State Management :: Serialize/Deserialize A Custom Class In And Out Of A Session

I'm trying to save a custom class in a session, but it just never gets saved, along with other sessions. Note that my SessionMode = STATESERVER, and i cannot change it to INPROC as per our business requirement. This is my class

/*******START*******/
[Serializable]
public class User : ISerializable
{
private int _userId;
public int UserId
{
get { return _userId; }
set { _userId = value; }
}
//custom class
private TDB.tdbUser _tdbUserDetails;
public TDB.tdbUser tdbUserDetails
{
get { return _tdbUserDetails; }
set { _tdbUserDetails = value; }
}
//collection of custom class Sites
private List<Sites> _assignedSites;
public List<Sites> AssignedSites
{
get { return _assignedSites; }
set { _assignedSites = value; }
}
public User()
{
//Constructor code
}
/******SERIALIZATION*****/
protected User(SerializationInfo info, StreamingContext context)
{
this._userId = (int)info.GetValue("_userId", typeof(int));
this._assignedSites = (List<Sites>)info.GetValue("_assignedSites", typeof(List<Sites>));
this._tdbUserDetails = (TDB.tdbUser)info.GetValue("_tdbUserDetails", typeof(TDB.tdbUser));
}
[SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.SerializationFormatter)]
void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context)
{
info.AddValue("_userId", this._userId);
info.AddValue("_assignedSites", this._assignedSites);
info.AddValue("_tdbUserDetails", this._tdbUserDetails);
}
}
/*******END*******/

The calling code is just this: (User)HttpContext.Current.Session["TDBUSER"];