Classes in C# are inheritable by default, as in Java, but can be marked sealed (final). Members (methods and properties) in C# are final by default, and must be explicitly marked virtual.
"I'd say it was just a mistake. I know many people (including myself) who believe that classes should indeed be sealed by default. There are at least a couple of people in the C# design team in that camp. The pendulum has swung somewhat away from inheritance since C# was first designed. (It has its place, of course, but I find myself using it relatively rarely.)"