How to deserialize an object data type in WCF

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XML

WCF

deserialization

I have a data contracts as follows:

[DataContract]
[KnownType(typeof(UserData))]
[KnownType(typeof(CardData))]
[XmlRoot(Namespace = Constants.XML_NMSP_TXN_MSG, ElementName = Constants.XML_TAG_COMMAND)]
public class MessageData
{
[XmlAttribute("ID")]
[DataMember]
public long CommandId { get; set; }
[DataMember]
public object Data { get; set; }
}

[DataContract]
public class UserData
{
[DataMember]
public string UserId { get; set; }
[DataMember]
public string FirstName { get; set; }
}

[DataContract]
public class CardData
{
[DataMember]
public string CardId { get; set; }
[DataMember]
public string CardNumber { get; set; }
}

And service contract as:
[OperationContract]
[WebInvoke(
Method = "POST",
RequestFormat = WebMessageFormat.Xml,
ResponseFormat = WebMessageFormat.Xml,
BodyStyle = WebMessageBodyStyle.Bare,
UriTemplate = "api/AddRequest/")]
string AddRequest(MessageData ReqData);

I am sending CardData and UserData as an object in MessageData.Data
The problem is I MessageData is not getting deserialized. Can anyone help me with this please?

Basically I want to use the AddRequest() method to send different data to the service. Is there another way to do it? Am i even doing it the right way?