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

Short Answer:

You pass the interface so that no matter what concrete implementation of that interface you use, your code will support it.

If you use a concrete implementation of list, another implementation of the same list will not be supported by your code.

Read a bit on inheritance and polymorphism.