> For the complete documentation index, see [llms.txt](https://raviram.gitbook.io/tdd/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://raviram.gitbook.io/tdd/tdd-exercises-2/shopping-cart/test-add-cart-item/test-assert-an-exception.md).

# 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.

   ```csharp
   [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**

3. **Add Validation Logic to ShoppingCart**

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

   ```csharp
   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);
       }
   }
   ```
4. **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)**

5. **Refactor if Necessary**
   * Review the code and tests for any possible improvements in readability or efficiency.


---

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