public void VariantsOf_Should_Throw_ArgumentException_If_ProductId_Is_Empty()
        {
            IQueryable <Product> products = new Product[0].AsQueryable();
            Guid productId = Guid.Empty;

            var ex = Assert.Throws <ArgumentException>(() => ProductExtensions.VariantsOf(products, productId));

            Assert.Equal(nameof(productId), ex.ParamName);
        }
        public void VariantsOf_Should_Throw_ArgumentNullException_If_Products_Is_Null()
        {
            IQueryable <Product> products = null;
            Guid productId = Guid.NewGuid();

            var ex = Assert.Throws <ArgumentNullException>(() => ProductExtensions.VariantsOf(products, productId));

            Assert.Equal(nameof(products), ex.ParamName);
        }