Generic interfaces which type implements another interface-CodePudding

Could somebody explain why it doesn't work.

I have two db context. And one common method with different return types and different queries.

public interface IDataFetcher<T> where T : IMarker
{
    public List<T> GetData();
}
public interface IFetchServiceOne<T> : IDataFetcher<T> where T : IMarker
{
//maybe some methods here
}
public interface IFetchServiceTwo<T> : IDataFetcher<T> where T : IMarker
{
//maybe some different methods here
}

Implementation:

public class FetchServiceOne<T> : IFetchServiceOne<T> where T : IMarker
{
    private readonly DBContext _dbContext;

    public FetchServiceOne(DBContext dbContext) => _dbContext = dbContext;

    public List<CrucialData> GetData()
    {
        var example = _dbContext.Test.ToList();
        return example;
    }
}
public class FetchServiceTwo<T> : IFetchServiceOne<T> where T : IMarker
{
    private readonly DBContext _dbContext;

    public FetchServiceTwo(DBContext dbContext) => _dbContext = dbContext;

    public List<CrucialDataTwo> GetData()
    {
        var example = _dbContext.Test2.ToList();
        return example;
    }
}
public class CrucialData: IMarker
{
//some properries
}
public class CrucialDataTwo: IMarker
{
//another properries
}

In the output I'm getting compile error:

Error (active) CS0738 'FetchService' does not implement interface member 'IDataFetcher.GetData()'. 'FetchService.GetData()' cannot implement 'IDataFetcher.GetData()' because it does not have the matching return type of 'List'.

CodePudding user response：

It looks like IFetchServiceOne and IFetchServiceTwo (and their implementations) should be "concrete" interfaces which implement closed versione of IDataFetcher. I.e.:

public interface IFetchServiceOne : IDataFetcher<CrucialData>
{
    //maybe some methods here
}

public class FetchServiceOne : IFetchServiceOne
{
    // ...

    public List<CrucialData> GetData()
    {
        // implementation ...
        return default;
    }
}

Otherwise you can't guarantee that T requested by user will be CrucialData for FetchServiceOne (i.e. FetchServiceOne<CrucialDataTwo>.GetData() ...)

Or just:

public class FetchServiceOne : IDataFetcher<CrucialData>
{
   // ...
}

Without need for introduction of the intermediate interface.

Note that for EF you can create a generic implementation via Set method if needed:

public class FetchServiceOne<T> : IDataFetcher<T> where T : IMarker
{
    private readonly DBContext _dbContext;

    public FetchServiceOne(DBContext dbContext) => _dbContext = dbContext;

    public List<T> GetData()
    {
        var example = _dbContext.Set<T>.ToList();
        return example;
    }
}

But note that this will fail at runtime if there is no matching DbSet property setup for provided T type.

CodePudding user response：

List<T> is neither co- nor contravariant. I.e., a List<A> is not assignment comapatible to a List<B>, even if A and B are.

Let GetData return a List<T> and also cast the IQueryable result to T before creating the list:

public class FetchServiceOne<T> : IFetchServiceOne<T> where T : IMarker
{
    ...

    public List<T> GetData()
    {
        return _dbContext.Test.Cast<T>().ToList();
    }
}

This compiles, but be aware that this is not type safe. If the user specifies a T that is not compatible to the DataSet, you will get a runtime error.