Recursive / nested locking in C# with the lock statement [duplicate]

Answer 1

Yes, Monitor support recursion, but you should be aware because this behavior differs from one synchronization primitive to another.

For example, ReaderWriterLockSlim by default doesn't support recursion and this code snippet throws exception:

public class MyClass
{
    ReaderWriterLockSlim rw = new ReaderWriterLockSlim();
    //You should explicitly stated that you want to use recursion
    ReaderWriterLockSlim rwWithRecursion = new ReaderWriterLockSlim (LockRecursionPolicy.SupportsRecursion);

    public void MethodA()
    {
        try {
           rw.EnterReadLock();
           // do something
           MethodB();
        }
        finally {
          rw.ExitReadLock();
        }
    }

    public void MethodB()
    {
        try {
           rw.EnterReadLock(); //throws LockRecursionException
        }
        finally {
          rw.ExitReadLock();
        }
    }
}

Answer 2

Yes, locks based on Monitor in .NET are recursive, and counted.

From the docs for Monitor.Enter:

It is legal for the same thread to invoke Enter more than once without it blocking; however, an equal number of Exit calls must be invoked before other threads waiting on the object will unblock.

Whether this is a good thing or not is up for debate...