I have recently solved a very similar issue in my own code by using a DI framework. To satisfy Dependency Inversion, the factory constructor should accept an interface (as the other answers have said), but to get the framework to inject the right type is tricky without having a massive list of arguments detailing each possible concretion.
SimpleInjector allows you to register all concretions of a given abstraction with:
Container.RegisterCollection(typeof(IProvider), new [] {typeof(TKnown).Assembly,...});
Your XML could list the (possibly external) assemblies where the concretions are defined and you could build the assembly array from there. Then your factory just needs to accept them all and pick one, perhaps based on the searchType you mentioned.
public class ProviderFactory
{
private List<IProvider> providers;
public ProviderFactory(IEnumerable<IProvider> providers)
{
this.providers = providers.ToList();
}
public IProvider GetProvider(string searchType)
{
// using a switch here would open the factory to modification
// which would break OCP
var provider = providers.SingleOrDefault(concretion => concretion.GetType().Name == searchType);
if (provider == null) throw new Exception("No provider found of that type. Are you missing an assembly in the RegisterCollection for IProvider?");
return provider;
}
I know I'm way late to the party on this but assuming other folks don't see this approach as problematic, it might be useful.