Question

Recently I have seen this code in a WebSite, and my question is the following:

        private bool mbTestFinished = false;

        private bool IsFinished()
        {
            lock( mLock )
            {
                return mbTestFinished;
            }
        }

        internal void SetFinished()
        {
            lock( mLock )
            {
                mbTestFinished = true;
            }
        }

In a multithread environment, is really necessary to lock the access to the mbTestFinished?

Answer 1

Yes, it is needed. .Net environment uses some optimizations, and sometimes if a memory location is accessed frequently, data is moved into CPU registers. So, in this case, if mbTestFinished is in a CPU register, then a thread reading it may get a wrong value. Thus using the volatile key ensures, all accesses to this variable is done at the memory location, not at the registers.
On the otherhand, I have no idea of the frequency of this occurence. This may occur at a very low frequency.