Under what conditions can a thread enter a lock (Monitor) region more than once concurrently?

Question

(question revised): So far, the answers all include a single thread re-entering the lock region linearly, through things like recursion, where you can trace the steps of a s

Answer 1

ThreadPool threads cannot be reused elsewhere just because they went to sleep; they need to finish before they're reused. A thread that is taking a long time in a lock region does not become eligible to run more code at some other independent point of control. The only way to experience lock re-entry is by recursion or executing methods or delegates inside a lock that re-enter the lock.