Question

I’ve noticed in other people’s code that methods returning generic collections will almost always return an interface (e.g. IEnumerable<T> or IList<T>) rather than a concrete implementation.

I have two related questions. Firstly, why (if at all) is it considered better to return an interface? Secondly, is there a collection interface that includes the Sort method (as List<T> does)?

Answer 1

From C# – List or IList

If you are exposing your class through
a library that others will use, you
generally want to expose it via
interfaces rather than concrete
implementations. This will help if you
decide to change the implementation of
your class later to use a different
concrete class. In that case the
user’s of your library won’t need to
update their code since the interface
doesn’t change.

If you are just using it internally,
you may not care so much, and using
List may be ok.