“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

Why not just return a List from your interface ...

public interface ISomeData
{
    List Data { get; }
}

If you know your consumers are going to both iterate over it (IEnumerable) and add to it (IList) then it seems logical to simply return a List<>.