ReaderWriterLockSlim: acquiring a read lock after an upgradeable lock doesn't throw LockRecursionException

Question

Concerning the ReaderWriterLockSlim:

Acquiring two locks subsequently within the same thread should actually throw a LockRecursionException (the recursion policy is set to NoRecursion).

My observation results:

• reader lock, then reader lock --> LockRecursionException
• reader lock, then upgradeable reader lock --> LockRecursionException
• reader lock, then writer lock --> LockRecursionException
• upgradeable reader lock, then reader lock --> no exception
• upgradeable reader lock, then upgradeable reader lock --> LockRecursionException
• upgradeable reader lock, then writer lock --> no exception
• writer lock, then reader lock --> LockRecursionException
• writer lock, then upgradeable reader lock --> LockRecursionException
• writer lock, then writer lock --> LockRecursionException

Is this behavior correct?

Answer 1

From the docs:

A thread in upgradeable mode can downgrade to read mode by first calling the EnterReadLock method and then calling the ExitUpgradeableReadLock method. This downgrade pattern is allowed for all lock recursion policies, even NoRecursion.

My understanding is that for the writing situation, entering a write lock is the normal way to move from upgradeable to write mode anyway, so has to be supported even under a policy of NoRecursion (there would seem to be little point to a non-upgradeable upgradeable lock :)