将dll动态加载到程序中的问题

I'm trying to add plugins to my program, and this looks good, except that I can't cast the correct type from the dll. I Have a solution with several projects on it. One of the project is a country Layer, that actually holds a CountryBase (defined as public abstract class CountryBase : CountryLayers.ICountryBase ) The Interface (public interface ICountryBase)I'm trying to add plugins to my program, and th