SharePoint and Office Open XML interaction question

Stream fs = mySPFile.OpenBinaryStream();

using (WordprocessingDocument ooxmlDoc = WordprocessingDocument.Open(fs, true))
{

    MainDocumentPart mainPart = wordDoc.MainDocumentPart;
    XmlDocument xmlMainDocument = new XmlDocument();
    xmlMainDocument.Load(mainPart.GetStream());

   // change the contents of the ooxmlDoc / xmlMainDocument

   Stream stream = mainPart.GetStream(FileMode.Open, FileAccess.ReadWrite);
   xmlMainDocument.Save(stream);
   // the stream should not be longer than the DocumentPart
   stream.SetLength(stream.Position);
}
mySPFile.SaveBinary(fs);
fs.Dispose();