Using nested Classes in C# and C++ - Why and When?

Question

In my self-directed efforts to bring my programming skills and habits into the 21st Century (migrating from Pascal & Fortran to C# and C++), I've been studying a fair amount of available source code. So far as I have been able to determine, Classes are unique 'standalone' entities (much like their Function ancestors).

I have, however, run across numerous instances where one or more Classes are nested within another Class. My 'gut instinct' in this regard is that doing so is simply due to extremely poor methodology - however, I'm not yet familiar enough with modern OOP methodology to truly make such a determination.

Hence, the following overlapping questions:

Is there legitimate reasoning for nesting one Class inside another? And, if so, what is the rationale in doing so as opposed to each Class being wholly independent?

(Note: The examples I've seen have been using C#, but it seems that this aspect applies equally to C++.)

Answer 1

Hmm this might call for a very suggestive answer and therefore many will disagree with my answer. I am one of those people who believe that there is no real need for nested classes and I am tended to agree with your statement:

My 'gut instinct' in this regard is that doing so is simply due to extremely poor methodology

Cases where people feel the need to design nested classes is where functionality that is tightly coupled to the behavior designed in the outer class. E.g. event handling can be designed in an inner class, or the Threading behavior can find its way to inner classes.

I rather refactor specific behavior out of the 'outer' class so that I end up with two smaller classes that both have clear responsibilities.

To me the main drawback of designing inner classes is that they tend to clutter functionality which is hard(er) to use with principals as TDD (test driven development).

If you are not relying on test driven principals, I think it will not harm you a lot. It is (like so many things) a matter of taste, not a matter of right or wrong. I learned that the topic can lead to long and exhausting discussion. Quite similar to whether you should use static helper classes that tend to do more than just 'be a helper' and are getting more and more state over time.

The discussions can become a lot more concrete if you ever run into a real life example. Until then it will be mostly people's "gut feeling".

Answer 2

Common uses for nested classes in C# include classes that are for internal use by your class that ou don't want exposed in your module's internal namespace. For example, you may need to throw an exception that you don't expose to the outside world. In that case you would want to make it a nested class so that others can't use it.

Another example is a binary tree's Node class. Not only would it be something you wouldn't want exposed outside of your class, but it might need access to private members for internal operations.