Why classes are not serializable by default in .Net?

Question

Developers have to \'opt in\' for making classes serializable by explicitly using SerializableAttribute. What could go wrong if classes were serializable by default

Answer 1

The Liskov Substitution Principle implies that if a class is serializable, all derived classes should be serializable as well. If classes were by default serializable, it would be very difficult to derive non-serializable classes without violating the Liskov Substitution Principle.