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Arrange Act Assert

Arrange/Act/Assert (AAA) is a pattern for arranging and formatting code in Unit Test methods.

It is a best practice to author your tests in more natural and convenient way. The idea is to develop a unit test by following these 3 simple steps:

  • Arrange – setup the testing objects and prepare the prerequisites for your test.
  • Act – perform the actual work of the test.
  • Assert – verify the result.

Lets illustrate this with an example. We will test our warehouse to ensure that it returns correct results when an order is placed.

The classes used for the next example are defined in the Testing Your Application with JustMock article.


First we need an order:

var order = new Order("Camera", 2); 
Dim order = New Order("Camera", 2) 

Now let’s mock the warehouse:

var warehouse = Mock.Create<IWarehouse>(); 
Dim warehouse = Mock.Create(Of IWarehouse)() 

We want to ensure that when an order of 2 cameras is placed the warehouse returns that it has availability in the inventory.

Mock.Arrange(() => warehouse.HasInventory("Camera", 2)).Returns(true); 
Mock.Arrange(Function() warehouse.HasInventory("Camera", 2)).Returns(true) 

That’s it. We set up the testing objects for our test. Now let’s act. Further, you can also check Create Mocks By Example.


Fill our order from the warehouse.


Next we need to ensure that our order was actually filled meaning that the warehouse really had inventory of 2 cameras.


We will use the Assert class from Microsoft.VisualStudio.TestTools.UnitTesting namespace (found in Microsoft.VisualStudio.QualityTools.UnitTestFramework assembly – automatically added as a reference from Visual Studio while creating a Test Project) to ensure that the IsFilled property of the order is set to true.


With this simple example we illustrated the use of the AAA pattern and showed how easy it is to test your code with JustMock. Notice that you don’t need to change even a single line of your original code to set up, execute and verify its correctness.

Verify Interaction

Now let's take it a little further and verify not only the final result, but also the interaction while executing the test.

We arranged that when the warehouse’s HasInventory method is called with specific parameters, it should return true, but we never ensured that this method is actually called. Let's change the Arrange method and mark that warehouse.HasInventory must be called.

Mock.Arrange(() => warehouse.HasInventory("Camera", 2)).Returns(true).MustBeCalled(); 
Mock.Arrange(Function() warehouse.HasInventory("Camera", 2)).Returns(true).MustBeCalled() 

To verify this we need to call Mock.Assert in the Assert phase with the warehouse object.


Verify Order of Calls

Furthermore you may want to ensure that a set of method calls are executed in a particular order. Let`s assume we have the following IFoo interface:

public interface IFoo 
    void Submit(); 
    void Echo(); 
Public Interface IFoo 
    Sub Submit() 
    Sub Echo() 
End Interface 

You use the Arrange method to define the methods invocation order.

public void ShouldVerifyCallsOrder() 
    // Arrange 
    var foo = Mock.Create<IFoo>(); 
    Mock.Arrange(() => foo.Submit()).InOrder(); 
    Mock.Arrange(() => foo.Echo()).InOrder(); 
    // Act 
    // Assert 
Public Sub ShouldVerifyCallsOrder() 
    ' Arrange 
    Dim foo = Mock.Create(Of IFoo)() 
    Mock.Arrange(Sub() foo.Submit()).InOrder() 
    Mock.Arrange(Sub() foo.Echo()).InOrder() 
    ' Act 
    ' Assert 
End Sub 

Again to verify this we need to call Mock.Assert in the Assert phase with the foo object.

Note that the InOrder option also supports asserting the order of mock calls regardless of the instance within the test scope. Imagine that you have to validate that the user has logged in before using their shopping cart in your application.

public interface IUserValidationService 
    int ValidateUser(string userName, string password); 
public interface IShoppingCartService 
    IList<string> LoadCart(int userID); 
Public Interface IUserValidationService 
    Function ValidateUser(ByVal userName As String, ByVal password As String) As Integer 
End Interface 
Public Interface IShoppingCartService 
    Function LoadCart(ByVal userID As Integer) As IList(Of String) 
End Interface 

Here we have defined the IUserValidationService and the IShoppingCartService services whose invocation order we are going to assert in the following test:

public void ShouldAssertInOrderForDifferentInstancesInTestMethodScope() 
    string userName = "Bob"; 
    string password = "Password"; 
    int userID = 5; 
    var cart = new List<string> {"Foo", "Bar"}; 
    // Arrange 
    var userServiceMock = Mock.Create<IUserValidationService>(); 
    var shoppingCartServiceMock = Mock.Create<IShoppingCartService>(); 
    Mock.Arrange(() => userServiceMock.ValidateUser(userName, password)).Returns(userID).InOrder().OccursOnce(); 
    Mock.Arrange(() => shoppingCartServiceMock.LoadCart(userID)).Returns(cart).InOrder().OccursOnce(); 
    // Act 
    userServiceMock.ValidateUser(userName, password); 
    // Assert 
Public Sub ShouldAssertInOrderForDifferentInstancesInTestMethodScope() 
    Dim userName As String = "Bob" 
    Dim password As String = "Password" 
    Dim userID As Integer = 5 
    Dim cart As IList(Of String) = {"Foo", "Bar"} 
    ' Arrange 
    Dim userServiceMock = Mock.Create(Of IUserValidationService)() 
    Dim shoppingCartServiceMock = Mock.Create(Of IShoppingCartService)() 
    Mock.Arrange(Function() userServiceMock.ValidateUser(userName, password)).Returns(userID).InOrder().OccursOnce() 
    Mock.Arrange(Function() shoppingCartServiceMock.LoadCart(userID)).Returns(cart).InOrder().OccursOnce() 
    ' Act 
    userServiceMock.ValidateUser(userName, password) 
    ' Assert 
End Sub 

In the arrange phase we defined that the ValidateUser call should be made only once and before the LoadCart service call. The LoadCart call should also occur only once and should follow the ValidateUser service call. We act and then assert our expectations.


Refer to the Asserting Occurrence topic to learn more about asserting occurrence. The example also uses the Returns option in order to ignore the actual call and return a custom value.

Benefits of Using Arrange Act Assert

  • Clearly separates what is being tested from the setup and verification steps.
  • Clarifies and focuses attention on a historically successful and generally necessary set of test steps.
  • Makes some test smells more obvious:
  • Assertions intermixed with "Act" code.
  • Test methods that try to test too many different things at once.

See Also

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