Question

In my Base repo I have this code which works fine:

abstract class BaseRepo <T> : IRepo <T>
{
    private ISession _session;

    public Entity GetById<Entity>(int Id)
    {
        return _session.Get<Entity>(Id);
    }

    // other methods
}

I want to add another method to return all rows for an object (entity). I want to do something like:

    public IList<Entity> GetAll<Entity>()
    {
        return _session.CreateCriteria<Entity>().List<Entity>;
    }

but I get an error saying:

The type 'Entity' must be a reference type in order to use it as parameter 'T' in the generic type or method 'NHibernate.ISession.CreateCriteria<T>()'

Here’s my DAL design for reference: Should I use generics to simplify my DAL?

Answer 1

CreateCriteria method requires you to use reference types – add constraint on your DAL method:

public IList<Entity> GetAll<Entity>()
     where Entity : class
{
    return _session.CreateCriteria<Entity>().List<Entity>();
}

This naturally implies that any Entity type you pass to this method must be a reference type.

I also suggest naming your generic type parameter TEntity – Entity alone is a bit confusing (as it’s perfectly fine name for say, entity base class).