Don't understand the need for Monitor.Pulse()

Question

According to MSDN, Monitor.Wait():

Releases the lock on an object and blocks the current thread until it reacquires the lock.

Answer 1

You use Enter / Exit to acquire exclusive access to a lock.

You use Wait / Pulse to allow co-operative notification: I want to wait for something to occur, so I enter the lock and call Wait; the notifying code will enter the lock and call Pulse.

The two schemes are related, but they're not trying to accomplish the same thing.

Consider how you'd implement a producer/consumer queue where the consumer can say "Wake me up when you've got an item for me to consume" without something like this.

Answer 2

I myself had this same doubt, and despite some interesting answers (some of them present here), I still kept searching for a more convincing answer.

I think an interesting and simple thought on this matter would be: I can call Monitor.Wait(lockObj) at a particular moment in which no other thread is waiting to acquire a lock on the lockObj object. I just want to wait for something to happen (some object's state to change, for instance), which is something I know that will happen eventually, on some other thread. As soon as this condition is achieved, I want to be able to reacquire the lock as soon as the other thread releases its lock.

By the definition of the Monitor.Wait method, it releases the lock and tries to acquire it again. If it didn't wait for the Monitor.Pulse method to be called before trying to acquire the lock again, it would simply release the lock and immediately acquire it again (depending on your code, possibly in loop).

That is, I think it's interesting trying to understand the need of the Monitor.Pulse method by looking at its usefulness in the functioning of the Monitor.Wait method.

Think like this: "I don't want to release this lock and immediately try to acquire it again, because I DON'T WANT to be ME the next thread to acquire this lock. And I also don't want to stay in a loop containing a call to Thread.Sleep checking some flag or something in order to know when the condition I'm waiting for has been achieved so that I can try to reacquire the lock. I just want to 'hibernate' and be awaken automatically, as soon as someone tells me the condition I'm waiting for has been achieved.".

Answer 3

Read the Remarks section of the linked MSDN page:

When a thread calls Wait, it releases the lock on the object and enters the object's waiting queue. The next thread in the object's ready queue (if there is one) acquires the lock and has exclusive use of the object. All threads that call Wait remain in the waiting queue until they receive a signal from Pulse or PulseAll, sent by the owner of the lock. If Pulse is sent, only the thread at the head of the waiting queue is affected. If PulseAll is sent, all threads that are waiting for the object are affected. When the signal is received, one or more threads leave the waiting queue and enter the ready queue. A thread in the ready queue is permitted to reacquire the lock.

This method returns when the calling thread reacquires the lock on the object. Note that this method blocks indefinitely if the holder of the lock does not call Pulse or PulseAll.

So, basically, when you call Monitor.Wait, your thread is in the waiting queue. For it to re-acquire the lock, it needs to be in the ready queue. Monitor.Pulse moves the first thread in the waiting queue to the ready queue and thus allows for it to re-acquire the lock.