Sealed class.
http://msdn.microsoft.com/en-us/library/88c54tsw%28v=vs.80%29.aspx
Abstract class
http://msdn.microsoft.com/en-us/library/ms173150.aspx
Polymorphism
http://msdn.microsoft.com/en-us/library/ms173152%28v=vs.80%29.aspx
Enumeration
http://msdn.microsoft.com/en-us/library/cc138362.aspx