A System.Threading.Timer Simple timer which requires a delegate to be supplied for execution when the timer expires. execution of the method provided by the delegate will be invoked on a ThreadPool Thread.
B System.Timers.Timer: Designed for use with worker threads in a multithreaded environment. Can move among threads to handle the raised Elapsed event May result in more accuracy than System.Windows.Forms.Timer instances.
C System.Windows.Forms.Timer Lower Resolution timer which requires a UI message pump on the creating thread.
D All of these