What is proper way to prevent two separate threads from running at same time?

Hi, you can try this:

        private object _locker = new object();

        private void timer_Elapsed(object sender, ElapsedEventArgs e)
        {
            lock (this._locker)
            {
                          ...do timer stuff....
            }
        }

        void DoStuff()
        {
            lock (this._locker)
            {
                  ...do my stuff...
            }
        }