The matching prefix. If there is no mapped prefix, the method returns String.Empty. If a null value is supplied, then null is returned.
This method finds the mapped prefix by walking the stack (that is, it looks globally). The supplied string must be atomized for the lookup to succeed. In other words, the supplied string object must exist in the namespace manager's name table (XmlNamespaceManager.NameTable).
The returned string is also atomized. For more information on atomized strings, see the System.Xml.XmlNameTable class.