如何在单元测试中确定哪个是 SUT，哪个是协作者？

Question

I was reading a book and below is some code:

[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
    // Arrange
    var store = new Store();
    store.AddInventory(Product.Shampoo, 10);
    var customer = new Customer();

    // Act
    bool success = customer.Purchase(store, Product.Shampoo, 5);

    // Assert
    Assert.True(success);
    Assert.Equal(5, store.GetInventory(Product.Shampoo));
}

[Fact]
public void Purchase_fails_when_not_enough_inventory()
{
    // Arrange
    var store = new Store();
    store.AddInventory(Product.Shampoo, 10);
    var customer = new Customer();

    // Act
    bool success = customer.Purchase(store, Product.Shampoo, 15);

    // Assert
    Assert.False(success);
    Assert.Equal(10, store.GetInventory(Product.Shampoo));
}

The author says Customer is the SUT and Store is the collaborator

I'm a little bit confused here, the assert phase also test the Store, isn't that Store is also a SUT?

Answer 1

The SUT stands for System Under Test , which practically means that object against which you perform your act .

In the assert phase you are checking your assumptions, where there is an actual and an expected value. You are validating the actual against the expected.

---

In the above mentioned code the Assert.Equal receives first the expected value and then the actual. And here the actual value is coming from the store. It is good because the Purchase method call indicates that the inventory should shrink because a purchase has been placed

Happy path:
- Given an inventory with 10 items
- When I purchase 5 items
- Then 5 items will remain

Unhappy path:
- Given an inventory with 10 items
- When I try to buy 15 items
- Then my purchase will fail and the inventory remains as it was.

To better emphasise the intent you could rewrite the test like this:

[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
    // Arrange
    const int initialItemCount = 10;
    const int intededPurchaseCount = 5;

    var store = new Store();
    var product = Product.Shampoo;
    store.AddInventory(product, initialItemCount);
    var customer = new Customer();

    // Act
    bool isSuccess = customer.Purchase(store, product, intededPurchaseCount );

    // Assert
    Assert.True(isSuccess);
    var expectedInventoryCount = initialItemCount - intededPurchaseCount;
    Assert.Equal(expectedInventoryCount, store.GetInventory(product));
}

[Fact]
public void Purchase_fails_when_not_enough_inventory()
{
    // Arrange
    const int initialItemCount = 10;
    const int intededPurchaseCount = 15;

    var store = new Store();
    var product = Product.Shampoo;
    store.AddInventory(product, initialItemCount);
    var customer = new Customer();

    // Act
    bool isSuccess = customer.Purchase(store, product, intededPurchaseCount);

    // Assert
    Assert.False(isSuccess);
    Assert.Equal(initialItemCount, store.GetInventory(product));
}

Answer 2

As Peter Csala points out, SUT stands for System Under Test. I can see how this can be confusing if you interpret the word system to mean 'an arrangement of things'. Under such an interpretation, I can see how the store could also be viewed as part of a wider system.

That is, however, not the way the term SUT is usually interpreted. Normally, the term denotes the unit you directly interact with. In object-oriented programming, this is usually an object on which you call a method.

I usually name test variables according to the roles they play in the test. Thus, I typically name the SUT variable sut .

Also, now that we're at it, I don't find it necessary to use comments to denote the arrange, act, and assert phases when blank lines already make that structure clear .

The following is closer to how I'd write the tests.

[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
    var store = new Store();
    store.AddInventory(Product.Shampoo, 10);
    var sut = new Customer();

    bool success = sut.Purchase(store, Product.Shampoo, 5);

    Assert.True(success);
    Assert.Equal(5, store.GetInventory(Product.Shampoo));
}

[Fact]
public void Purchase_fails_when_not_enough_inventory()
{
    var store = new Store();
    store.AddInventory(Product.Shampoo, 10);
    var sut = new Customer();

    bool success = sut.Purchase(store, Product.Shampoo, 15);

    Assert.False(success);
    Assert.Equal(10, store.GetInventory(Product.Shampoo));
}

This makes it unambiguously clear which object is the SUT. It's the variable named sut .