What is Error in this

Hi

I am trying to inherit Interface in inner class . Pls help me what is error in this

public class Vehicle
    {

        internal class CAR : IVehicle
        {
            public int GetPrice()
            {

                return 300000;
            }
        }

        class BUS : IVehicle
        {
            public int GetPrice()
            {

                return 600000;
            }
        }
        class VAN : IVehicle
        {
            public int GetPrice()
            {

                return 5000000;
            }
        }

        public interface ICar
        {
            int GetCarPrice();
        }

        public interface IBus
        {
            int GetBusPrice();
        }

        public interface IVan
        {
            int GetVanPrice();
        }

        public interface IVehicle : ICar, IBus, IVan
        {
            int GetPrice();
        }

        class DiscountPrice
        {
            IVehicle CalculateDiscount(String ClassName)
            {
                IVehicle objIreport = null;
                Type objectType = Type.GetType(ClassName);

                if (objectType.IsClass)
                {
                    objIreport = (IVehicle)Activator.CreateInstance(objectType);
                }

                return objIreport;
            }
        }
    }