S7131 — A write lock should not be released when a read lock has been acquired and vice versa

When using ReaderWriterLock and ReaderWriterLockSlim for managing read and write locks, you should not release a read lock while holding a write lock and vice versa, otherwise you might have runtime exceptions. The locks should be always correctly paired so that the shared resource is accessed safely.

This rule raises if:

• you call ReaderWriterLock.AcquireWriterLock or ReaderWriterLock.UpgradeToWriterLock and then use ReaderWriterLock.ReleaseReaderLock
• you call ReaderWriterLockSlim.EnterWriteLock or ReaderWriterLockSlim.TryEnterWriteLock and then use ReaderWriterLockSlim.ExitReadLock
• you call ReaderWriterLock.AcquireReaderLock or ReaderWriterLock.DowngradeFromWriterLock and then use ReaderWriterLock.ReleaseWriterLock
• or you call ReaderWriterLockSlim.EnterReadLock, ReaderWriterLockSlim.TryEnterReadLock, ReaderWriterLockSlim.EnterUpgradeableReadLock or ReaderWriterLockSlim.TryEnterUpgradeableReadLock and then use ReaderWriterLockSlim.ExitWriteLock

Why is this an issue?

If you use the ReaderWriterLockSlim class, you will get a LockRecursionException. In the case of ReaderWriterLock, you’ll get a runtime exception for trying to release a lock that is not owned by the calling thread.

Code examples

Noncompliant code example

public class Example
{
    private static ReaderWriterLock rwLock = new();

    public void Writer()
    {
        rwLock.AcquireWriterLock(2000);
        try
        {
            // ...
        }
        finally
        {
            rwLock.ReleaseReaderLock(); // Noncompliant, will throw runtime exception
        }
    }

    public void Reader()
    {
        rwLock.AcquireReaderLock(2000);
        try
        {
            // ...
        }
        finally
        {
            rwLock.ReleaseWriterLock(); // Noncompliant, will throw runtime exception
        }
    }
}

Compliant solution

public class Example
{
    private static ReaderWriterLock rwLock = new();

    public static void Writer()
    {
        rwLock.AcquireWriterLock(2000);
        try
        {
            // ...
        }
        finally
        {
            rwLock.ReleaseWriterLock();
        }
    }

    public static void Reader()
    {
        rwLock.AcquireReaderLock(2000);
        try
        {
            // ...
        }
        finally
        {
            rwLock.ReleaseReaderLock();
        }
    }
}

Resources

Documentation

• Microsoft Learn - ReaderWriterLock Class
• Microsoft Learn - ReaderWriterLockSlim