Duh, this works:
var transferResult = session.PutFiles(s.toString(), r.toString(), false, transferOptions);
var transferResult = session.PutFiles(s.toString(), r.toString(), false, transferOptions);
var transferResult = session.PutFiles("D:\\test.xml", "/", false, transferOptions);
var s = new String("D:\\test.xml");
var r = new String("/");
var transferResult = session.PutFiles(s, r, false, transferOptions);