How should concrete types return an instance of an interface in a loosely-coupled way?

Question

I have an IDirectory interface and a Directory class that implements it.
I need to create an IDirectoryEnumerator which just has a method that looks like so:

IEnumerable<IDirectory> GetDirectories();

So I created a DirectoryEnumerator class, but I'm not sure which IDirectory to return in GetDirectories(), since I want to keep my code loosely-coupled.

Should I use generics with constraints to IDirectory somehow?
Should I get it somehow through a DI container?
Should something like that be tightly-coupled and I should focus on a specific concrete type?
Or is there another better option?

Note: concrete types of IDirectory in my example don't have any default constructors and are immutable.

Answer 1

You could let the concrete IDirectoryEnumerator implementation accept some sort of factory (e.g. IDirectoyFactory) through the constructor, and then use a parametrized method in that factory to create concrete types of IDirectory.

This also makes testing easier and enables you to use a DI container to inject the dependencies.

EXAMPLE:

public interface IDirectoryEnumerator
{
    IEnumerable<IDirectory> GetDirectories();
}

public class DirectoryEnumImpl : IDirectoryEnumerator
{

    private readonly IDirectoryFactory _directoryFactory;

    public DirectoryEnumImpl(IDirectoryFactory directoryFactory)
    {
        _directoryFactory = directoryFactory;
    }

    public IEnumberable<IDirectory> GetDirectories()
    {
        // you can use the factory here
    }
}

public interface IDirectoryFactory
{
    IDirectory CreateDirectory(DirectoryType directoryType);
}

public class DirectoryFactoryImpl : IDirectoryFactory
{
    IDirectory CreateDirectory(DirectoryType directoryType)
    {
        if (directoryType == DirectoryType.DIR_A)
            return new Dir_A();
        // the same goes for other types.
    }
}

public enum DirectoryType {
    DIR_A, DIR_B, // etc ...
}