Question

I need to design my own custom GenericCollection class. Now i have plenty of options to derive it using IEnumerable, ICollection, and IList, where later offers some added functionalities.

I am little confused that if i go with IEnumerable<T> i might require declaring the object to actually hold the collection like in this case _list.

public class GenericCollection<T> : IEnumerable<T>
{
    private List<T> _list;
    //...
}

But if i go with ICollection<T> or IList<T>, i do not require to declare the List object as it is implicitly available.

public class GenericCollection<T> : IList<T>
{
    // no need for List object
    //private List<T> _list; 
    //...
}

What is the difference between these two approaches with respect to performance?

In which scenario each one is preferred especially when it comes to designing your own collection. I am interested in the light weight collection with good performance. I think this can be achieved using IEnumerable<T> but how exactly along with some strong reasons to go with it?

I have reviewed some existing posts but none is giving required information.

Returning 'IList' vs 'ICollection' vs 'Collection'

Answer 1

IEnumerable, ICollection, and IList (generally, any type with an I prefix) are just interfaces. They let you expose what your class will do, but unlike if you inherit a class, interfaces do not provide you a default implementation of any of the things they say you must do.

As far as choosing which interface, here’s a quick guide:

• An IList is an ICollection that can be accessed by index.
• An ICollection is an IEnumerable with easy access to things like Add, Remove, and Count.
• An IEnumerable is anything that can be enumerated, even if the list of those things doesn’t exist until you enumerate it.

Some classes that you might want to extend (or keep as a private field that runs most of the logic) for your collection are List<T>, Collection<T>, (which implements IList<T>, but with easier access to overriding implementation, see Collection<T> versus List<T> what should you use on your interfaces? for the big differences between these two) ObservableCollection<T>, or collections that are not lists, like Dictionary<T, U> and HashSet<T>. For more info on any of these, look up the MSDN documentation on the class.