C# thread locking and using a lock object; what should the correct scope be for the lock object?

Question

Is there a difference between using a lock object that is declared as a field of a class as opposed to local scope?

For example: Is there a difference between using lockObject1 or lockObject2 in the below example?

public class Test()
{
   private Object lockObject1 = new Object();

   public void FooBar()
   {
       lock(lockObject1)
       {
          //Thread safe area 1
       }

       var lockObject2 = new Object();
       lock(lockObject2)
       {
           //Thread safe area 2
       }
   }
}

It seems like most examples always seem to glaze over the importance of scoping of the chosen lock object.

Every time that method is called, a new lockObject2 is created, thus each thread will be locked on their own instance, which means the lock is essentially doing nothing — Rob
– Rob ♦, Commented Mar 16, 2011 at 5:10

DocMax · Accepted Answer · 2011-03-16 05:07:49Z

9

The local lock object will not really be providing any thread safety since multiple threads running FooBar will each have their own object rather than sharing a single object for locking. (Sadly, I have seen this very thing in code headed toward production before I raised the issue and got it corrected in time.)

answered Mar 16, 2011 at 5:07

DocMax

12.2k7 gold badges48 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

7wp Over a year ago

So that means, lockObject1 would only provide thread safety if the same instance of the Test class is used across all threads right? If a new Test class is instantiated in every thread then were back to square one like the local lockObject2 ?

DocMax Over a year ago

Yes, that is correct. Use a non-static to provide thread safety within an instance and a static to provide thread safety between multiple instances.

antsyawn · Accepted Answer · 2011-03-16 05:29:22Z

5

You should always use a private static member variable if you want to lock all instances of an object. Use a non-static private member if you want to lock a specific instance.

Never use a publicly visible member, or 'this', as you never know when someone else is going to use it for their own lock, and possibly cause deadlocks.

public class Test()
{
   private static Object lockObjectAll = new Object();
   private Object lockObjectInstance = new Object();

   public void FooBar()
   {
       lock (lockObjectAll)
       {
          // Thread safe for all instances
       }

       lock (lockObjectInstance)
       {
          // Thread safe for 'this' instance
       }
   }
}

answered Mar 16, 2011 at 5:29

antsyawn

98110 silver badges17 bronze badges

Comments

Fun Mun Pieng · Accepted Answer · 2011-03-16 05:23:12Z

2

your lockObject2 won't protect anything since, it's not going to be locked anywhere else. Do note that multiple calls to FooBar by different threads will not be protected since it's a new instance each time FooBar is called. Also, do note that lockObject1 isn't much better. It allows locking between calls on the same instance. This is good if it's only member variables being accessed. But if some global resource is being accessed, lockObject1 won't protect fully since each instance of the class has its own instance. As suggested by @Haoest, making lockObject1 static should fix this problem.

edited Mar 16, 2011 at 5:23

answered Mar 16, 2011 at 5:12

Fun Mun Pieng

6,9293 gold badges32 silver badges31 bronze badges

Comments

Haoest · Accepted Answer · 2011-03-16 05:10:13Z

1

Make lockObject1 static and you should be alright.

lockObject2 should also be static field to be effective.

answered Mar 16, 2011 at 5:10

Haoest

14k30 gold badges96 silver badges110 bronze badges

3 Comments

Rob Over a year ago

lockObject1 doesn't necessarily need to be static

7wp Over a year ago

@Rob if the FooBar method was static, then lockObject1 would have to be static, right?

Rob Over a year ago

Yes, but it wouldn't even compile if the method was static and it tried to access a private member

Collectives™ on Stack Overflow

C# thread locking and using a lock object; what should the correct scope be for the lock object?

4 Answers 4

2 Comments

Comments

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

Comments

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related