Requests:
    <?xml version="1.0" encoding="UTF-8" ?>
    <message id="123" company="CompanyA">
            <query>
                <product>someProduct</product>
                <dateFrom>2012-01-01</dateFrom>
                <dateTo>2012-01-10</dateTo>
                <status>10</status>
            </query>
    </message>

    <?xml version="1.0" encoding="UTF-8" ?>
    <message id="456" company="CompanyB">
            <query>
                <price>99.95</price>
                <quantity>50</quantity>
            </query>
    </message>

    Response:
        <message Id="789" company="CompanyC">
                <file>
                    <fileName>abc123.pdf</fileName>
                    <data>LLSHDFLJHSJKLDFHJLKS</data>
                </file>
        </message>

[DataContract, XmlRoot("message")]
public class Message
{
   [DataMember, XmlAttribute]
   public string id { get; set; }
   [DataMember, XmlAttribute]
   public string company { get; set; }
}

public class ExtendedMessage:Message
{
   [DataMember, XmlElement]
   public string whantEver { get; set; }
}

public class ExtendedMessage2:Message
{
   [DataMember, XmlElement]
   public int someNumber { get; set; }
}