Question

class A
{
}

class B : A
{
}

I know that B b = new A(); is not possible, but what is the explanation behind it?

Answer 1

By deriving from A, you specify that instances of B are not only B, they’re A also. This is called inheritance in OOP. The power of inheritance is in being able to abstract away general properties/behaviour to a common class and then derive specialized classes from it. The specialized classes can change existing functionality (called overriding) or add new functionality.

However, inheritance works only in one direction, not both. Objects of class A cannot be treated as B because B may (and often does!) contain more functionality than A. Or, in other words, B is more specific while A is more general.

Therefore, you can do A a = new B(); but not B b = new A();