public static IEnumerable<Product> GetProformaFormularies(int? productDepartmentId = null, int? productCategoryId = null, IEnumerable<int?> productCategoryIds = null, IEnumerable<string> sections = null) {
            var retList = (from obj in products
                            where (productDepartmentId == null || obj.ProductDepartmentId == productDepartmentId) &&
                                    (productCategoryId == null || obj.ProductCategoryId == productCategoryId) &&
                                    (productCategoryIds == null || productCategoryIds.Contains(obj.ProductCategoryId)) &&
                                    (sections == null || sections.Contains(obj.sections))
                            select obj).ToList();
            return retList;
        }

        private static List<Product> products;

        public class Product {
            public int ProductDepartmentId, ProductCategoryId;
            public string sections;
        }