Why can't we lock on a value type?

Question

I was trying to lock a Boolean variable when I encountered the following error :

\'bool\' is not a reference type as require

Answer 1

Because value types don't have the sync block that the lock statement uses to lock on an object. Only reference types carry the overhead of the type info, sync block etc.

If you box your reference type then you now have an object containing the value type and can lock on that object (I expect) since it now has the extra overhead that objects have (a pointer to a sync block that is used for locking, a pointer to the type information etc). As everyone else is stating though - if you box an object you will get a NEW object every time you box it so you will be locking on different objects every time - which completely defeats the purpose of taking a lock.

This would probably work (although it's completely pointless and I haven't tried it)

int x = 7;
object boxed = (object)x;

//thread1:
lock (boxed){
 ...
}
//thread2:
lock(boxed){
...
}

As long as everyone uses boxed and the object boxed is only set once you would probably get correct locking since you are locking on the boxed object and it's only being created once. DON'T do this though.. it's just a thought exercise (and might not even work - like I said, I haven't tested it ).

As to your second question - No, the value is not copied for each thread. Both threads will be using the same boolean, but the threads are not guaranteed to see the freshest value for it (when one thread sets the value it might not get written back to the memory location immediately, so any other thread reading the value would get an 'old' result).