Test: Assert an Exception

Write and test a validation feature in the ShoppingCart class to prevent the addition of items with negative quantities or prices.

Part 1: Writing the Failing Test

1. Create the Test for Negative Quantity

   • In your test project, under ShoppingCartTests.cs, write a new test method to ensure that an exception is thrown when attempting to add an item with a negative quantity.

   [Fact]
   public void AddItem_WithNegativeQuantity_ThrowsArgumentException()
   {
       var cart = new ShoppingCart();
       var item = new Item("Apple", -1, 0.75);
   
       Action act = () => cart.AddItem(item);
   
       act.Should().Throw<ArgumentException>().WithMessage("*negative*");
   }

2. Run the Test

   • Execute the test. It should fail, indicating that the ShoppingCart class currently does not handle this scenario.

Part 2: Implementing the Validation

1. Add Validation Logic to ShoppingCart

   • Modify the AddItem method in the ShoppingCart class to include validation logic that checks for negative quantities and prices.

   public class ShoppingCart
   {
       // ... existing code ...
   
       public void AddItem(Item item)
       {
           if (item.Quantity < 0 || item.Price < 0)
           {
               throw new ArgumentException("Quantity and price must be non-negative.");
           }
           Items.Add(item);
       }
   }

2. Re-run the Test

   • Run the test again. It should now pass, confirming that the ShoppingCart class correctly handles negative quantities.

Part 3: Refactoring (Optional)

1. Refactor if Necessary

   • Review the code and tests for any possible improvements in readability or efficiency.