Should I Prefer a Closed or Open List<> System?

Question

Hey guys,

I've got a class in my project that stores a List<> of elements. I'm trying to figure out whether I should allow the user to add to that List directly (e.g. Calling the native add/remove methods) or lock it down by declaring the List private and only allowing a handful of methods I choose to actually alter the List.

It's a framework, so I'm trying to design it as robustly as possible, but I also want to keep it as simple and error-free as possible.

What's the best practice in this situation?

Thanks, Tyler

Answer 1

Keeping the list private gives you more control and may make it more robust (you can check values before accepting them into the list)

But keep it public is the simplest.

Answer 2

You could sub class the List and override the methods that modify the lists contents. In the override, you could either do additional processing to prevent the list from being modified, or you could fire an event so that you can be notified when the list changes. You could even just not call the base implementation in the overrides, and throw a NotSupportedException, if you wanted to disable the method altogether.

Answer 3

It really depends on what you are doing.

Personally, when I have a list in a custom class, and its providing, say, a list of business entities to a databound control, I would make it private and expose a couple of simple public methods to update it, and I could put extra code in there, like moving data around or whatever.

However, If the list is a result of some query, it may be better to expose the whole thing, and be able to use all your extension methods etc to work with it.

Its about whats best for the framework in context, in my opinion

Answer 4

You should always keep your variables private. See Jon Skeets advice for details. Basically because you want to keep control on how you handle the data. The user does not need to know if it is a List or something else.

If you're using .NET 3.5 or above you can privately use a List<> and make a public property ReadonlyCollection. This is also thread-safe, but might require a bit of work on your side.

For that kind of questions, I can recommend FxCop. It analyzes your code and gives you hints on design, performance, etc.

Answer 5

For a framework, I'd recommend to encapsulate the list completely and create methods to retrieve and add elements to it.

If the need arises to check the elements which are added, or events need to be fired for some actions, you'll be able to do that.

If you prefer to store those elements differently for whatever reason, you can.

On the other hand. if you allow to access the list directly, it will be difficult to go back and encapsulate it, or to change that and use something else. Code which uses this framework may depend exactly on that direct access to the list.

Answer 6

@Dylan Vester: As you should be favoring composition over inheritence, don't inherit the list. You may want to change it to a different data structure later, or perform some sort of validation/checking before adding to your list.