How do I define the return type of an interface method to be another interface?

Question

I\'m new to interfaces and abstract classes. I want to create a couple of interfaces to define core methods and variables for the objects for a shopping cart system. Then I want

Answer 1

This is a matter of contra/covariance.

2 changes are required to make this compile.

1) Remove the "set" option on the property of the interface. (It's only implementing a get; property, which makes the most sense, in any case)

2) Change Cart to:

public abstract class Cart : ICart
{


private List _cartItems = new List();

public List CartItems 
{ ...

I also highly recommend changing your interface to expose IList instead of List. The design guidelines (and FxCop) recommend not exposing List in the public interface. List is an implementation detail - IList is the appropriate interface/return type.