Basic object reference and use question using c#

Question

I am new the realm of Object Orientation and programming. There are some things I am still trying to understand.

For instance, I have the following code:

 public abstract class ParentA
    {
        public virtual void MethodA()
        {
            Console.WriteLine("Doing somethin...");
        }
    }
    public class DerivedClassA : ParentA
    {
        public override void MethodA()
        {
            Console.WriteLine("Doing something from derived...");
        }
    }

Now, I see some code where the class is instatiated like this:

ParentA p = new DerivedClassA();
            p.MethodA();

Why not just instatiate the actually class you want to use and use it's members?

 DerivedClassA d = new DerivedClassA();
            d.MethodA();

I see this used a lot interfaces as well where is written like this:

public interface Animal
    {
        void Bark();
    }
    public class Dog : Animal
    {
        public void Bark()
        {
            Console.WriteLine("bark");
        }
    }

and then used in this manner:

Animal a = new Dog();
            a.Bark();

Why not just do this??:

Dog d = new Dog();
            d.Bark();

When does it matter?

Thanks for the help

:)

Answer 1

You're right; that does look odd, doesn't it?

The code:

Animal animal = new Dog();

is reasonably rare; normally if you knew you were making a Dog then you'd type the variable as Dog. What is more common is:

Animal animal = petStore.ObtainInexpensivePet();

where you don't know exactly what is going to come back; maybe a kitten, maybe an iguana, but you know it will at least be an Animal. It's the pet store that is creating the dog object, not you.