.Net apps sharing a file in a peer-to-peer network?

Can anyone tell me a bit about this?  I have a peer-to-peer network but the laptop is belly up at the moment and I am unable to do any experiments.

If two apps try to share a file using StreamWriters will the latecomer be denied access?

If a StreamWriter is accessing a file will other StreamReaders be allowed?

How about if the file is on a Server instead of a peer?

I vaguely recall a FileMode parameter which could be used to control file sharing but maybe I am thinking of the native Windows API.  Or maybe even another operating system.

Thanks,  Bob