April 16, 2013 Posted in programming | autofixture

General-purpose customizations with AutoFixture

If you’ve been using AutoFixture in your tests for more than a while, chances are you’ve already come across the concept of customizations. If you’re not familiar with it, let me give you a quick introduction:

A customization is a group of settings that, when applied to a given Fixture object, control the way AutoFixture will create instances for the types requested through that Fixture.

At this point you might find yourself feeling an irresistible urge to know everything there’s to know about customizations. If that’s the case, don’t worry. There are a few resources online where you learn more about them. For example, I wrote about how to take advantage of customizations to group together test data related to specific scenarios.

In this post I’m going to talk about something different which, in a sense, is quite the opposite of that: how to write general-purpose customizations.

A (user) story about cooking

It’s hard to talk about test data without a bit of context. So, for the sake of this post, I thought we would pretend to be working on a somewhat realistic project. The system we’re going to build is an online catalogue of food recipies. The domain, at the very basic level, consists of three concepts:

Cookbook
Recipes
Ingredients

Now, let’s imagine that in our backlog of requirements we have one where the user wishes to be able to search for recepies that contain a specific set of ingredients. Or in other words:

As a foodie, I want to know which recipes I can prepare with the ingredients I have, so that I can get the best value for my groceries.

From the tests…

As usual, we start out by translating the requirement at hand into a set of acceptance tests. In order do that, we need to tell AutoFixture how we’d like the test data for our domain model to be generated.

For this particular scenario, we need every Ingredient created in the test fixture to be randomly chosen from a fixed pool of objects. That way we can ensure that all recepies in the cookbook will be made up of the same set of ingredients.

Here’s how such a customization would look like:

public class RandomIngredientsFromFixedSequence : ICustomization
{
    private readonly Random randomizer = new Random();
    private IEnumerable<Ingredient> sequence;

    public void Customize(IFixture fixture)
    {
        InitializeIngredientSequence(fixture);
        fixture.Register(PickRandomIngredientFromSequence);
    }

    private void InitializeIngredientSequence(IFixture fixture)
    {
        this.sequence = fixture.CreateMany<Ingredient>();
    }

    private Ingredient PickRandomIngredientFromSequence()
    {
        var randomIndex = this.randomizer.Next(0, sequence.Count());
        return sequence.ElementAt(randomIndex);
    }
}

Here we’re creating a pool of ingredients and telling AutoFixture to randomly pick one of those every time it needs to create an Ingredient object by using the Fixture.Register method.

Since we’ll be using Xunit as our test runner, you can take advantage of the AutoFixture Data Theories to keep our tests succinct by using AutoFixture in a declarative fashion. In order to do so, we need to write an xUnit Data Theory attribute that tells AutoFixture to use our new customization:

public class CookbookAutoDataAttribute : AutoDataAttribute
{
    public CookbookAutoDataAttribute()
        : base(new Fixture().Customize(
                   new RandomIngredientsFromFixedSequence())))
    {
    }
}

If you prefer to use AutoFixture directly in your tests, the imperative equivalent of the above is:

var fixture = new Fixture();
fixture.Customize(new RandomIngredientsFromFixedSequence());

At this point, we can finally start writing the acceptance tests to satisfy our original requirement:

public class When_searching_for_recipies_by_ingredients
{
    [Theory, CookbookAutoData]
    public void Should_only_return_recipes_with_a_specific_ingredient(
        Cookbook sut,
        Ingredient ingredient)
    {
        // When
        var recipes = sut.FindRecipies(ingredient);
        // Then
        Assert.True(recipes.All(r => r.Ingredients.Contains(ingredient)));
    }

    [Theory, CookbookAutoData]
    public void Should_include_new_recipes_with_a_specific_ingredient(
        Cookbook sut,
        Ingredient ingredient,
        Recipe recipeWithIngredient)
    {
        // Given
        sut.AddRecipe(recipeWithIngredient);
        // When
        var recipes = sut.FindRecipies(ingredient);
        // Then
        Assert.Contains(recipeWithIngredient, recipes);
    }
}

Notice that during these tests AutoFixture will have to create Ingredient objects in a couple of different ways:

indirectly when constructing Recipe objects associated to a Cookbook
directly when providing arguments for the test parameters

As far as AutoFixture is concerned, it doesn’t really matter which code path leads to the creation of ingredients. The algorithm provided by the RandomIngredientsFromFixedSequence customization will apply in all situations.

…to the implementation

After a couple of Red-Green-Refactor cycles spawned from the above tests, it’s not completely unlikely that we might end up with some production code similar to this:

// Cookbook.cs
public class Cookbook
{
    private readonly ICollection<Recipe> recipes;

    public Cookbook(IEnumerable<Recipe> recipes)
    {
        this.recipes = new List<Recipe>(recipes);
    }

    public IEnumerable<Recipe> FindRecipies(params Ingredient[] ingredients)
    {
        return recipes.Where(r => r.Ingredients.Intersect(ingredients).Any());
    }

    public void AddRecipe(Recipe recipe)
    {
        this.recipes.Add(recipe);
    }
}

// Recipe.cs
public class Recipe
{
    public readonly IEnumerable<Ingredient> Ingredients;

    public Recipe(IEnumerable<Ingredient> ingredients)
    {
        this.Ingredients = ingredients;
    }
}

// Ingredient.cs
public class Ingredient
{
    public readonly string Name;

    public Ingredient(string name)
    {
        this.Name = name;
    }
}

Nice and simple. But let’s not stop here. It’s time to take it a bit further.

An opportunity for generalization

Given the fact that we started working from a very concrete requirement, it’s only natural that the RandomIngredientsFromFixedSequence customization we came up at with encapsulates a behavior that is specific to the scenario at hand. However, if we take a closer look we might notice the following:

The only part of the algorithm that is specific to the original scenario is the type of the objects being created. The rest can easily be applied whenever you want to create objects that are picked at random from a predefined pool.

An opportunity for writing a general-purpose customization has just presented itself. We can’t let it slip.

Let’s see what happens if we extract the Ingredient type into a generic argument and remove all references to the word “ingredient”:

public class RandomFromFixedSequence<T> : ICustomization
{
    private readonly Random randomizer = new Random();
    private IEnumerable<T> sequence;

    public void Customize(IFixture fixture)
    {
        InitializeSequence(fixture);
        fixture.Register(PickRandomItemFromSequence);
    }

    private void InitializeSequence(IFixture fixture)
    {
        this.sequence = fixture.CreateMany<T>();
    }

    private T PickRandomItemFromSequence()
    {
        var randomIndex = this.randomizer.Next(0, sequence.Count());
        return sequence.ElementAt(randomIndex);
    }
}

Voilà. We just turned our scenario-specific customization into a pluggable algorithm that changes the way objects of any type are going to be generated by AutoFixture. In this case the algorithm will create items by picking them at random from a fixed sequence of T.

The CookbookAutoDataAttribute can easily changed to use the general-purpose version of the customization by closing the generic argument with the Ingredient type:

public class CookbookAutoDataAttribute : AutoDataAttribute
{
    public CookbookAutoDataAttribute()
        : base(new Fixture().Customize(
                   new RandomFromFixedSequence<Ingredient>())))
    {
    }
}

The same is true if you’re using AutoFixture imperatively:

var fixture = new Fixture();
fixture.Customize(new RandomFromFixedSequence<Ingredient>());

Wrapping up

As I said before, customizations are a great way to set up test data for a specific scenario. Sometimes these configurations turn out to be useful in more than just one situation.

When such opportunity arises, it’s often a good idea to separate out the parts that are specific to a particular context and turn them into parameters. This allows the customization to become a reusable strategy for controlling AutoFixture’s behavior across entire test suites.

Enrico Campidoglio

Hi, I'm Enrico Campidoglio. I'm a freelance programmer, trainer and mentor focusing on helping teams develop software better. I write this blog because I love sharing stories about the things I know. You can read more about me here, if you like.