类型实现另一个接口的通用接口

Question

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'.

Answer 1

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

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 (ie 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 this will fail at runtime if there is no matching DbSet property setup for provided T type.