When a thread is blocked on Monitor.Enter(obj), is it put in obj's ready queue?

Question

If so, that would imply that the blocked thread can enter when another thread calls Monitor.Wait(obj). This seems a little odd to me in that it must contend with other threads in the ready queue.

If not, can it only un-block when Monitor.Exit(obj) is called? Or is it in obj's waiting queue?

This isn't clearly documented in the MSDN Library class documentation.

Answer 1

Yes, the blocked thread can enter when another thread calls Monitor.Wait. Why would it not be able to?

In particular, it has to be able to - as the thread which is currently blocking may be the one which is going to call Pulse.

Consider a producer/consumer queue:

Producer                Consumer

                        Enter lock
Enter lock (block)
                        Check: queue is empty
                        Wait (block)
Unblocked
Add item to queue
Pulse
Exit lock
                        Unblocked (enters lock again)
                        Check: queue is now not empty
                        Fetch item
                        Exit lock

This scenario wouldn't work if the producer thread were still waiting for an Exit call.