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Why seal a class?
I haven't seen a lot of classes with this modifier, however I'm wondering in which kind of scenarios this would be useful.
In case you do not wand anyone to extend a class.
Classic example is a String class, it's self-sufficient and implements flyweight pattern, that might be easily destroyed if anyone could overwrite it's methods/properties.
You want to use the sealed class if you provide your API that doesn't need to be changed, for any of above reasons.