How could an instance of the base class hold an instance of the derived class?

Question

I have been a .Net coder (can not say I am a programmer) for 2 years. There is one question that I can not understand for years, that is how could an instance of the base class hold an instance of the derived class?

Suppose we have two classes:

class BaseClass
{
    public A propertyA;
    public B propertyB;
}

class DerivedClass ：BaseClass
{
    public C propertyC;

}

How could this happen:

BaseClass obj = new DerivedClass ()

I mean, the memory model of BaseClass, has no space for the newly added propertyC, so how could it still hold the value of propertyC?

On the other side, how could this can not happen:

DerivedClass  obj = new BaseClass()

I thought this is the correct way since the memory model of DerivedClass has all the spaces for the BaseClass and even more. But this is not true, why?

I know I am asking a really stupid question, but could someone give me a more detail answer of this? It would be better from the perspective of the memory or compiler.

Answer 1

Answer copied verbatim from what another user, jk, wrote on a exactly same question here on Stackoverflow.

If I tell you I have a dog, you can safely assume that I have a dog.

If I tell you I have a pet, you don't know if that animal is a dog, it could be a cat or maybe even a giraffe. Without knowing some extra information you can't safely assume I have a dog.

similarly a derived object is a base class object (as its a sub class), so can be pointed to by a base class pointer. however a base class object is not a derived class object so can't be assigned to a derived class pointer.

(The creaking you will now hear is the analogy stretching)

Suppose you now want to buy me a gift for my pet.

In the first scenario you know it is a dog, you can buy me a leash, everyone is happy.

In the second scenario I haven't told you what my pet is so if you are going to buy me a gift anyway you need to know information I havent told you (or just guess), you buy me a leash, if it turns out I really did have a dog everyone is happy.

However if I actually had a cat then we now know you made a bad assumption (cast) and have an unhappy cat on a leash (runtime error)

Answer 2

Since BaseClass is a reference type, obj is not a BaseClass object. It is a reference to a BaseClass object. Specifically, it is a reference to the BaseClass part of the DerivedClass.