OCP Principle

Jeremy Miller (aka The Shade Tree Developer) has a very good article about OCP principle printed by MSDN magazine. This is a valuable article for any developer that strives to work according to law of "measure twice, cut once", and not just cook spaghetti. http://msdn.microsoft.com/en-us/magazine/cc546578.aspx 

This is not the only principle to consider, but one of many that are not widely explained in textbooks during your school years. OCP, LSP, DIP, ISP, and much more. I am more than happy ALT.NET folks are bringing these subjects over the new brand shiny new technologies under the hood of .NET