 1

我也是这么想的，但在C# ;-p

using System;
using System.Threading;

class Program
{
    static void Main()
    {
        ReaderWriterLockSlim sync = new ReaderWriterLockSlim();

        using (sync.Read())
        {
           // etc    
        }
    }


}
public static class ReaderWriterExt
{
    sealed class ReadLockToken : IDisposable
    {
        private ReaderWriterLockSlim sync;
        public ReadLockToken(ReaderWriterLockSlim sync)
        {
            this.sync = sync;
            sync.EnterReadLock();
        }
        public void Dispose()
        {
            if (sync != null)
            {
                sync.ExitReadLock();
                sync = null;
            }
        }
    }
    public static IDisposable Read(this ReaderWriterLockSlim obj)
    {
        return new ReadLockToken(obj);
    }
}

Answer 2

到目前为止，所有发布的解决方案都有死锁的风险。像这样的using块：

ReaderWriterLockSlim sync = new ReaderWriterLockSlim();
using (sync.Read())
{
  // Do stuff
}

被转换成类似这样的内容：

ReaderWriterLockSlim sync = new ReaderWriterLockSlim();
IDisposable d = sync.Read();
try
{
  // Do stuff
}
finally
{
  d.Dispose();
}

这意味着在sync.Read()和try块之间可能会发生ThreadAbortException (或类似的)。当这种情况发生时，finally块永远不会被调用，锁也永远不会被释放！

有关更多信息和更好的实现，请参阅：Deadlock with ReaderWriterLockSlim and other lock objects。简而言之，更好的实现归根结底是将锁移动到try块中，如下所示：

ReaderWriterLockSlim myLock = new ReaderWriterLockSlim();
try
{
    myLock.EnterReadLock();
    // Do stuff
}
finally
{
    // Release the lock
    myLock.ExitReadLock();
}

类似于接受答案中的包装器class将是：

  /// <summary>
  /// Manager for a lock object that acquires and releases the lock in a manner
  /// that avoids the common problem of deadlock within the using block
  /// initialisation.
  /// </summary>
  /// <remarks>
  /// This manager object is, by design, not itself thread-safe.
  /// </remarks>
  public sealed class ReaderWriterLockMgr : IDisposable
  {
    /// <summary>
    /// Local reference to the lock object managed
    /// </summary>
    private ReaderWriterLockSlim _readerWriterLock = null;

    private enum LockTypes { None, Read, Write, Upgradeable }
    /// <summary>
    /// The type of lock acquired by this manager
    /// </summary>
    private LockTypes _enteredLockType = LockTypes.None;

    /// <summary>
    /// Manager object construction that does not acquire any lock
    /// </summary>
    /// <param name="ReaderWriterLock">The lock object to manage</param>
    public ReaderWriterLockMgr(ReaderWriterLockSlim ReaderWriterLock)
    {
      if (ReaderWriterLock == null)
        throw new ArgumentNullException("ReaderWriterLock");
      _readerWriterLock = ReaderWriterLock;
    }

    /// <summary>
    /// Call EnterReadLock on the managed lock
    /// </summary>
    public void EnterReadLock()
    {
      if (_readerWriterLock == null)
        throw new ObjectDisposedException(GetType().FullName);
      if (_enteredLockType != LockTypes.None)
        throw new InvalidOperationException("Create a new ReaderWriterLockMgr for each state you wish to enter");
      // Allow exceptions by the Enter* call to propogate
      // and prevent updating of _enteredLockType
      _readerWriterLock.EnterReadLock();
      _enteredLockType = LockTypes.Read;
    }

    /// <summary>
    /// Call EnterWriteLock on the managed lock
    /// </summary>
    public void EnterWriteLock()
    {
      if (_readerWriterLock == null)
        throw new ObjectDisposedException(GetType().FullName);
      if (_enteredLockType != LockTypes.None)
        throw new InvalidOperationException("Create a new ReaderWriterLockMgr for each state you wish to enter");
      // Allow exceptions by the Enter* call to propogate
      // and prevent updating of _enteredLockType
      _readerWriterLock.EnterWriteLock();
      _enteredLockType = LockTypes.Write;
    }

    /// <summary>
    /// Call EnterUpgradeableReadLock on the managed lock
    /// </summary>
    public void EnterUpgradeableReadLock()
    {
      if (_readerWriterLock == null)
        throw new ObjectDisposedException(GetType().FullName);
      if (_enteredLockType != LockTypes.None)
        throw new InvalidOperationException("Create a new ReaderWriterLockMgr for each state you wish to enter");
      // Allow exceptions by the Enter* call to propogate
      // and prevent updating of _enteredLockType
      _readerWriterLock.EnterUpgradeableReadLock();
      _enteredLockType = LockTypes.Upgradeable;
    }

    /// <summary>
    /// Exit the lock, allowing re-entry later on whilst this manager is in scope
    /// </summary>
    /// <returns>Whether the lock was previously held</returns>
    public bool ExitLock()
    {
      switch (_enteredLockType)
      {
        case LockTypes.Read:
          _readerWriterLock.ExitReadLock();
          _enteredLockType = LockTypes.None;
          return true;
        case LockTypes.Write:
          _readerWriterLock.ExitWriteLock();
          _enteredLockType = LockTypes.None;
          return true;
        case LockTypes.Upgradeable:
          _readerWriterLock.ExitUpgradeableReadLock();
          _enteredLockType = LockTypes.None;
          return true;
      }
      return false;
    }

    /// <summary>
    /// Dispose of the lock manager, releasing any lock held
    /// </summary>
    public void Dispose()
    {
      if (_readerWriterLock != null)
      {
        ExitLock();
        // Tidy up managed resources
        // Release reference to the lock so that it gets garbage collected
        // when there are no more references to it
        _readerWriterLock = null;
        // Call GC.SupressFinalize to take this object off the finalization
        // queue and prevent finalization code for this object from
        // executing a second time.
        GC.SuppressFinalize(this);
      }
    }

    protected ~ReaderWriterLockMgr()
    {
      if (_readerWriterLock != null)
        ExitLock();
      // Leave references to managed resources so that the garbage collector can follow them
    }
  }

使用方法如下：

ReaderWriterLockSlim myLock = new ReaderWriterLockSlim();

using (ReaderWriterLockMgr lockMgr = new ReaderWriterLockMgr(myLock))
{
    lockMgr.EnterReadLock();
    // Do stuff
}

另外，来自Joe Duffy's Blog

接下来，锁对异步异常(如线程中止和内存不足)的健壮性不强。如果其中一个发生在锁的某个方法中，锁状态可能会被破坏，从而导致随后的死锁、未处理的异常，以及(可悲的)由于在内部使用自旋锁而导致的100% CPU挂起。因此，如果你打算在一个经常使用线程中止或试图在硬OOM中幸存下来的环境中运行你的代码，你不会对这个锁感到满意。

Answer 3

这不是我的发明，但它确实让头发不那么灰白了。

internal static class ReaderWriteLockExtensions
{
    private struct Disposable : IDisposable
    {
        private readonly Action m_action;
        private Sentinel m_sentinel;

        public Disposable(Action action)
        {
            m_action = action;
            m_sentinel = new Sentinel();
        }

        public void Dispose()
        {
            m_action();
            GC.SuppressFinalize(m_sentinel);
        }
    }

    private class Sentinel
    {
        ~Sentinel()
        {
            throw new InvalidOperationException("Lock not properly disposed.");
        }
    }

    public static IDisposable AcquireReadLock(this ReaderWriterLockSlim lock)
    {
        lock.EnterReadLock();
        return new Disposable(lock.ExitReadLock);
    }

    public static IDisposable AcquireUpgradableReadLock(this ReaderWriterLockSlim lock)
    {
        lock.EnterUpgradeableReadLock();
        return new Disposable(lock.ExitUpgradeableReadLock);
    }

    public static IDisposable AcquireWriteLock(this ReaderWriterLockSlim lock)
    {
        lock.EnterWriteLock();
        return new Disposable(lock.ExitWriteLock);
    }
} 

使用方法：

using (m_lock.AcquireReadLock())
{
    // Do stuff
}