Question

I have two internal properties that use lazy-loading of backing fields, and are used in a multi-threaded application, so I have implemented a double-checking lock scheme as per this MSDN article

Now, firstly assuming that this is an appropriate pattern, all the examples show creating a single lock object for an instance. If my two properties are independent of each other, would it not be more efficient to create a lock instance for each property?

It occurs to me that maybe there is only one in order to avoid deadlocks or race-conditions. A obvious situation doesn’t come to mind, but I’m sure someone can show me one… (I’m not very experienced with multi-threaded code, obviously)

private List<SomeObject1> _someProperty1;
private List<SomeObject2> _someProperty2;
private readonly _syncLockSomeProperty1 = new Object();
private readonly _syncLockSomeProperty2 = new Object();

internal List<SomeObject1> SomeProperty1
{
  get
  {
    if (_someProperty1== null)
    {
      lock (_syncLockSomeProperty1)
      {
        if (_someProperty1 == null)
        {
          _someProperty1 = new List<SomeObject1>();
        }
      }
    }
    return _someProperty1;
  }

  set
  {
    _someProperty1 = value;
  }
}

internal List<SomeObject2> SomeProperty2
{
  get
  {
    if (_someProperty2 == null)
    {
      lock (_syncLockSomeProperty2)
      {
        if (_someProperty2 == null)
        {
          _someProperty2 = new List<SomeObject2>();
        }
      }
    }
    return _someProperty2;
  }

  set
  {
    _someProperty2 = value;
  }
}

Answer 1

Yes, if they are independent of each other, this would indeed be more efficient, as access to one wont’ block access to the other. You’re also on the money about the risk of a deadlock if that independence turned out to be false.

The question is, presuming that _someProperty1 = new List<SomeObject1>(); isn’t the real code for assigning to _someProperty1 (hardly worth the lazy-load, is it?), then the question is: Can the code that fills SomeProperty1 ever call that which fills SomeProperty2, or vice-versa, through any code-path, no matter how bizarre?

Even if one can call the other, there can’t be a deadlock, but if they both can call each other (or 1 call 2, 2 call 3 and 3 call 1, and so on), then a deadlock can definitely happen.

As a rule, I’d start with broad locks (one lock for all locked tasks) and then make the locks narrower as an optimisation as needed. In cases where you have, say, 20 methods which need locking, then judging the safety can be harder (also, you begin to fill memory just with lock objects).

Note that there are two issues with your code also:

First, you don’t lock in your setter. Possibly this is fine (you just want your lock to prevent multiple heavy calls to the loading method, and don’t actually care if there are over-writes between the set, and the get), possibly this is a disaster.

Second, depending on the CPU running it, double-check as you write it can have issues with read/write reordering, so you should either have a volatile field, or call a memory barrier. See http://blogs.msdn.com/b/brada/archive/2004/05/12/130935.aspx

Edit:

It’s also worth considering whether it’s really needed at all.

Consider that the operation itself should be thread-safe:

1. Do a bunch of stuff is done.
2. Have an object created based on that bunch of stuff.
3. Assign that object to the local variable.

1 and 2 will only happen on one thread, and 3 is atomic. Therefore, the advantage of locking is:

1. If performing step 1 and/or 2 above have their own threading issues, and aren’t protected from them by their own locks, then locking is 100% necessary.

2. If it would be disastrous for something to have acted upon a value obtained in step 1 and 2, and then later to do so with step 1 and 2 being repeated, locking is 100% necessary.

3. Locking will prevent the waste of 1 and 2 being done multiple times.

So, if we can rule out case 1 and 2 as an issue (takes a bit of analysis, but it’s often possible), then we’ve only preventing the waste in case 3 to worry about. Now, maybe this is a big worry. However, if it would rarely come up, and also not be that much of a waste when it did, then the gains of not locking would outweigh the gains of locking.

If in doubt, locking is probably the safer approach, but its possible that just living with the occasional wasted operation is better.