Question

I have an abstract class “MainClass” which composes “Animal” class. I derive two classes TypeA and TypeB from abstract class which contains common functionality. TypeA and TypeB classes need to be extended to include their own specific functionality.

For example, TypeA would require to add cat functionality under Animal class. So that test application will be accessing the cat class like this typeA._animals._cat?

I know types cannot be added at runtime but is there any other design pattern that could solve my problem?

public abstract class MainClass
{
    public Animal _animals;
}

public class Animal
{
    public Tiger _tiger;
}

public class Tiger
{
    public int type { get { return "Tiger" ; } }
}

public class Cat
{
    public int type { get { return "Car" ; } }
}

public class Leopard
{
     public int type { get { return "Leopard" ; } }
}

public class TypeA : MainSession
{
    //Would like to add type Cat to Animal class
}

public class TypeB : MainSession
{
    //Would like to add type Leopard to Animal class  
}

Answer 1

The first thing you should do is to create an inheritance:

public interface IAnimal
{
    string SpeciesName {get; }
}

And implement it:

public class Tiger : IAnimal
{
    public string SpeciesName { get { return "Tiger" ; } }
}


public class Cat : IAnimal
{
    public string SpeciesName { get { return "Cat" ; } }
}

Let’s use it:

public abstract class MainSession
{
    private List<IAnimal> _animals;

    public IEnumerable<IAnimal> Animals {get { return _animals; }}

    proteced void AddAnimal(IAnimal animal)
    {
        _animals.Add(animal);
    }
}

public class TypeA : MainSession
{
    public TypeA()
    {
        AddAnimal(new Tiger());
    }
}

public class TypeB : MainSession
{
    public TypeB()
    {
        AddAnimal(new Leopard());
    }
}

You could also convert the AddAnimal into a factory method:

public abstract class MainSession
{
    private List<IAnimal> _animals;

    public IEnumerable<IAnimal> Animals {get { return _animals; }}

    protected IAnimal CreateAnimal(string speciesName)
    {
        // Either use reflection to find the correct species,
        // or a simple switch like below:
        switch (speciesName)
        {
            case "tiger":
                return new Tiger();
                break;
        }
    }
}

Having the factory in MainSession breaks Single Responsibility Principle, so I would break it out into a separate class.