using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(new DogSignTextBuilder().BuildSignText(new Dog("Fido")));

    }

}

​

public interface IAnimalVisitor<out T>

{

    T Visit(Dog a);

    T Visit(Cat a);

}

​

public interface IAnimal

{

    string Name { get; }

​

    T Accept<T>(IAnimalVisitor<T> v);

}

​

public abstract class AnimalBase : IAnimal

{

    public string Name { get; set;}

​

    protected AnimalBase(string name)

    {

        Name = name;

    }

​

    public abstract T Accept<T>(IAnimalVisitor<T> v);

}

​

public class Dog : AnimalBase

{

    public Dog(string name) : base(name) { }

​

    public override T Accept<T>(IAnimalVisitor<T> v)

    {

        return v.Visit(this);

    }

}

​

public class Cat : AnimalBase

{

    public Cat(string name) : base(name) { }

​

    public override T Accept<T>(IAnimalVisitor<T> v)

    {

        return v.Visit(this);

    }

}

​

public class AnimalSpeaker

{

    private class SpeakerVisitor : IAnimalVisitor<string>

    {

        public string Visit(Dog a)

        {

            return "Woof";

        }

​

        public string Visit(Cat a)

        {

            return "Meow";

        }

    }

​

    private readonly SpeakerVisitor _SpeakerVisitor = new SpeakerVisitor();

​

    public string Speak(IAnimal a)

    {

        return a.Accept(_SpeakerVisitor);

    }

}

​

public abstract class AnimalSignTextBuilderBase<TAnimal>

    where TAnimal : IAnimal

{

    private readonly AnimalSpeaker _AnimalSpeaker = new AnimalSpeaker();

​

    public string BuildSignText(TAnimal a)

    {

        var spokenText = _AnimalSpeaker.Speak(a);

        return string.Format("{0} says {1}.",a.Name, spokenText);

    }

}

​

public class DogSignTextBuilder : AnimalSignTextBuilderBase<Dog> { }

public class CatSignTextBuilder : AnimalSignTextBuilderBase<Cat> { }