Threadsafe collection without lock

Question

I am preparing myself for an interview and I came across the followign question. I tried but I could not find anything which can create a class containing thread safe collec

Answer 1

My solution. Basically mimic locking using Interlocked.Exchange and an AutoResetEvents. Did some simple tests and it seems working.

    public class SharedStringClass
    {
        private static readonly int TRUE = 1;
        private static readonly int FALSE = 0;

        private static int allowEntry;

        private static AutoResetEvent autoResetEvent;

        private IList internalCollection;

        public SharedStringClass()
        {
            internalCollection = new List();
            autoResetEvent = new AutoResetEvent(false);
            allowEntry = TRUE;
        }

        public void AddString(string strToAdd)
        {
            CheckAllowEntry();

            internalCollection.Add(strToAdd);

            // set allowEntry to TRUE atomically
            Interlocked.Exchange(ref allowEntry, TRUE);
            autoResetEvent.Set();
        }

        public string ToString()
        {
            CheckAllowEntry();

            // access the shared resource
            string result = string.Join(",", internalCollection);

            // set allowEntry to TRUE atomically
            Interlocked.Exchange(ref allowEntry, TRUE);
            autoResetEvent.Set();
            return result;
        }

        private void CheckAllowEntry()
        {
            while (true)
            {
                // compare allowEntry with TRUE, if it is, set it to FALSE (these are done atomically!!)
                if (Interlocked.CompareExchange(ref allowEntry, FALSE, TRUE) == FALSE)
                {
                    autoResetEvent.WaitOne();
                }
                else
                {
                    break;
                }
            }
        }
    }