productbl.cs

public class ProductBL
    {
        private readonly ProductDAL dal;

        public ProductBL(ProductDAL dal)
        {
            this.dal = dal;
        }

        public List<Product> GetProducts()
        {
            // Get products, make some manipulation with the products,
            // e.g. sorting, filtering...
            return dal.GetProducts();
        }
    }