C# Lawyer: Don't Lock Value Type

Please read the following sample code with Monitor.Enter and Monitor.Exit. Does the lock work?

No! The code is totally wrong. You are not supposed to lock a value type, which doesn't have the SyncBlock field. Monitor.Enter(counter) is causing the int to be boxed first and then you are getting the lock on the box object, not the int itself. The lock doesn't work as you expected, since a different object(a different SyncBlock) will be used each time.

    class LockMe
    {
        private static int counter = 0;

        public static void Lock()
        {
            Monitor.Enter(counter); 
            counter++;
            Monitor.Exit(counter);
        }
    }    

See also

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