When to use custom collections?

Question

I see a lot of APIs that use custom collections instead of just using strongly-typed packaged generics.

For instance, the API will have a Object1Collection instead of just a List<Object1> (or whatever the appropriate generic would be).

When do you know to use one or the other? I understand you'd create a custom collection if you want to extend the capabilities of a pre-shipped generic collection, but what if that's not the case?

Is it prudent to create the custom collection from the beginning, with thoughts that in the future that object might scale up and require further capabilities, and it may be easier to just do that initially? Is that justification enough?

And if you just want to extend functionality, instead of define collection functionality, you would just derive your custom collection from a base generic collection class, otherwise inherit the interface accordingly? Is that typical?

Answer 1

In general, rarely if ever.

You'll occasionally make things that wrap collections and work on collections of things, but you will rarely make a class that is a generic, reusable collection. More often you'll have some class that represents some concept which uses a collection, rather than is a collection.

Why? Because frankly there's only so many data structures in the world, and they've pretty much all been implemented. Maybe you can come up with the next hash table, and if you do, then make a new collection type for it.

If you want to add functionality, either wrap the existing collection or add extension methods to it.

If you see a specific ObjectCollection, I would assume that the API was made during .NET 1.0 (no generics) or for some API that has onerous interoperability requirements put in it.

Answer 2

Custom collections don't really make sense for general programming. Thats why List<T> exists. But they sure make lots of sense in domain modelling. For example, you would like to have special behavior that happens when you add a new item into a collection. The best way to do it is to create custom collection class, derive from Collection<T> and overload the Add method.

Also, there is question of what actually is a "custom collection". Is having Add method and implementing IEnumerable<T> enough or does the class have to derive from Collection<T>?

So as Telastyn said. You are generally not going to implement new collections in a way of creating new ways of storing data. That problem has long been solved. But you sure can (and should) create custom collections as a way of expressing domain behavior on those collections.