Why use IList or List?

Question

I know there has been a lot of posts on this but it still confuses me why should you pass in an interface like IList and return an interface like IList back instead of the c

Answer 1

Inside the method, you should use var, instead of IList or List. When your data source changes to come from a method instead, your onlySomeInts method will survive.

The reason to use IList instead of List as parameters, is because many things implement IList (List and [], as two examples), but only one thing implements List. It's more flexible to code to the interface.

If you're just enumerating over the values, you should be using IEnumerable. Every type of datatype that can hold more than one value implements IEnumerable (or should) and makes your method hugely flexible.