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    <blockquote> <p>In C# you have to mark method as virtual to make it possible to override. Does it mean that in C# you should mark all methods virtual (except a few ones that you don't want to be overridden), since most likely you don't know in what way your class can be inherited?</p> </blockquote> <p>No. If the language designers thought that virtual should have been the default then <strong>it would have been the default</strong>.</p> <p>Overridablility is a <em>feature</em>, and like all features it has <em>costs</em>. The costs of an overrideable method are considerable: there are big design, implementation and testing costs, particularly if there is any "sensitivity" to the class; virtual methods are ways of introducing untested third-party code into a system and that has a security impact.</p> <p>If you don't know how you intend your class to be inherited then <em>don't publish your class</em> because <em>you haven't finished designing it yet.</em> Your extensibility model is definitely something you should know ahead of time; it should deeply influence your design and testing strategy. </p> <p>I advocate that all classes be <em>sealed</em> and all methods be <em>non-virtual</em> until you have a real-world customer-focussed reason to unseal or to make a method virtual. </p> <p>Basically your question is "I am ignorant of how my customers intend to consume my class; should I therefore make it arbitrarily extensible?" No; you should <em>become knowledgable</em>! You wouldn't ask "I don't know how my customers are going to use my class, so should I make all my properties read-write? And should I make all my methods read-write properties of delegate type so that my users can replace any method with their own implementation?" No, don't do any of those things until you have <em>evidence</em> that a user actually needs that capability! Spend your valuable time designing, testing and implementing features that users actually want and need, and do so from a position of knowledge.</p>
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    1. POShould I mark all methods virtual?
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    1. USEric Lippert
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    1. CO+100 for "all classes be sealed and all methods be non-virtual until you have a real-world customer-focussed reason to unseal or to make a method virtual."
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      1. UStallseth
    2. COI agree with this answer, but I find it funny that you advocate that classes should be `sealed` by default, which they are not.
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      1. USMatthew
    3. CO-1 as I don't really agree. I personnaly stick with the open/close principle. I don't know how my classes are going to be extended, so I make an educated guess. Later on, I can refine my choices, based on customer feedback. That way customer's not blocked, and we both can keep moving on.
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      1. USClement Herreman
 

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