Problem with Inheritance.

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            var animalFactory = new AnimalFactory();

            // Search for some fishes.
            var fishService = animalFactory.GetAnimalService("fish");
            var fishes = fishService.SearchForAnimals(new FishSearchOptions {ScalesType = "shiny green"});

            // Search for meows.
            var catService = animalFactory.GetAnimalService("cat");
            var kittyCats = catService.SearchForAnimals(new MamalSearchOptions {HairColour = "golden"});
        }
    }

    #region Main Animal and Searching Interfaces

    /// <summary>
    /// I am an Animal! Here me roar! (fish can roar?)
    /// </summary>
    public interface IAnimal
    {
        string Name { get; set; }
        int Age { get; set; }
    }

    public class BaseAnimal : IAnimal
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }

    /// <summary>
    /// How we can search for animals.
    /// </summary>
    public interface IAnimalSearchOptions
    {
        string Name { get; set; }
    }

    /// <summary>
    /// Search results. We might return the fist 5 results, out of 150.
    /// </summary>
    public class SearchResult<T> where T: IAnimal
    {
        IList<IAnimal> Animals { get; set; }
        int TotalCount { get; set; }
    }

    /// <summary>
    /// The main search interface for finding animals.
    /// </summary>
    public interface IAnimalService<T, in TU> 
        where T: IAnimal
        where TU: IAnimalSearchOptions
    {
        SearchResult<T> SearchForAnimals(TU searchOptions);
    }

    #endregion

    #region Mamals

    public class MamalItem : BaseAnimal
    {
        // -- Mamal specific properties --
        public string HairColour { get; set; }
    }

    public class MamalSearchOptions : IAnimalSearchOptions
    {
        public string Name { get; set; }

        /// <summary>
        /// Filter results by hair colour.
        /// </summary>
        public string HairColour { get; set; }
    }

    public class MamalService : IAnimalService<MamalItem, MamalSearchOptions>
    {
        public SearchResult<MamalItem> SearchForAnimals(MamalSearchOptions searchOptions)
        {
            throw new NotImplementedException();
        }
    }

    #endregion

    #region Fishes

    public class Fish : BaseAnimal
    {
        // Fish specific properties
        public string ScalesType { get; set; }
    }

    public class FishSearchOptions : IAnimalSearchOptions
    {
        public string Name { get; set; }


        // Filter by scale type.
        public string ScalesType { get; set; }
    }

    public class FishService : IAnimalService<Fish, FishSearchOptions>
    {
        public SearchResult<Fish> SearchForAnimals(FishSearchOptions searchOptions)
        {
            throw new NotImplementedException();
        }
    }

    #endregion

    public class AnimalFactory
    {
        private static IDictionary<string, IAnimalService<IAnimal, IAnimalSearchOptions>> _animals;

        public AnimalFactory()
        {
            _animals = new Dictionary<string, IAnimalService<IAnimal, IAnimalSearchOptions>>
                {
                    {"Cat", new MamalService() },
                    {"Dog", new MamalService() },
                    {"GoldFish", new FishService() }
                };
        }

        public IAnimalService<IAnimal, IAnimalSearchOptions> GetAnimalService(string name)
        {
            if (string.IsNullOrEmpty(name))
            {
                throw new ArgumentNullException("name");
            }

            return _animals[name];
        }
    }
}