Question

I’m refactoring some code and want to classes a bit higher in the inheritance chain be a bit more strict with their parameters. As I’m not sure I’m explaining this correctly, here’s what I’ve got:

public interface ISvdPredictor
{
    List<string> Users { get; set; }
    List<string> Artists { get; set; }
    float PredictRating(ISvdModel model, string user, string artist);
    float PredictRating(ISvdModel model, int userIndex, int artistIndex);
}

ISvdPredictor uses ISvdModel:

public interface ISvdModel
{
    float[,] UserFeatures { get; set; }
    float[,] ArtistFeatures { get; set; }
}

Now I want to implement another variation:

public interface IBiasSvdPredictor : ISvdPredictor
{
    float PredictRating(IBiasSvdModel model, string user, string artist);
    float PredictRating(IBiasSvdModel model, int userIndex, int artistIndex);
}

Which uses IBiasSvdModel which derives from ISvdModel:

public interface IBiasSvdModel : ISvdModel
{
    float GlobalAverage { get; set; }
    float[] UserBias { get; set; }
    float[] ArtistBias { get; set; }
}

IBiasSvdPredictor will not work with ISvdModel.

The problem is that when I implement IBiasSvdPredictor I’d have to implement 2 pairs of PredictRating methods. One from ISvdPredictor and the other from IBiasSvdPredictor. What do I need to do to be able to just implement those from IBiasSvdPredictor?

I’ve tried generics as well, but couldn’t restrict the PredictRating for BiasSvdPredictor to IBiasSvdModel using the where directive. I may be doing this all wrong so any suggestion might help. I think you get what I’m trying to do.

EDIT: If anyone needs more context see https://github.com/gligoran/RecommendationSystem. I’m writing this code for my thesis for BSc.

Answer 1

You could use generics and constraints.

public interface ISvdModel
{
    float[,] UserFeatures { get; set; }
    float[,] ArtistFeatures { get; set; }
}

public interface IBiasSvdModel : ISvdModel
{
    float GlobalAverage { get; set; }
    float[] UserBias { get; set; }
    float[] ArtistBias { get; set; }
}

public interface ISvdPredictor<in TSvdModel>
    where TSvdModel : ISvdModel // Require that TSvdModel implements ISvdModel
{
    List<string> Users { get; set; }
    List<string> Artists { get; set; }

    float PredictRating(TSvdModel model, string user, string artist);
    float PredictRating(TSvdModel model, int userIndex, int artistIndex);
}

// I would actually avoid declaring this interface. Rather, see comment on the class.
public interface IBiasSvdPredictor : ISvdPredictor<IBiasSvdModel> { }

class BiasSvdPredictor : IBiasSvdPredictor // Preferred : ISvdPredictor<IBiasSvdModel>
{
    // ...
    public float PredictRating(IBiasSvdModel model, string user, string artist) { }
    public float PredictRating(IBiasSvdModel model, int userIndex, int artistIndex) { }
}