Class declared inside of another class in C#

Question

I am working on some legacy code and have come across something that I'm not sure of. We have a class y that is declared inside of another class x. Class y is only ever used inside of class x but my question is why wouldn't you create a separate class file and put class y in there instead of declaring it inside of class x? Isn't this violating OOP's or is it just a matter of style since it is only ever used inside of this class. I'm refactoring some of this code and my first reaction would be to separate class y out into it's own file.

namespace Library {    public class x    {       // methods, properties, local members of class x        class y       {          // methods, properties, local members of class y       }    } } 

Answer 1

You create an inner class because it is only ever used within the scope of class x and it logically fits in the factoring/architecture of class x.

Class y might also be privy to implementation details of class x that are not meant to be known to the public.

Answer 2

This has permissions implications. A top-level "class y" would be "internal" - however, here "y" is private to "x". This approach is helpful for implementation details (for example cache rows etc). Likewise, y has access to all private state of x.

There are also implications with generics; x<T>.y is generic "of T", inherited from the outer class. You can see this here, where Bar has full use of T - and note that any static fields of Bar are scoped per-T.

class Foo<T> {     void Test(T value) {         Bar bar = new Bar();         bar.Value = value;     }     class Bar {         public T Value { get; set; }     } } 

Often people incorrectly think they need to define Bar as Bar<T> - this is now (effectively) doubly generic - i.e. Foo<TOld, T> - where TOld is the (now unavailable) T from Foo<T>. So don't do that! Or if you want it to be doubly-generic, pick different names. Fortunately, the compiler warns you about this...