public class FileFactory
    {
        private static readonly string path = ConfigurationManager.ConnectionStrings["DBType"].ConnectionString;
        private static readonly string namespacepath = "xxx"
        public static IFileDal CreateFile()
        {
            string className = namespacepath + path + ".FileDal";
            return (IFileDal)Assembly.Load("DLL name").CreateInstance(className);
        }
    }