public XElement Serialize()
 {
     return(new XElement(UblNames.Cac + nameof(ExternalReference),
                         URI.Serialize(nameof(URI)),
                         DocumentHash.Serialize(nameof(DocumentHash)),
                         HashAlgorithmMethod.Serialize(nameof(HashAlgorithmMethod)),
                         ExpiryDate.Serialize(nameof(ExpiryDate)),
                         ExpiryTime.Serialize(nameof(ExpiryTime)),
                         MimeCode.Serialize(nameof(MimeCode)),
                         FormatCode.Serialize(nameof(FormatCode)),
                         EncodingCode.Serialize(nameof(EncodingCode)),
                         CharacterSetCode.Serialize(nameof(CharacterSetCode)),
                         FileName.Serialize(nameof(FileName)),
                         Description.Serialize(nameof(Description))
                         ));
 }