Using Fluent Builder Pattern to Configure Test Objects

Depending on the complexity of the domain model, configuring mock objects for specific cenarios can make the resulting test code to get messy.

Consider the situation below with C#, NUnit and Moq framework

[Test]
[ExpectedException(InvalidPaymentAgreementException)]
public void PaymentAgreementMustNotBeCreatedWhenThePaymentOptionIsNotValidForTheDebtType()
{
  Mock somePaymentOptionMock = new Mock();
  Mock debTypeMock = new Mock();
  debtTypeMock
    .Setup(debtType.GetPaymentOptions())
    .Returns(new List(){somePaymentOptionMock.Object});

  Mock anotherPaymentOptionMock = new Mock();

  Mock debtMock = new Mock();
  debtMock.Setup(debt.DebtType).Returns(debTypeMock.Object);            

  PaymentAgreement paymentAgreement = new PaymentAgreement(
    new PaymentAgreementCreationParameter()
    {
      AgreementYear = SystemDate.Get().Value.Year,
      AgreementNumber = 1,
      AgreementCreationDate = SystemDate.Get().Value.Date,
      NumberOfInstallments = 1,
      AgreementValue = 100.0m,
      Debts = new List() { debtMock.Object },
      SelectedPaymentOption =  anotherPaymentOptionMock.Object
    });
}

A developer who reads the unit test can take a consirable amount of time to understand what is actually being tested even if a well-designed mock framework such as Moq is in use. Most of this work is about configuring mock objects. Besides the code size, the unit test doesn´t speak the language of the domain (business) and thus it becomes a mass of meaningless mock configuration.
I am on my way to learn how to apply TDD effectvely in software development and I quickly realized that the quality of unit tests are very important in order to this kind of methodology to succeed.

Accidentally some days I read an interesting post from Andrian about Rich Domain Tests at http://adrianhummel.wordpress.com/ and I decided to apply his idea.
After reading his post I got to the conclusion that one way to solve or at least minimize this unit-test-messy-code-problem was by using a fluent builder pattern to configure mock objects by using a builder whose interface could be describe how I am configuring a mock object with an interface closer to domain language.

The unit test below is a rewritten from the example above with the concepts described here:

[Test]
[ExpectedException(InvalidPaymentAgreementException)]
public void PaymentAgreementMustNotBeCreatedWhenThePaymentOptionIsNotValidForTheDebtType()
{
  PaymentOption somePaymentOption = PaymentOptionMockBuilder
                                    .Begin()
                                    .BuildPaymentOption();

  DebtType debtType = DebtTypeMockBuilder
                      .Begin()  
                      .AddPaymentOptionOf(somePaymentOption)
                      .BuildDebtType();
            
  PaymentOption anotherPaymentOption = PaymentOptionMockBuilder
                                       .Begin()
                                       .BuildPaymentOption();          
            
  Debt debt = DebtMockBuilder
              .Begin()
              .WithDebtTypeOf(debtType)
              .BuildDebt();

  PaymentAgreement paymentAgreement = new PaymentAgreement(
    new PaymentAgreementCreationParameter()
    {
      AgreementYear = SystemDate.Get().Value.Year,
      AgreementNumber = 1,
      AgreementCreationDate = SystemDate.Get().Value.Date,
      NumberOfInstallments = 1,
      AgreementValue = 100.0m,
      Debts = new List &lt Debt &gt () { debt},
      SelectedPaymentOption =  anotherPaymentOption
    });

}

The fluent builder for Debt is DebtMockBuilder and can be programmed as follows:

    public class DebtMockBuilder
    {
        private Mock _debtMock;

        public static DebtMockBuilder Begin()
        {
            DebtMockBuilder builder = new DebtMockBuilder();
            builder._debtMock = new Mock();
            return builder;
        }

        public Debt BuildDebt()
        {
            return _debtMock.Object;
        }

        public DebtMockBuilder WithDebtTypeOf(DebtType debtType)
        {
            this._debtMock.Setup(debt => debt.DebtType).Returns(debtType);
            return this;
        }
    }

PaymentOptionMockBuilder follows the same idea.

Although I see that some improvements are needed I could see the following advantages from fluent mock builders:

• The test code was easier to understand because domain terms were applied instead of specific API language.

• Specific mock framework calls were encapsulated which theoretically can let programmers to use another mock framework in other projects or using more than one mock framework in the same test (I dont know why someone would do such a thing...).

• Finally mock configuration becomes more flexible since existing methods don´t have to be modified to include new configuration but only a new configuration method is needed. Thus mock configuration can evolve as needed without loosing the domain interface.