namespace Foobar
{
    public interface IAddress
    {
        char CountryCode { get; set; }
        string Name { get; set; }
        string Surname { get; set; }
        string Address { get; set; }
        string City { get; set; }
        short Plz { get; set; }
        short HouseNumber { get; set; }
    }

    public static class IAddressExtensions
    {
        public static void SetCountryCode(this IAddress address)
        {
            // ...
        }
    }

    public class Address : IAddress
    {
        public char CountryCode { get; set; }
        public string Name { get; set; }
        public string Surname { get; set; }
        public string Address { get; set; }
        public string City { get; set; }
        public short Plz { get; set; }
        public short HouseNumber { get; set; }
    }
}