The child/descendant class object references can not point to the parent/ancestor class objects. Why and how ????

Suppose Dog and Cat inherit from Animal.

Then a Dog is an Animal but an Animal isn't necessarily a Dog - it could be a Cat.

So, you can do this:

  Animal a = new Dog();

but you can't do this

  Dog d = a;

though, if you know that 'a' really refers to a Dog you can do this and not get an error at runtime:

  Dog d = (Dog)a; 

The C# compiler allows the first and third lines because it knows that the Dog class inherits everything (except its constructor(s) and destructor) that the Animal class has and so you can access all these inherited members just as well via an Animal reference as a Dog reference.

However, it doesn't allow the second line because the Animal variable 'a' could actually refer to a different Animal entirely (e.g. a Cat) which lacks some of the members which the Dog class adds to those inherited from Animal. So, if this were allowed and you tried to access such a member through the Dog reference 'd', you'd get an error at runtime.