Question

I am using the ReaderWriterLock class to lock a Quotes collection which is a SortedDictionary. I am thinking of using a while loop until a thread can acquire the reader lock in case it is temporarily locked for writing. First question, my tests are working fine, but is there a drawback to this approach. Second question, what’s the optimal/best-practice way of doing this?

        public void RequestQuote(string symbol, QuoteRequestCallback qrc)
        {
            // add the call back on a list and take care of it when the quote is available
            while (!AcquireReaderLock(100)) Thread.Sleep(150);
            if (Quotes.ContainsKey(symbol))
            {
                qrc(Quotes[symbol]);
                rwl.ReleaseReaderLock();
            }
            else
            {
                rwl.ReleaseReaderLock();
                lock (requestCallbacks)
                    requestCallbacks.Add(new KeyValuePair<string, QuoteRequestCallback>(symbol, qrc));
                // request symbol to be added
                AddSymbol(symbol);
            }
        }


        private bool AquireReaderLock(int ms)
        {
            try
            {
                rwl.AcquireReaderLock(ms);
                return true;
            }
            catch (TimeoutException)
            {
                return false;
            }
        }

        private bool AquireWriterLock(int ms)
        {
            try
            {
                rwl.AcquireWriterLock(ms);
                return true;
            }
            catch (TimeoutException)
            {
                return false;
            }
        }

Answer 1

Do you have a second piece of code locks requestCallbacks before locking with the AcquireReaderLock() method? If so, it can deadlock with this.

Adding loops and sleep delays to avoid deadlocks won’t work in the general case. A strict hierarchy of lock acquisition will work in the general case.