Question

I have an IRepository interface that inherits from IRepository<TObject>. I also have a SqlRepository class that inherits from SQLRepository<TObject>, which in turn implements IRepository<TObject>. Why can’t I instantiate a instance of SqlRepository as an IRepository?

public class MyObject : IObject {
    ...
}

public interface IRepository<TObject> where TObject : IObject, new() {
    ...
}

public interface IRepository : IRepository<MyObject> { }

public class SqlRepository<TObject> : IRepository<TObject> where TObject : IObject, new() {
    ...
}

public class SqlRepository : SqlRepository<MyObject> { }

public class Controller {
    private IRepository _repository;

    public Controller() {
        _repository = new SqlRepository();
    }
}

The example above fails when trying to assign a new SqlRepository to _repository in the Controller class, with the following error message.

Argument '1': cannot convert from 'SqlRepository' to 'IRepository'

Which basic principle of inheritance have I failed to grasp, please help.

Answer 1

Because IRepository is an IRepository<MyObject> and not the other way around.

Basically:

SqlRepository is a SqlRepository<MyObject>
which is an IRepository<MyObject>.
To make that work, you should either inherit IRepository<TObject> from IRepository or make SqlRepository implement IRepository depending in your intention.

This issue is not generics-specific at all. Assume:

interface IMovable { }
interface IDrivable : IMovable { } 
class Ball : IMovable { }

It’s obvious that Ball is not an IDrivable while it is an IMovable.