c# lock question: lock(this) vs lock(SyncRoot)

Question

I have a class with a field of type collection.

Questions:

1. if I lock(this), do I effectively lock the collection too?
2. what is more efficient, to do lock(this) or to create a SyncRoot object and do lock(SyncRoot)?

Answer 1

Don't lock on this. It could be the case that someone else has used the instance as a lock object too. Use specifically designated lock objects.

1) if I lock(this), do I effectively lock the collection too?

No.

2) what is more efficient, to do lock(this) or to create a SyncRoot object and do lock(SyncRoot) ?

Efficient? Focus on semantics. locking on this is dangerous. Don't do it. The difference in performance, if any, is not material.

Seriously, it's akin to asking, what will get me to my destination faster, driving 100 MPH the wrong way down the freeway, or walking?

Answer 2

Always use lock(_syncRoot).

Where _syncRoot is a private field (just has to be an object).

This is no difference in terms of efficiency, but you're better to have a private field that you're in control of to lock on. If you lock on this, another object may also be locking on it.

See Why is lock(this) {...} bad? for a much better explanation. Also have a look at the msdn article on lock.

By locking on a collection, you aren't doing anything to stop it from being changed. A misunderstanding you might have, is that lock doesn't do anything special to stop that object being changed, it only works if every critical piece of code also calls lock.