lock(X) vs lock(typeof(X))

Question

What is the difference between locking on a type of a class vs locking on the class itself?

For example:

private readonly object xmpp = new object();

lock (xmpp)
{
    ...
}

vs

lock (typeof(Xmpp))
{
    ...
}

They are both wrong. This is well covered here: http://stackoverflow.com/questions/251391/why-is-lockthis-bad — Hans Passant, Nov 18 '11 at 15:48
i didn't think it was identical as the post you refer to as I am locking on an object, not on itself. Updated the code slightly, think that compiles properly — Firedragon, Nov 18 '11 at 15:53
I think lock on typeof() is incorrect, it is quite meaningless. — OKEEngine, Nov 18 '11 at 15:57
@ChiaMing It's not meaningless to lock on typeof, which is unfortunate, because it it was meaningless it wouldn't compile, which would be a good thing given the problems locking on types brings. — Jon Hanna, Nov 18 '11 at 17:13
I mean, it is not practical. it is more like if + goto vs while. — OKEEngine, Nov 18 '11 at 17:57

sehe · Accepted Answer · 2011-11-18T18:28:54.583

19

Always lock on a private lock object:

 public class X
 {
      private readonly Object _lock = new Object();

      // ...
            lock (_lock)
            {
            }

If you must synchronize access to class static members, use the same pattern:

 public class X
 {
      private readonly static Object s_lock = new Object();

edited Nov 18 '11 at 18:28

answered Nov 18 '11 at 15:48

sehe

5

I would just like to point out WHY you should always use a private lock object... Because only your class and code should have access to that lock object, if you lock something publicly accessible someone else could potentially lock your objects and break your code. This really only applies when writing 3rd party libraries, but it's a good pattern and good habit to get in to everywhere. – Bradley Uffner Nov 18 '11 at 15:56
Thanks for the comments both of you. Very helpful in my understanding – Firedragon Nov 18 '11 at 15:57
4

I would also recommend adding the `readonly` keyword to the locking object, for clarity's sake. Not that you would intentionally assign a value, *but* those bugs are always fun. – Bryan Crosby Nov 18 '11 at 18:27

1 Answers1