xmlWriter and HttpRequest

 I want to create an xml file in code. I don't want to save it to the server.
Does any know the correct syntax for the
using (xmlWriter writer = xmlwriter.create(?????????) <-- what goes here when you just want it in memory and not to a file
Also:
Then do I attach "writer" from above to the HttpWebRequest or StreamWriter?
Thanks for getting me started, John Feeney
...