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

 AssemblyExtensions Extensions for Assembly that are not available in .NET Standard

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

 MemberInfoExtensions Extensions to the various MemberInfo derived classes

 TypeExtensions Provides NUnit specific extensions to aid in Reflection across multiple frameworks