I recently had to update an abstract base class on some OSS that I was using so that it was more testable by making them virtual (I could not use an interface as it combined two). This got me thinking whether I should mark all of the methods that I needed virtual, or if I should mark every public method/property virtual. I generally agree with Roy Osherove that every method should be made virtual, but I came across this article that got me thinking about whether this was necessary or not. I am going to limit this down to abstract classes for simplicity, however (whether all concrete public methods should be virtual is especially debatable, I am sure).

I could see where you might want to allow a sub-class to use a method, but not want it overriding the implementation. However, as long as you trust that Liskov's Substitution Principle will be followed, then why would you not allow it to be overriden? By marking it abstract, you are forcing a certain override anyway, so, it seems to me that all public methods inside of an abstract class should indeed be marked virtual.

However, I wanted to ask in case there was something I might not be thinking. Should all public methods within an abstract class be made virtual?