Weird Behavior Using MOQ - unit-testing

Question

I am experiencing some weird behavior using MOQ in my unit tests:
Given the following test:
[Fact]
public void ShoppingCart_ShouldIncrementQuantity_WhenAddingDuplicateItem()
{
var cart = new ShoppingCart();
var item1 = GetMockItem("Test");
var item2 = GetMockItem("Test", quantity: 2);
cart.AddItem(item1.Object);
cart.AddItem(item2.Object);
cart.Items.Single(x => x.Sku == "Test").Quantity
.Should().Be(3);
}
private Mock<IShoppingCartItem> GetMockItem(string sku, decimal price = 10, int quantity = 1)
{
var mock = new Mock<IShoppingCartItem>();
mock.Setup(x => x.Sku).Returns(sku);
mock.Setup(x => x.Price).Returns(price);
mock.Setup(x => x.Quantity).Returns(quantity);
return mock;
}
And this is the code under test:
public void AddItem(IShoppingCartItem item)
{
Enforce.ArgumentNotNull(item, "item");
var existingItem = this.Items.SingleOrDefault(x => x.Sku == item.Sku);
if (existingItem != null)
{
existingItem.Quantity += item.Quantity;
}
else
{
this.Items.Add(item);
}
}
I am getting this result: Test 'Titan.Tests.ShoppingCartTests.ShoppingCart_ShouldIncrementQuantity_WhenAddingDuplicateItem' failed: Expected 3, but found 1.
I am baffled or I am just having a dumb moment!

Answer 1

The problem here is that you haven't told Moq what to do when the Quantity property is set.
By default, Moq doesn't just assume that all of your properties should be simple getter/setters. It's up to you to decide what to do with them.
You have a couple of options.
Use SetupAllProperties() to tell Moq to treat properties as simple getter/setters.
private Mock<IShoppingCartItem> GetMockItem(string sku, decimal price = 10, int quantity = 1)
{
var mock = new Mock<IShoppingCartItem>();
mock.SetupAllProperties();
// Set the properties like normal properties. Moq will do the right thing.
mock.Object.Sku = sku;
mock.Object.Price = price;
mock.Object.Quantity = quantity;
return mock;
}
Use SetupSet to handle the case where the Quantity property is set, and in its callback, re-setup the property getter, so that it returns the new value.
private Mock<IShoppingCartItem> GetMockItem(string sku, decimal price = 10, int quantity = 1)
{
var mock = new Mock<IShoppingCartItem>();
mock.Setup(x => x.Sku).Returns(sku);
mock.Setup(x => x.Price).Returns(price);
mock.Setup(x => x.Quantity).Returns(quantity);
// You can call Setups from within Setups
mock.SetupSet(x => x.Quantity).Callback(q => mock.Setup(x => x.Quantity).Returns(q));
return mock;
}
Alternately, you could also change your design so that you aren't modifying public properties.

Answer 2

The mocked property of the first item is set up to always return 1. It doesn't matter you add 2 to it then, it will always return 1.
Edit: your += is ignored because your cart stores mocked objects. The one that gets to the cart first is mocked to ALWAYs returns 1.