Null readonly lock object

Question

How can a static readonly object become null? (I've set the _lock object as static and not static, but always readonly.)

The validate method works fine for a few times, then after it's called 2-3 times the _lock object is null. Is this an indicator that the lock is owned by another thread?

Answer 1

Besides reflection, another of the ways that this (more specifically, a null reference exception on a static variable assigned via initializer) can happen is if you have a static constructor defined elsewhere in your class that for some reason sets the value to null, eg:

class Program
{
    class A
    {
        private static readonly object _lock = new object();

        public void Validate()
        {
            lock (_lock) // NullReferenceException here...
            {
                Console.WriteLine("Not going to make it here...");
            }
        }

        static A()
        {
            Console.WriteLine(_lock.ToString());
            Console.WriteLine("Now you can see that _lock is set...");
            _lock = null;
        }
    }

    static void Main(string[] args)
    {
        var a = new A();
        a.Validate();
    }
}