Does overriding violate the Open/Closed principle?

Question

The open/closed principle states that a class should be open for extension but closed for modification.

I thought that the modification part referred strictly to altering the source code of the base class. But I had an argument with someone saying that this also involves overriding methods from the base class.

It this interpretation correct?

Answer 1

Virtual methods allow replacing behavior of a base class in a derived class, without having to alter the base class and this means you adhere to the Open/Closed principle since you can extend the system without having to modify existing code.

Base classes (that are not purely abstract) however, tend to violate the Dependency Inversion Principle, since the derived class takes a dependency on the base class, which is a concrete component instead of being an abstraction. Remember, the DIP states that:

High-level modules should [...] depend on abstractions.

Besides this, base classes tend to violate the Interface Segregation Principle as well in case they define multiple public (or protected) methods that are not all used by the derived type. This is a violation of the ISP because:

no client should be forced to depend on methods it does not use

Answer 2

An override is a lot like a callback that anyone can register. It's like:

if (IsOverridden) CallCallback();
else DefaultImplementation(); //possibly empty

In that sense there is no modification. You are just reconfiguring the object to call the callback instead of doing the default behavior.

It's just like the click event of a button. You wouldn't consider subscribing to an event a modification. It's extension.