LongLivedMarshalByRefObject A MarshalByRefObject that lives forever

 NUnitNullType This class is used as a flag when we get a parameter list for a method/constructor, but we do not know one of the types because null was passed in.

 AdditionalTypeExtensions Type extensions that apply to all target frameworks

 AttributeHelper Provides a platform-independent methods for getting attributes for use by AttributeConstraint and AttributeExistsConstraint.