Generics - Using parent class to specify type in generics

Question

I am looking at some covariance/contravariance stuff, I have a much wider question but it all boils down to this:

GenericRepository<BaseEntity> repo = new GenericRepository<ProductStyle>(context);

This doesn't work, even though BaseEntity is the parent abstract class of ProductStyle, is there a way of achieving this?

Answer 1

The only way of doing that is with an out generic restriction (which will make it hard to save objects, but fine to retrieve them), on an interface (not a class). If you have:

interface IGenericRepository<out T> {...}

then an IGenericRepository<ProductStyle> can be assigned to a variable of type IGenericRepository<BaseEntity>, since all ProductStyle are also BaseEntity, and we have restricted ourselves to covariant / out usage:

IGenericRepository<BaseEntity> tmp = GetRepo<ProductStyle>(context);
// note that the right-hand-side returns IGenericRepository<ProductStyle>
...
private IGenericRepository<T> GetRepo(...) {...}

Note, however, that this covariant / out usage makes it impossible to do things like:

interface IGenericRepository<out T>
{
    T Get(int id); // this is fine, but:
    void Save(T value); // does not compile; this is not covariantly valid
}