Disposable reader/writer locks for C#

ReaderWriterLockSlimExtensions.cs

using System;
using System.Threading;

namespace RwLock
{
    public static class ReaderWriterLockSlimExtensions
    {
        private abstract class LockToken : IDisposable
        {
            protected ReaderWriterLockSlim RwLock { get; private set; }

            protected LockToken(ReaderWriterLockSlim rwLock)
            {
                RwLock = rwLock;
            }

            public void Dispose()
            {
                if (RwLock == null) return;
                Exit();
                RwLock = null;
            }

            protected abstract void Exit();
        }

        private sealed class ReadToken : LockToken
        {
            public ReadToken(ReaderWriterLockSlim rwLock) : base(rwLock)
            {
                rwLock.EnterReadLock();
            }

            protected override void Exit()
            {
                RwLock.ExitReadLock();
            }
        }

        private sealed class WriteToken : LockToken
        {
            public WriteToken(ReaderWriterLockSlim rwLock) : base(rwLock)
            {
                rwLock.EnterWriteLock();
            }

            protected override void Exit()
            {
                RwLock.ExitWriteLock();
            }
        }

        public static IDisposable Read(this ReaderWriterLockSlim rwLock)
        {
            return new ReadToken(rwLock);
        }

        public static IDisposable Write(this ReaderWriterLockSlim rwLock)
        {
            return new WriteToken(rwLock);
        }
    }
}