Arrange, Act and Assert syntax for testing

This post is a summary of a lightningtalk I held for some of my colleagues at Objectware back in August 2008.

What is Arrange, Act, Assert (AAA)?

AAA is a way to organize your tests. The contents of the tests are divided in to three parts:

1. Arrange: Do the necessary setup required for running the test. Usually multiple lines of code.
2. Act: Execute the code which should be tested. Should be one line of code.
3. Assert: Verify that the code behaved as expected. Should contain only one assertion.

Example of a test without AAA

[TestMethod]
public void GetCustomerById()
{
// Arrange
const int CustomerId = 5;
var repository = new CustomerRepository();

// Act & Assert
Assert.AreEqual(CustomerId, repository.GetCustomerById(CustomerId));
}

The same test written in AAA style:

[TestMethod]
public void GetCustomerById()
{
// Arrange
const int CustomerId = 5;
var repository = new CustomerRepository();

// Act
Customer customer = repository.GetCustomerById(CustomerId);

// Assert
Assert.AreEqual(CustomerId, customer);
}

The example above is simple, but the value of using AAA style becomes more apparent for complex tests.

Why using AAA syntax

• The tests become more robust, easier to read and to maintain
• Being able to use AAA syntax is essential when writing Behaviour Driven Development style specifications/tests

AAA and mocking

In the following example, which parts of the code are arranging, acting and asserting?

The record/playback style should not be used, since it results in tests which are very hard to read and understand.

The same test as above written in AAA style:

[TestMethod]
public void ShouldGenerateCreditInvoiceDocumentWhenNegativeAmountNew()
{
// Arrange
ICustomerService customerService = MockRepository.GenerateStub<ICustomerService>();
IDocumentService documentService = MockRepository.GenerateStub<IDocumentService>();
double amount = -1000;
Customer customer = new Customer() { Id = 12345, Name = "Name1" };
customerService.Stub(stub => stub.GetCustomer(customer.Id)).Return(customer);
 InvoicingProcess invoicingProcess = new InvoicingProcess(customerService, documentService);

// Act
 invoicingProcess.Invoice(customer.Id, amount);

// Assert
documentService.AssertWasCalled(stub => stub.CreateDocument(customer, DocumentType.CreditInvoice));
}

Mocking frameworks supporting AAA syntax

The following mocking frameworks supports AAA:

• Rhino Mocks 3.5
• Moq
• NUnit
• Typemock Isolator 5.0