A system and method for constraining generic types is disclosed. In brief, the subject invention provides a pattern type for defining a set of one or more parameter constraints. Rather than being confined to a predetermined set of hard-coded constraints in an execution engine, a pattern type can be specified as an extensible constraint mechanism for generic type parameters. According to one aspect of the invention, the pattern type can manifest itself as a class including one or more properties and/or features. The shape of the pattern type can then be compared or matched to instantiated type parameters to determine constraint satisfaction.