what is deamon thread in c#?

A Deamon Thread will run until it completes or until all User Threads have completed.

Daemon threads make excellent "behind the scenes" processes for things that must happen in the background but can be terminated when the application ends - garbage collection is a great example of a process that could be handled with a daemon thread. Once all User Threads complete, any Deamon Threads that are still running are automatically halted and the application terminates.