There are some design guidelines about testable code in “The Art of Unit Testing”. The first one is “Make methods virtual by default”. I’m curious to know your idea about non-virtual-by-default behavior in C#. I’ve read about Hejlsberg opinions but I think one the most important reasons could be that it may lead us to “composition over inheritance” principal.
Could “composition over inheritance” be one of those reasons which make non-virtual-by-default preferred over virtual-by-default?
Regarding this subject, please consider test-driven point of view; where we want to write testable code. While we are encouraged to make all members virtual by default (at the mentioned book), we can follow "composition over inheritance" and keep going non-virtual-by-default. Isn't it better?