Dispose ReaderWriterLockSlim after ExitWriterLock

Question

I have a function to clean up some objects as well as the ReaderWriterLockSlim. But I need the ReaderWriterLockSlim to lock as writer lock to prevent the other thread read the data while I am doing the clean up.

ConcurrentDictionary<string, ReaderWriterLockSlim> RwLocks = new ConcurrentDictionary<string, ReaderWriterLockSlim>();

private ReaderWriterLockSlim GetRwLock(string key)
{
    return RwLocks.GetOrAdd(key, _ => new ReaderWriterLockSlim());
}

public void CleanUp(string key)
{
    ReaderWriterLockSlim rwLock = this.GetRwLock(key);

    try 
    {
        rwLock.EnterWriterLock();
        // do some other clean up
        this.RwLocks.TryRemove(key, out _);
    }
    finally
    {
        rwLock.ExitWriterLock();
        // It is safe to do dispose here?
        // could other thread enter the read lock or writer lock here?
        // and the dispose will throw exceptions?
        // What is the best practice to do the dispose?
        rwLock.Dispose();
    }
}

---

I have an idea to wrap the ReaderWriterLockSlim. Do you think it could solve the problem or have any potential risk

public class ReaderWriterLockSlimWrapper
{
    private ReaderWriterLockSlim rwLock;

    private volatile bool disposed;

    public ReaderWriterLockSlimWrapper()
    {
        rwLock = new ReaderWriterLockSlim();
        disposed = false;
    }

    private void DisposeInternal()
    {
        if (!rwLock.IsReadLockHeld && !rwLock.IsUpgradeableReadLockHeld && !rwLock.IsWriteLockHeld)
        {
            rwLock.Dispose();
        }
    }

    public void Dispose()
    {
        disposed = true;

        DisposeInternal();
    }

    public void EnterReadLock()
    {
        rwLock.EnterReadLock();
    }

    public void ExitReadLock()
    {
        rwLock.ExitReadLock();

        if (disposed)
        {
            DisposeInternal();
        }
    }

    public void EnterWriteLock()
    {
        rwLock.EnterWriteLock();
    }

    public void ExitWriteLock()
    {
        rwLock.ExitWriteLock();

        if (disposed)
        {
            DisposeInternal();
        }
    }
}

Answer 1

You haven't described the specific scenario where you intend to use your two mechanisms, neither for the first one with the CleanUp/GetRwLock methods, nor for the second one with the ReaderWriterLockSlimWrapper class. So I guess the question is:

Are my two mechanisms safe to use in all possible multithreaded scenarios, where thread-safety and atomicity of operations is mandatory?

The answer is no, both of your mechanisms are riddled with race conditions, and offer no guarantees about atomicity. Using them in a multithreaded scenario would result in undefined behavior, including but not limited to:

1. Unexpected exceptions.
2. Violations of the policies that a correctly used ReaderWriterLockSlim is normally expected to enforce. In order words it is possible that two threads will acquire a writer lock for the same key concurrently to each other, or concurrently with others threads that have acquired a reader lock for the same key, or both.

Explaining why your mechanisms are flawed is quite involved. A general explanation is that whenever you use the pattern if (x.BooleanProperty) x.Method(); in a multithreaded environment, although the BooleanProperty and the Method might be individually thread-safe, you are allowing a second thread to preempt the current thread between the two invocations, and invalidate the result of the first check.

As a side note, be aware that the ReaderWriterLockSlim is not a cross-process synchronization primitive. So even if you fix your mechanisms and then attempt to use them in a web application, the policies might still be violated because the web server might decide at random moments to recycle the current process and start a new one. In that case the web application might by running concurrently on two processes, for a period of time that spans a few seconds or even minutes.