// This is a variant type. At each single time it can only hold one case (a value)
// from a predefined set of cases. All classes that implement this interface
// consitute the set of the valid cases of the variant. So at each time a variant can
// be an instance of one of the classes that implement this interface. In order to
// add a new case to the variant there must be another class that implements
// this interface.
public interface ISomeAnimal
{
    // This method introduces the currently held case to whoever uses/processes
    // the variant. By processing we mean that the case is turned into a resulting
    // value represented by the generic type TResult.
    TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor);
}

// This is the awkward part,the visitor that is required every time we want to
// to process the variant. For each possible case this processor has a corresponding
// method that turns that case to a resulting value.
public interface ISomeAnimalProcessor<TResult>
{
    TResult ProcessCat(Cat cat);
    TResult ProcessFish(Fish fish);
}

// A case that represents a cat from the ISomeAnimal variant.
public class Cat : ISomeAnimal
{
    public CatsHead Head { get; set; }
    public CatsBody Body { get; set; }
    public CatsTail Tail { get; set; }
    public IEnumerable<CatsLeg> Legs { get; set; }
    public TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor)
    {
        // a processor has a method for each case of a variant,for this
        // particular case (being a cat) we always pick the ProcessCat method
        return processor.ProcessCat(this);
    }
}

// A case that represents a fish from the ISomeAnimal variant.
public class Fish : ISomeAnimal
{
    public FishHead Head { get; set; }
    public FishBody Body { get; set; }
    public FishTail Tail { get; set; }
    public TResult GetProcessed<TResult>(ISomeAnimalProcessor<TResult> processor)
    {
        // a processor has a method for each case of a variant,for this
        // particular case (being a fish) we always pick the ProcessCat method
        return processor.ProcessFish(this);
    }
}

public static class AnimalPainter
{
    // Now,in order to process a variant,in this case we want to
    // paint a picture of whatever animal it prepresents,we have to
    // create a new implementation of ISomeAnimalProcessor interface
    // and put the painting logic in it. 
    public static void AddAnimalToPicture(Picture picture,ISomeAnimal animal)
    {
        var animalToPictureAdder = new AnimalToPictureAdder(picture);
        animal.GetProcessed(animalToPictureAdder);
    }

    // Making a new visitor every time you need to process a variant:
    // 1. Requires a lot of typing.
    // 2. Bloats the type system.
    // 3. Makes the code harder to maintain.
    // 4. Makes the code less readable.
    private class AnimalToPictureAdder : ISomeAnimalProcessor<Nothing>
    {
        private Picture picture;

        public AnimalToPictureAdder(Picture picture)
        {
            this.picture = picture;
        }

        public Nothing ProcessCat(Cat cat)
        {
            this.picture.AddBackground(new SomeHouse());
            this.picture.Add(cat.Body);
            this.picture.Add(cat.Head);
            this.picture.Add(cat.Tail);
            this.picture.AddAll(cat.Legs);
            return Nothing.AtAll;
        }

        public Nothing ProcessFish(Fish fish)
        {
            this.picture.AddBackground(new SomeUnderwater());
            this.picture.Add(fish.Body);
            this.picture.Add(fish.Tail);
            this.picture.Add(fish.Head);
            return Nothing.AtAll;
        }
    }

}