how manualresetevent work ?

First of all, correct question would ask about threads, not BackgroundWorker, which is just one of the ways to create a separate thread.

This is not the purpose of System.Threading.EventWaitHandle (manual or auto-reset are just to options or reset; this class is derived into two classes, System.Threading.ManualResetEvent and System.Threading.AutoResetEvent only to embed the difference in reset; if you want, you can directly use System.Threading.EventWaitHandle where all functionality is, so let's describe only this class.

So, wait state of a thread calling one of EventWaitHandle.WaitOne methods can end with resetting of the instance of the wait handle used in the call. (Other reasons for waking up the thread are Abort, Interrupt and, optionally, the timeout expiration). If you signal the event handle with Set, all threads in the queue will be released.

Therefore, all you can do to have selective wake up of a thread, it to keep all threads in different wait queues of different instances of wait handlers, one wait handler per thread.

This would be the solution, but it makes me concerned about the whole purpose. It's quite possible that you rather need to review the design of your threaded code.

—SA