A constructor is a class member invoked when an object is created to initialize its state. In tests, putting shared fixture setup in a constructor reduces repetition but couples every test to the same arrangement. section 3.3 section 11.1
Prefer arrangements that preserve independent modification and readable tests:
- Avoid placing test-specific fixture setup in the test class constructor; changing one arrangement can invalidate unrelated tests. section 3.3
- Extract genuinely common setup into private factory methods so each test states its own arrangement explicitly. section 3.3
- Use a base-class constructor for genuinely shared integration infrastructure, such as creating and disposing a database. section 3.3
- Treat private constructors by purpose: avoid testing implementation details, but preserve contracts required by an ORM or encapsulation. section 11.1
Refactoring shared test setup
Replace constructor-owned state such as `_store` and `_sut` with local arrangements: `Store store = CreateStoreWithInventory(Product.Shampoo, 10);` and `Customer sut = CreateCustomer();`. Each test then exposes its assumptions while retaining reusable construction logic. section 3.3
Listing 3.7. Extracting the initialization code into the test constructor
public class CustomerTests
{
private readonly Store _store; #1
private readonly Customer _sut;
public CustomerTests() #2
{ #2
_store = new Store(); #2
_store.AddInventory(Product.Shampoo, 10); #2
_sut = new Customer(); #2
} #2
[Fact]
public void Purchase_succeeds_when_enough_inventory()
{
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()
{
bool success = _sut.Purchase(_store, Product.Shampoo, 15);
Assert.False(success);
Assert.Equal(10, _store.GetInventory(Product.Shampoo));
}
}
Shows the fragile pattern of initializing a store and system under test in the test constructor. section 3.3
Listing 11.3. A class with a private constructor
public class Inquiry
{
public bool IsApproved { get; private set; }
public DateTime? TimeApproved { get; private set; }
private Inquiry( #1
bool isApproved, DateTime? timeApproved) #1
{
if (isApproved && !timeApproved.HasValue)
throw new Exception();
IsApproved = isApproved;
TimeApproved = timeApproved;
}
public void Approve(DateTime now)
{
if (IsApproved)
return;
IsApproved = true;
TimeApproved = now;
}
}
Shows a private constructor enforcing the invariant that approved inquiries require an approval time. section 11.1
