How to tell if an instance of a parent class was created from an instance of a particular child

Question

I find it easier to ask this question through a code example.

class Parent {}
class Child : Parent {}
...
...
Child firstChild = new Child();
Child secondChild = new Child();
Parent firstParent = (Parent)firstChild;
Parent secondParent = (Parent)secondChild;

If I wasn't privy to the above assignments, how would I determine if firstParent was created from the instance firstChild without accessing/comparing their fields or properties?

Answer 1

Well, firstParent is not created (there is no "new" keyword being used) but cast from the firstChild:

Parent firstParent = (Parent)firstChild;

To test use Object.ReferenceEquals (i.e. firstParent and firstChild are just the same instance)

if (Object.ReferenceEquals(firstParent, firstChild)) { ... }