Note: I cannot just write where T : IRealmObject, because it will broke interface implementation

That's all you can do

can't I show that T is acceptable Type somehow else?

like cast it

That's whatwhere is for

Where is compile time. You're trying to fix a compile problem with a run time check. No bueno

You can change All<T> to All<IRealm...> Then also use OfType I guess

That would not be efficient, I'm presuming

public IQueryable<T> Select<T>()
        {
            if (!typeof(T).IsSubclassOf(typeof(IRealmObject)))
                throw new TypeAccessException($"{typeof(T).Name} is not a subclass of {typeof(IRealmObject).Name}");

            return realm?.All<IRealmObject>().OfType<T>();
        }

At least it works this way