EDIT: I believe classes should be designed to be open. With some restrictions, but shouldn't be closed for inheritance.
Why: "Final classes" or "sealed classes" seems to me weird. I never have to mark one of my own classes as "final" ("sealed"), because I may have to inheret that classes, later.
I have buy / download third party libraries with classes, (most visual controls), and really grateful none of those libraries used "final", even if supported by the programming language, in which they where coded, because I ended extending those classes, even if I have compiled libraries without the source code.
Sometimes, I have to deal with some ocassional "sealed" classes, and ended making a new class "wrapper" containg the given class, that has similar members.
class MyBaseWrapper {
protected FinalClass _FinalObject;
public virtual DoSomething()
{
// these method cannot be overriden,
// its from a "final" class
// the wrapper method can be open and virtual:
FinalObject.DoSomething();
}
} // class MyBaseWrapper
class MyBaseWrapper: MyBaseWrapper {
public override DoSomething()
{
// the wrapper method allows "overriding",
// a method from a "final" class:
DoSomethingNewBefore();
FinalObject.DoSomething();
DoSomethingNewAfter();
}
} // class MyBaseWrapper
Scope clasifiers allow to "seal" some features without restricting inheritance.
When I switched from Structured Progr. to O.O.P., I started using private class members, but, after many projects, I ended using protected, and, eventually, promote those properties or methods to public, if necessarily.
P.S. I don't like "final" as keyword in C#, I rather use "sealed" like Java, or "sterile", following the inheritance metaphor. Besides, "final" is an ambiguos word used in several contexts.