Casting IMongoQueryable<T> to IMongoQuerable<TInterface>

I have an object model where a large number of entities share a common structure, defined in interfaces. Here’s a sample object and the interfaces

public class PhoneBookCategory : IIdItem, INamedItem
{
    public string Id { get; set; }

    public string Name { get; set; }
}

public interface IIdItem
{
    string Id { get; set; }
}

public interface INamedItem
{
    string Name { get; set; }
}

I then have a geric search method that works on the Name property. I’m starting with an IMongoQueryable<T>. I then check if T is an INamedItem:

if (typeof(INamedItem).IsAssignableFrom(typeof(T))

If true, I’d like to run a query on an IMongoQueryable<INamedItem>^. But.. it appears that an ? IMongoQueryableis not anIMongoQueryble even if T implements INamedItem. So, how'd I go t about it? The funny thing is, ìMongoQueryable<T> IS an IMongoQueryable<PhoneBookCategory> so casting works as expected. But I don’t want to write a search for every type of T (my app has a large number of them), I’d like to handle every T that is an INamedItem in one go.