What are the best practices regarding the use and structure of inner classes in C#.
For instance if I have a very large base class and two large inner classes should I split them up into separate (partial class) codefiles or leave them as one very large unwieldy codefile?
Also is it bad practice to have an abstract class, with a public inherited inner class?
Answer
Typically I reserve inner-classes for one of two purposes:
Public classes which derive from their parent class where the parent class is an abstract base implementation with one or more abstract methods and each subclass is an implementation which serves a specific implementation. after reading Framework Design and Guidelines I see that this is marked as "Avoid", however I use it in scenarios similar to enums--althogh that's probably giving a bad impression as well
The inner classes are private and are units of business logic or otherwise tightly coupled to their parent class in a manner in which they are fundamentally broken when consumed or used by any other class.
For all other cases I try to keep them in the same namespace and the same accessibility level as their consumer/logical parent--often with names that are a little less friendly than the "main" class.
On big projects you'd be surprised how often you may find yourself initially building a strongly-coupled component just because it's first or primary purpose makes it seem logical--however unless you have a very good or technical reason to lock it down and hide it from sight then there is little harm in exposing the class so that other components can consume it.
Edit Keep in mind that even though we're talking about sub-classes they should be more-or-less well designed and loosely coupled components. Even if they are private and invisible to the outside world keeping a minimal "surface area" between classes will greatly ease the maintainability of your code for future expansion or alteration.