A Quantum Immortal

List<int> tmp;
lock (_list)
{
    tmp = _list;
    _list = new List<int>(1);
}
 
// use tmp object like it's owned exclusively by this thread

using System;
using System.Collections.Generic;
using System.Threading;
 
namespace tstlock
{
    public class LockTester
    {
        private List<int> _list = new List<int>(1);
 
        public void Run()
        {
            var threads = new Thread[40];
            for (int i = 0; i < threads.Length; ++i)
            {
                threads[i] = new Thread(ThreadFunc);
                threads[i].Start();
            }
            foreach (var thread in threads)
                thread.Join();
        }
 
        private void ThreadFunc()
        {
            for (int i = 0; i < 1000000; ++i)
            {
                List<int> tmp;
                lock (_list)
                {
                    tmp = _list;
 
                    // at this point _list is always supposed to be an empty list
                    // because all the additions to it are after the new list is allocated
                    _list = new List<int>(1);
                }
 
                var array = new []{1,2,3,54,5,6};
                tmp.AddRange(array);
                if (tmp.Count != array.Length)
                {
                    throw new Exception("Bug - tmp is not local as we thought");
                }
            }
        }
    }
}