Is returning IList worse than returning T[] or List?

Question

The answers to questions like this: List or IList always seem to agree that returning an interface is better than returning a concrete implementation of a

Answer 1

Returning an interface is not necessarily better than returning a concrete implementation of a collection. You should always have a good reason to use an interface instead of a concrete type. In your example it seems pointless to do so.

Valid reasons to use an interface could be:

1. You do not know what the implementation of the methods returning the interface will look like and there may be many, developed over time. It may be other people writing them, from other companies. So you just want to agree on the bare necessities and leave it up to them how to implement the functionality.

2. You want to expose some common functionality independent from your class hierarchy in a type-safe way. Objects of different base types that should offer the same methods would implement your interface.

One could argue that 1 and 2 are basically the same reason. They are two different scenarios that ultimately lead to the same need.

"It's a contract". If the contract is with yourself and your application is closed in both functionality and time, there is often no point in using an interface.