public void ShouldChangeQuantityOfItemInBasketGivenExistingItemDetail() { //arrange var customerId = 123456789; var itemDetail = new { ProductId = 1234, Quantity = 1 }; var newQuantity = 25; var sut = new CustomerBasket(customerId); sut.AddItem(itemDetail.ProductId, itemDetail.Quantity); //act sut.ChangeQuantity(itemDetail.ProductId, newQuantity); //assert var result = sut.Items.Single(); Assert.Equal(itemDetail.ProductId, result.ProductId); Assert.Equal(newQuantity, result.Quantity.Amount); }