Checking whether the current thread owns a lock

Question

Suppose I have the following code:

public class SomeClass()
{
    private readonly object _lock = new object();

    public void SomeMethodA()
    {
     lock (_lock)
     {
      SomeHelperMethod();

      //do something that requires lock on _lock
     }
    }

    public void SomeMethodB()
    {
     lock (_lock)
     {
      SomeHelperMethod();

      //do something that requires lock on _lock
     }
    }

    private void SomeHelperMethod()
    {
     lock (_lock)
     {
      //do something that requires lock on _lock
     }
    }
}

Locking inside SomeHelperMethod seems redundant and wasteful, since all callers have already taken a lock. However, simply removing the lock from SomeHelperMethod seems dangerous since we might later refactor the code and forget to lock the _lock object prior to calling SomeHelperMethod.

Ideally I could work around this by asserting that the current thread owns a lock on _lock inside SomeHelperMethod:

private void SomeHelperMethod()
{
    Debug.Assert(Monitor.HasLock(_lock));

    //do something that requires lock on _lock
}

But there does not appear to be such a method. Monitor.TryEnter does not help because locks are re-entrant. Therefore, if the current thread already owns the lock, TryEnter will still succeed and return true. The only time it will fail is if another thread owns the lock and the call times out.

So, does such a method exist? If not, why? It does not seem dangerous at all to me, since it merely tells you whether the current thread (not another thread) owns a lock or not.

Answer 1

OK, I now understand your problem. Conceptually I am now thinking of a Semaphore object. You can easily check the semaphore value (a value greater or equal than 0 means the resource is available). Also, this does not actually lock the resource that being done by incrementing or decrementing the semaphore value. The Semaphore object is present in the .NET framework but i did not use it so I cannot provide a proper example. If necessary I can build a simple example and post it here. Let me know.

Alex.

L.E. I am unsure now if you actually have control to the synchronization mechanisms in the application. If you only have access to an object that could be locked or not, my solution could prove (once again) of no use.

Answer 2

I wish that was possible, that would make my code much, much cleaner.

Answer 3

There's a fundamental problem with your request. Suppose there were a IsLockHeld() method and you'd use it like this:

if (not IsLockHeld(someLockObj)) then DoSomething()

This cannot work by design. Your thread could be pre-empted between the if() statement and the method call. Allowing another thread to run and lock the object. Now DoSomething() will run, even though the lock is held.

The only way to avoid this is to take the lock and see if it worked. Monitor.TryEnter().

You see the exact same pattern at work in Windows. There is no way to check if a file is locked by another process, other than trying to open the file. For the exact same reason.

I'll upvote Quassnoi's answer now...

Answer 4

Not the best thing to do but ...

bool OwnsLock(object someLockObj)
{
  if (Monitor.TryEnter(someLockObj))
  {
    Monitor.Exit();
    return false;
  }
  return true;
}

Debug.Assert(OwnsLock(_someLockObject), "_someLockObject should be locked when this method is called")

Answer 5

Locking or re-locking are effectively free. The downside of this low cost is that you don't have as many features as the other synchronisation mechanisms. I would simply lock whenever a lock is vital, as you have above.

If you desparately want to omit 'unnecessary' locks without the risk involved with potential refactoring, add comments. If someone changes your code and it breaks, there is a comment there that explains why. If they still can't figure it out, that's their problem, not yours. A further alternative is to create a class for use as a locking object that contains a boolean you can flick on and off. However, the overheads introduced by doing this (including try/finally blocks, which are not free), are probably not worth it.

Answer 6

I have no experience whatsoever with .NET programming, but in my own code (Delphi) I have once implemented such assertions by having custom Acquire() and Release() methods in a critical section class, with a private member variable for the id of the thread having acquired the cs. After EnterCriticalSection() the return value of GetCurrentThreadID was written, and before LeaveCriticalSection() the field value was reset.

But I don't know whether .NET allows a similar thing, or whether you could implement this in your program...