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

The same test written in AAA style:

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

Why using AAA syntax

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:

Mocking frameworks supporting AAA syntax

The following mocking frameworks supports AAA:

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