Changing Collection type from Interface to Implementing class

Question

If I have an interface with a method that returns a collection of something, is it possible to return an implementation of the collection type instead?

For example:

public interface IVehicle
{
    IEnumerable<Part> GetParts(int id);
}

public class Car : IVehicle
{
    List<Part> GetParts(int id)
    {
        //return list of car parts
    }
}

public class Train : IVehicle
{
    IEnumerable<Part> GetParts(int id)
    {
        //return IEnumerable of train parts
    }
}

If not, why not?

At least to me, this makes sense.

Strangely no. I can see use cases for this, though. Maybe C# has different syntax to be able to do this? — Jessica, Dec 06 '18 at 10:09

score 5 · Accepted Answer · answered Dec 06 '18 at 10:16

You are of course free to return any implementation of an IEnumerable<Part> as an implementation detail of your GetParts method (e.g. Train may return a List<Part> easily). But the method signatures have to match exactly between an interface definition and a class's implementation of that method.

Here (unlike e.g. overloading) the method signature includes the return type of the method. So no, you cannot write Car as shown or anything similar. You are of course free to have a GetParts method that does return a List<Part> but that will not satisfy the interface requirement - which you may choose to provide an explicit implementation for instead:

public class Car : IVehicle
{
    List<Part> GetParts(int id)
    {
        //return list of car parts
    }
    IEnumerable<Part> IVehicle.GetParts(int id) => this.GetParts(id);
}

score 1 · Answer 2 · answered Dec 06 '18 at 10:20

1

No, C# does not support covariance for inherited methods return type.

Does C# support return type covariance?

answered Dec 06 '18 at 10:20

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Changing Collection type from Interface to Implementing class

2 Answers2