.net - Lock aqcuired and further attempts to lock do not block: are C# locks re-entrant?

Question

I've written a test of what I think should be a valid case for a deadlock. It appears that once the lock has been acquired by an instance of the a class, that instance doesn't need to re-acquire the lock anymore even if I explicitly try to call another method that should lock again.

Here is the class:

internal class Tester
{
    private readonly object _sync = new object();

    public Tester() { }

    public void TestLock()
    {
        lock (_sync)
        {
            for (int i = 0; i < 10; i++)
            {
                Deadlock(i);
            }
        }

    }

    private void Deadlock(int i)
    {
        lock (_sync)
        {
            Trace.WriteLine(i + " no deadlock!");
        }
    }
}

Output:

0 no deadlock!
1 no deadlock!
2 no deadlock!
3 no deadlock!
4 no deadlock!
5 no deadlock!
6 no deadlock!
7 no deadlock!
8 no deadlock!
9 no deadlock!

I would have thought that this would cause a deadlock... can anybody shed some light on this?

Answer 1

Locks in .NET are reentrant. Only acquisitions from other threads are blocked. When the same thread locks the same object multiple times, it simply increments a counter, and decrements it when released. When the counter hits zero, the lock is actually released for access from other threads.