RwLock

using System;
using System.Threading;

public sealed class RwLock : IDisposable
{
    private readonly ReaderWriterLockSlim _innerLock = new ReaderWriterLockSlim();
    private bool _disposed = false;

    public IDisposable Read()
    {
        return new Reader(this._innerLock);
    }

    public IDisposable Write()
    {
        return new Writer(this._innerLock);
    }

    public void Dispose()
    {
        if (!_disposed)
        {
            _disposed = true;
            _innerLock.Dispose();
        }
    }

    private class Reader : IDisposable
    {
        private readonly ReaderWriterLockSlim _rwLock;
        public Reader(ReaderWriterLockSlim rwLock)
        {
            this._rwLock = rwLock;
            this._rwLock.EnterReadLock();
        }

        public void Dispose()
        {
            _rwLock.ExitReadLock();
        }
    }

    private class Writer : IDisposable
    {
        private readonly ReaderWriterLockSlim _rwLock;
        public Writer(ReaderWriterLockSlim rwLock)
        {
            this._rwLock = rwLock;
            this._rwLock.EnterWriteLock();
        }

        public void Dispose()
        {
            _rwLock.ExitWriteLock();
        }
    }
}

RwLock.fs

// Reed Copsey wrote this
type RwLock() =
    let rwLock = ReaderWriterLockSlim();

    member __.Read() =
        rwLock.EnterReadLock()
        { new IDisposable with
            member __.Dispose() = rwLock.ExitReadLock()
        }
    member __.Write() =
        rwLock.EnterWriteLock()
        { new IDisposable with
            member __.Dispose() = rwLock.ExitWriteLock()
        }
    interface IDisposable with
        member __.Dispose() =
            rwLock.Dispose()

Sample.cs

using (_rwLock.Read())
{
    _innerList.CopyTo(array, arrayIndex);
}