Object Data Type Serialization In Wcf Using DataContract Attribute

I have created a WCF Rest API Service using wcf rest starter kit preview 2 which automatically return JSON or XML data depending on the requested content type. everything is working fine but as my service methods return multiple data type some methods returns a object of a class or collection of class, bool, int or collection type. so I planed to return a same Response in all methods for this I Create a new class as following:

[DataContract(Namespace = "")]
public class ServiceResponse
{
[DataMember]
public bool IsError
{ get; set; }
[DataMember]
public string ResponseMessage
{ get; set; }
[DataMember]
public Object ResponseData
{ get; set; }
}

and User class as following:

[DataContract(Namespace = "")]
public class User
{
[DataMember]
public string UserName
{ get; set; }
[DataMember]
public int UserID
{ get; set; }
}

In ServiceResponse Class I have declared an Object Type property ResponseData and my all method has return type of ServiceResponse. My problem is that when I set any string,bool,int type as ResponseData it is serialized but when I set collection type or an object of another class which also has DataContarct attribute this class is not serialized see below code :

public ServiceResponse GetUser(int userID)
{
User user = getUser(userID); get user data from database
ServiceResponse response=new ServiceResponse();
var response = new ServiceResponse
{
IsError = false,
ResponseMessage = string.Empty,
ResponseData = user; // Setting Result Data here
};
return response;
}

when I called above method(GetUser) I got Null response due to serialization problem, but following code works fine

public ServiceResponse TestMethod(string testMessage)
{
ServiceResponse response=new ServiceResponse();
var response = new ServiceResponse
{
IsError = false,
ResponseMessage = string.Empty,
ResponseData = testMessage; // Setting Result Data here
};
return response;
}