Purpose of final and sealed

Question

Why would anyone want to mark a class as final or sealed?

Answer 1

According to Wikipedia, "Sealed classes are primarily used to prevent derivation. They add another level of strictness during compile-time, improve memory usage, and trigger certain optimizations that improve run-time efficiency."

Also, from Patrick Smacchia's blog:

• Versioning: When a class is originally sealed, it can change to unsealed in the future without breaking compatibility. (…)

• Performance: (…) if the JIT compiler sees a call to a virtual method using a sealed types, the JIT compiler can produce more efficient code by calling the method non-virtually.(…)

• Security and Predictability: A class must protect its own state and not allow itself to ever become corrupted. When a class is unsealed, a derived class can access and manipulate the base class’s state if any data fields or methods that internally manipulate fields are accessible and not private.(…)

Those are all pretty good reasons - I actually wasn't aware of the performance benefit implications until I looked it up just now :)

The versioning and security points seem like a huge benefit in terms of code confidence, which is very well justified on any kind of large project. It's no drop-in for unit testing, of course, but it would help.

Answer 2

Because creating a type for inheritance is much harder work than most folks think. It is best to mark all types this way by default as this will prevent others from inheriting from a type that was never intended to be extended.

Whether or not a type should be extended is a decision of the developer who created it, not the developer who comes along later and wants to extend it.