您将如何简化进入和退出 ReaderWriterLock

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

我也是这么想的，但在 C# 中 ;-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);
    }
}

到目前为止发布的所有解决方案都有死锁的风险。像这样的使用块：

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

转换为如下所示的内容：

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

这意味着同步之间可能会发生 ThreadAbortException(或类似异常(。Read(( 和 try 块。当这种情况发生时，最终块永远不会被调用，锁永远不会被释放！

有关详细信息和更好的实现，请参阅：与 ReaderWriterLockSlim 和其他锁对象的死锁。简而言之，更好的实现归结为将锁移动到try块中，如下所示：

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

像接受答案中的包装类这样的包装类将是：

  /// <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
    }
  }

按如下方式进行用法：

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

另外，来自乔·达菲的博客

接下来，锁对异步异常(如线程中止和内存不足情况(不可靠。 如果在其中一个锁的方法中间发生其中一种情况，则锁状态可能会损坏，从而导致随后的死锁、未经处理的异常，并且(可悲的是(由于内部使用旋转锁，固定的 100% CPU。 因此，如果您要在经常使用线程中止或尝试在硬 OOM 中生存的环境中运行代码，那么您不会对此锁感到满意。

这不是我的发明，但它肯定使头发变得不那么白白。

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);
    }
} 

如何使用：

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

我最终做到了，但我仍然对设计中的更好方法或缺陷持开放态度。

Using m_Lock.ReadSection
    Return m_List.Count
End Using

这使用以下扩展方法/类：

<Extension()> Public Function ReadSection(ByVal lock As ReaderWriterLockSlim) As ReadWrapper
    Return New ReadWrapper(lock)
End Function

Public NotInheritable Class ReadWrapper
    Implements IDisposable
    Private m_Lock As ReaderWriterLockSlim
    Public Sub New(ByVal lock As ReaderWriterLockSlim)
        m_Lock = lock
        m_Lock.EnterReadLock()
    End Sub
    Public Sub Dispose() Implements IDisposable.Dispose
        m_Lock.ExitReadLock()
    End Sub
End Class

由于锁的目的是保护某些内存，我认为将该内存包装在"锁定"对象中会很有用，并且只能通过各种锁令牌访问它(如 Mark 所述(：

// Stores a private List<T>, only accessible through lock tokens
//  returned by Read, Write, and UpgradableRead.
var lockedList = new LockedList<T>( );

using( var r = lockedList.Read( ) ) {
  foreach( T item in r.Reader )
    ...
}

using( var w = lockedList.Write( ) ) {
  w.Writer.Add( new T( ) );
}

T t = ...;
using( var u = lockedList.UpgradableRead( ) ) {
  if( !u.Reader.Contains( t ) )
    using( var w = u.Upgrade( ) )
      w.Writer.Add( t );
}

现在，访问内部列表的唯一方法是调用相应的访问器。

这对List<T>特别有效，因为它已经有了ReadOnlyCollection<T>包装器。对于其他类型，您总是可以创建一个Locked<T,T>，但是您就失去了可读/可写类型的区别。

一项改进可能是将R和W类型定义为一次性包装器本身，这将防止(无意中(错误，例如：

List<T> list;
using( var w = lockedList.Write( ) )
  list = w.Writable;
//BAD: "locked" object leaked outside of lock scope
list.MakeChangesWithoutHoldingLock( );

但是，这将使Locked使用起来更加复杂，并且当前版本确实为您提供了与手动锁定共享成员时相同的保护。

---

sealed class LockedList<T> : Locked<List<T>, ReadOnlyCollection<T>> {
  public LockedList( )
    : base( new List<T>( ), list => list.AsReadOnly( ) )
  { }
}
public class Locked<W, R> where W : class where R : class {
  private readonly LockerState state_;
  public Locked( W writer, R reader ) { this.state_ = new LockerState( reader, writer ); }
  public Locked( W writer, Func<W, R> getReader ) : this( writer, getReader( writer ) ) { }
  public IReadable Read( ) { return new Readable( this.state_ ); }
  public IWritable Write( ) { return new Writable( this.state_ ); }
  public IUpgradable UpgradableRead( ) { return new Upgradable( this.state_ ); }

  public interface IReadable : IDisposable { R Reader { get; } }
  public interface IWritable : IDisposable { W Writer { get; } }
  public interface IUpgradable : IReadable { IWritable Upgrade( );}

  #region Private Implementation Details
  sealed class LockerState {
    public readonly R Reader;
    public readonly W Writer;
    public readonly ReaderWriterLockSlim Sync;
    public LockerState( R reader, W writer ) {
      Debug.Assert( reader != null && writer != null );
      this.Reader = reader;
      this.Writer = writer;
      this.Sync = new ReaderWriterLockSlim( );
    }
  }
  abstract class Accessor : IDisposable {
    private LockerState state_;
    protected LockerState State { get { return this.state_; } }
    protected Accessor( LockerState state ) {
      Debug.Assert( state != null );
      this.Acquire( state.Sync );
      this.state_ = state;
    }
    protected abstract void Acquire( ReaderWriterLockSlim sync );
    protected abstract void Release( ReaderWriterLockSlim sync );
    public void Dispose( ) {
      if( this.state_ != null ) {
        var sync = this.state_.Sync;
        this.state_ = null;
        this.Release( sync );
      }
    }
  }
  class Readable : Accessor, IReadable {
    public Readable( LockerState state ) : base( state ) { }
    public R Reader { get { return this.State.Reader; } }
    protected override void Acquire( ReaderWriterLockSlim sync ) { sync.EnterReadLock( ); }
    protected override void Release( ReaderWriterLockSlim sync ) { sync.ExitReadLock( ); }
  }
  sealed class Writable : Accessor, IWritable {
    public Writable( LockerState state ) : base( state ) { }
    public W Writer { get { return this.State.Writer; } }
    protected override void Acquire( ReaderWriterLockSlim sync ) { sync.EnterWriteLock( ); }
    protected override void Release( ReaderWriterLockSlim sync ) { sync.ExitWriteLock( ); }
  }
  sealed class Upgradable : Readable, IUpgradable {
    public Upgradable( LockerState state ) : base( state ) { }
    public IWritable Upgrade( ) { return new Writable( this.State ); }
    protected override void Acquire( ReaderWriterLockSlim sync ) { sync.EnterUpgradeableReadLock( ); }
    protected override void Release( ReaderWriterLockSlim sync ) { sync.ExitUpgradeableReadLock( ); }
  }
  #endregion
}