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Should I seal all classes I know shouldn't ever be used as a base class even when there are no tangible performance or security concerns, or is this just adding cruft?
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A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes prevent derivation. Because they can never be used as a base class, some run-time optimizations can make calling sealed class members slightly faster.
Sealed Classes allow us to fix type hierarchies and forbid developers from creating new subclasses. They are useful when we have a very strict inheritance hierarchy, with a specific set of possible subclasses and no others.
A sealed class, in C#, is a class that cannot be inherited by any class but can be instantiated. The design intent of a sealed class is to indicate that the class is specialized and there is no need to extend it to provide any additional functionality through inheritance to override its behavior.
sealed classes will be at least a tiny bit faster, but sometimes can be waayyy faster... if the JIT Optimizer can inline calls that would have otherwise been virtual calls. So, where there's oft-called methods that are small enough to be inlined, definitely consider sealing the class.
A class which is extensible implements the feature that it can be extended -- that's a feature like any other feature of the class, and should be treated like one, no different from a method. All features should be thought through carefully to ensure that they meet the goals of the customer using the feature. Features need to be designed, implemented, reviewed for security problems, debugged, documented and maintained.
All that costs effort, and effort usually requires the outlay of money. Whose money are you spending? They might have an opinion on whether you should do this feature or not.
Basically, you have three choices:
1) Spend the money to do the feature so that you have confidence that it is correct, robust, secure and meets user needs.
2) Do none of the above but ship the feature anyway and hope that shipping an undesigned, rapidly implemented, untested, undocumented, unmaintained feature with unknown security risks doesn't harm you, your employer or your customers.
3) Seal the class. Unseal it later if you find that (1) was the right choice.
I say that (3) is good value for the money. I always seal every class I write that was not designed for extensibility.
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