internal class A { } internal class B { public class C : A { } } Note that effective accessiblity of C is 'internal', so it is OK that it is derived from internal class. Apply refactoring 'move to outer scope' to nested class C. internal class A { } internal class B { } public class C : A { } /* error CS0060: Inconsistent accessibility: base class 'A' is less accessible than class 'C' */ Preferred result: internal class A { } internal class B { } internal class C : A { } /* accessibility OK */