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

Using IList instead of List makes writing unit tests significantly easier. It allows you to use a 'Mocking' library to pass and return data.

The other general reason for using interfaces is to expose the minimum amount of knowledge necessary to the user of an object.

Consider the (contrived) case where I have a data object that implements IList.

public class MyDataObject : IList
{
    public void Method1()
    {
       ...
    }
    // etc
}

Your functions above only care about being able to iterate over a list. Ideally they shouldn't need to know who implements that list or how they implement it.

In your example, IEnumerable is a better choice as you thought.