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 Shared rwLock As New ReaderWriterLock()

    Public Sub Writer()
        rwLock.AcquireWriterLock(2000)
        Try
            ' ...
        Finally
            rwLock.ReleaseReaderLock() ' Noncompliant, will throw runtime exception
        End Try
    End Sub

    Public Sub Reader()
        rwLock.AcquireReaderLock(2000)
        Try
            ' ...
        Finally
            rwLock.ReleaseWriterLock() ' Noncompliant, will throw runtime exception
        End Try
    End Sub

End Class

Compliant solution

Public Class Example

    Private Shared rwLock As New ReaderWriterLock()

    Public Shared Sub Writer()
        rwLock.AcquireWriterLock(2000)
        Try
            ' ...
        Finally
            rwLock.ReleaseWriterLock()
        End Try
    End Sub

    Public Shared Sub Reader()
        rwLock.AcquireReaderLock(2000)
        Try
            ' ...
        Finally
            rwLock.ReleaseReaderLock()
        End Try
    End Sub

End Class

Resources

Documentation

• Microsoft Learn - ReaderWriterLock Class
• Microsoft Learn - ReaderWriterLockSlim