I need your help to extend my base class, here is the similar structure i have.

    public class ShowRoomA
    {
            public audi AudiModelA { get; set; }
            public benz benzModelA { get; set; }
    }   
    public class audi
    {
            public string Name { get; set; }
            public string AC { get; set; }
            public string PowerStearing { get; set; }
    }
    public class benz
    {
            public string Name { get; set; }
            public string AC { get; set; }
            public string AirBag { get; set; }
            public string MusicSystem { get; set; }
    }
//My Implementation class like this
    class Main()
    {
      private void UpdateDetails()
      {
        ShowRoomA ojbMahi = new ShowRoomA();
        GetDetails( ojbMahi ); // this works fine
      }
      private void GetDetails(ShowRoomA objShowRoom)
      {
         objShowRoom = new objShowRoom();
         objShowRoom.audi = new audi();
         objShowRoom.audi.Name = "AUDIMODEL94CD698";
         objShowRoom.audi.AC = "6 TON";
         objShowRoom.audi.PowerStearing = "Electric";

         objShowRoom.benz= new benz();
         objShowRoom.audi.Name = "BENZMODEL34LCX";
         objShowRoom.audi.AC = "8 TON";
         objShowRoom.audi.AirBag = "Two (1+1)";
         objShowRoom.audi.MusicSystem = "Poineer 3500W";
      } 
    }
// Till this cool.
// Now I got requirement for ShowRoomB with replacement of old audi and benz with new models and new other brand cars also added.
// I don't want to modify GetDetails() method. by reusing this method additional logic i want to apply to my new extended model.
// Here I struck in designing my new model of ShowRoomB (base of ShowRoomA) ... I have tried some thing like... but not sure.

    public class audiModelB:audi
    {
            public string JetEngine { get; set; }
    }
    public class benzModelB:benz
    {
            public string JetEngine { get; set; }
    }
    public class ShowRoomB
    {
            public audiModelB AudiModelB { get; set; }
            public benzModelB benzModelB { get; set; }
    } 

// My new code to Implementation class like this
    class Main()
    {
      private void UpdateDetails()
      {
        ShowRoomB ojbNahi = new ShowRoomB();
        GetDetails( ojbNahi ); // this is NOT working! I know this object does not contain base class directly, still some what i want fill my new model with old properties. Kindly suggest here 
      }
    }

Can any one please give me solutions how to achieve my extending requirement for base class "ShowroomA"

Really appreciated your time and suggestions.

Thanks in advance,