Does XPathDocument use the memory stream it is constructed with after construction?
// Take the xml message and turn it into an object
var bytes = Encoding.ASCII.GetBytes(message);
var memoryStream = new MemoryStream(bytes);

XPathDocument xPathDocument = new XPathDocument(memoryStream);

// Take the xml message and turn it into an object
var bytes = Encoding.ASCII.GetBytes(message);
var memoryStream = new MemoryStream(bytes);

XPathDocument xPathDocument = null;
using(memoryStream)
{
    XPathDocument xPathDocument = new XPathDocument(memoryStream);
}