This is (as richcb has pointed out) not a question.
It is basic research, that anyone which access to Google (or their course notes) should be able to locate very, very quickly.
A very quick Google
based on your question text gave almost 7 million results:
IO (Files) in C#[
^]
You could have been looking at this 25 minutes ago.
In future, please try to do at least basic research yourself, and not waste your time or ours.