“Interface not implemented” when Returning Derived Type

Question

The following code:

public interface ISomeData
{
    IEnumerable Data { get; }
}

public class MyData : ISomeData
{
    private List

        

Answer 1

I would choose option 2:

The point to define an interface in your code is to define an contract, and so you and other people who implement your interface know what to agree on. Whether you define IEnumerable or List in your interface is really an contract issue and belong to framework design guideline. Here is a whole book to discuss this.

Personally , I would expose IEnumerable and implement MyData to IEnumerable, and you can cast it back to List in RandomAccess() method.