The Specification pattern has been added to NGenerics. In my previous post on the Specification Pattern, we explored creating specification functionality using extension methods. It's actually been implemented with the operator methods (And, Or, Xor) on the actual interface, with an abstract class forming the base of all specifications. The deal-breaker for this approach was the need to add operators |, &, | and ^ in order to trim down the syntax a little bit. With the Specification Pattern in NGenerics, you can now do this :

400: Invalid request

Alternatively, you can also use the operators defined in AbstractSpecification<T> :

400: Invalid request

The part I'm looking into now is subsumption and partially satisfied specifications. From Martin Fowler and Eric Evans' paper :

Subsumption allows you to compare specifications to see if satisfying one implies satisfaction of the other. It is also sometimes possible to use subsumption to implement satisfaction. If a candidate object can produce a specification that characterizes it, the testing with a specification then becomes a comparison of similar specifications — removing the coupling of specification mechanism entirely from the domain.