1. Initialize an instance of the Fixture class.
2. Configure the way different types of objects involved in the test should be created by using the Build<T> method.
3. Create the actual objects with the CreateAnonymous<T> or CreateMany<T> methods.

As a result, my unit tests had started to look a lot like this:

[Test]
public void WhenGettingAListOfPublishedPostsThenItShouldOnlyIncludeThose()
{
    // Step 1: Initialize the Fixture
    var fixture = new Fixture();

    // Step 2: Configure the object creation
    var draft = fixture.Build<Post>()
        .With(a => a.IsDraft = true)
        .CreateAnonymous();
    var publishedPost = fixture.Build<Post>()
        .With(a => a.IsDraft = false)
        .CreateAnonymous();
    fixture.Register<IEnumerable<Post>>(() => new[] { draft, publishedPost });

    // Step 3: Create the anonymous objects
    var posts = fixture.CreateMany<Post>();

   // Act and Assert...
}

In this particular configuration, AutoFixture will satisfy all requests for IEnumerable<Post> types by returning the same array with exactly two Post objects: one with the IsDraft property set to True and one with the same property set to False.

At that point I felt pretty satisfied with the way things were shaping up: I had managed to replace entire blocks of boring object initialization code with a couple of calls to the AutoFixture API, my unit tests were getting smaller and all was good.

Duplication creeps in

After a while though, the configuration lines created in Step 2 started to repeat themselves across multiple unit tests. This was naturally due to the fact that different unit tests sometimes shared a common set of object states in their test scenario. Things weren’t so DRY anymore and suddenly it wasn’t uncommon to find code like this in the test suite:

[Test]
public void WhenGettingAListOfPublishedPostsThenItShouldOnlyIncludeThose()
{
    var fixture = new Fixture();
    var draft = fixture.Build<Post>()
        .With(a => a.IsDraft = true)
        .CreateAnonymous();
    var publishedPost = fixture.Build<Post>()
        .With(a => a.IsDraft = false)
        .CreateAnonymous();
    fixture.Register<IEnumerable<Post>>(() => new[] { draft, publishedPost });
    var posts = fixture.CreateMany<Post>();

    // Act and Assert...
}

[Test]
public void WhenGettingAListOfDraftsThenItShouldOnlyIncludeThose()
{
    var fixture = new Fixture();
    var draft = fixture.Build<Post>()
        .With(a => a.IsDraft = true)
        .CreateAnonymous();
    var publishedPost = fixture.Build<Post>()
        .With(a => a.IsDraft = false)
        .CreateAnonymous();
    fixture.Register<IEnumerable<Post>>(() => new[] { draft, publishedPost });
    var posts = fixture.CreateMany<Post>();

    // Different Act and Assert...
}

See how these two tests share the same initial state even though they verify completely different behaviors? Such blatant duplication in the test code is a problem, since it inhibits the ability to make changes.
Luckily a solution was just around the corner as I discovered customizations.

Customizing your way out

A customization is a pretty general term. However, put in the context of AutoFixture it assumes a specific definition:

What that means is that I could take all the boilerplate configuration code produced during Step 2 and move it out of my unit tests into a single place, that is a customization. That allowed me to specify only once how different objects needed to be created for a given scenario, and reuse that across multiple tests.

public class MixedDraftsAndPublishedPostsCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        var draft = fixture.Build<Post>()
            .With(a => a.IsDraft = true)
            .CreateAnonymous();
        var publishedPost = fixture.Build<Post>()
            .With(a => a.IsDraft = false)
            .CreateAnonymous();
        fixture.Register<IEnumerable<Post>>(() => new[] { draft, publishedPost });
    }
}

As you can see, ICustomization is nothing more than a role interface that describes how a Fixture should be set up. In order to apply a customization to a specific Fixture instance, you’ll simply have to call the Fixture.Customize(ICustomization) method, like shown in the example below.
This newly won encapsulation allowed me to rewrite my unit tests in a much more terse way:

[Test]
public void WhenGettingAListOfDraftsThenItShouldOnlyIncludeThose()
{
    // Step 1: Initialize the Fixture
    var fixture = new Fixture();

    // Step 2: Apply the customization for the test scenario
    fixture.Customize(new MixedDraftsAndPublishedPostsCustomization());

    // Step 3: Create the anonymous objects
    var posts = fixture.CreateMany<Post>();

    // Act and Assert...
}

The configuration logic now exists only in one place, namely a class whose name clearly describes the kind of test data it will produce.
If applied consistently, this approach will in time build up a library of customizations, each representative of a given situation or scenario. Assuming that they are created at the proper level of granularity, these customizations could even be composed to form more complex scenarios.

Conclusion

Customizations in AutoFixture are a pretty powerful concept in of themselves, but they become even more effective when mapped directly to test scenarios. In fact, they represent a natural place to specify which objects are involved in a given scenario and the state they are supposed to be in. You can use them to remove duplication in your test code and, in time, build up a library of self-documenting modules, which describe the different contexts in which the system’s behavior is being verified.

The good old fashioned way

Before I jump into the details of what exactly has been changed and how, allow me to set up a little bit of stage:

Put in simple terms, this essentially means that AutoFixture will come up with test values at run time that can be considered “random” within some predefined bounds. These bounds are imposed at the lowest level by the variable’s own data type: a string is a string, a number is a number and so on. More constraints, however, can be added at a higher level, based on any semantics the variable may have in the specific test scenario. For example a string can’t be longer than 20 characters or a number must be between 1 and 100.

AutoFixture comes with a set of built-in generation algorithms that can produce test values for all the primitive types included in the .NET Framework. The algorithm for numeric types has historically been based on individually incremented sequences, one for each numeric data type. Let’s look at an example that illustrates this:

var fixture = new Fixture();
Console.WriteLine("Byte specimen is {0}, {1}",
    fixture.CreateAnonymous<byte>(),
    fixture.CreateAnonymous<byte>());
Console.WriteLine("Int32 specimen is {0}, {1}",
    fixture.CreateAnonymous<int>(),
    fixture.CreateAnonymous<int>());
Console.WriteLine("Single specimen is {0}, {1}",
    fixture.CreateAnonymous<float>(),
    fixture.CreateAnonymous<float>());

// The output will be:
// Byte specimen is 1, 2
// Int32 specimen is 1, 2
// Single specimen is 1, 2

The key point here is that AutoFixture will only guarantee unique numeric specimens within the scope of a specific data type. Now, you may wonder how this would be a problem. Well, it certainly isn’t in itself, but if you asked AutoFixture to give you an anonymous instance of a class with multiple properties of different numeric types, you would get something like this:

public class NumericBag
{
    public byte ByteValue { get; set; }
    public int Int32Value { get; set; }
    public float SingleValue { get; set; }
}

var fixture = new Fixture();
var specimen = fixture.CreateAnonymous<NumericBag>();
Console.WriteLine("ByteValue property is {0}", specimen.ByteValue);
Console.WriteLine("Int32Value property is {0}", specimen.Int32Value);
Console.WriteLine("SingleValue property is {0}", specimen.SingleValue);

// The output will be:
// ByteValue property is 1
// Int32Value property is 1
// SingleValue property is 1

We can agree that the end result doesn’t exactly live up to the expectation of anonymous values being “random”. Starting from version 2.2, however, this behavior is due to change.

The fresh new way

AutoFixture has taken a different approach to numeric specimen generation and will now by default return unique values across all numeric types. Running our first example in AutoFixture 2.2 will therefore yield a very different result:

var fixture = new Fixture();
Console.WriteLine("Byte specimen is {0}, {1}",
    fixture.CreateAnonymous<byte>(),
    fixture.CreateAnonymous<byte>());
Console.WriteLine("Int32 specimen is {0}, {1}",
    fixture.CreateAnonymous<int>(),
    fixture.CreateAnonymous<int>());
Console.WriteLine("Single specimen is {0}, {1}",
    fixture.CreateAnonymous<float>(),
    fixture.CreateAnonymous<float>());

// The output will be:
// Byte specimen is 1, 2
// Int32 specimen is 3, 4
// Single specimen is 5, 6

In other words, AutoFixture is being a little more “non-deterministic” when it comes to numeric test values. Take for example the following scenario:

public class NumericBag
{
    public byte ByteValue { get; set; }
    public int Int32Value { get; set; }
    public float SingleValue { get; set; }
}

var fixture = new Fixture();
var specimen = fixture.CreateAnonymous<NumericBag>();
Console.WriteLine("ByteValue property is {0}", specimen.ByteValue);
Console.WriteLine("Int32Value property is {0}", specimen.Int32Value);
Console.WriteLine("SingleValue property is {0}", specimen.SingleValue);

// The output will be:
// ByteValue property is 1
// Int32Value property is 2
// SingleValue property is 3

See how all the numeric properties on the generated object have different values? That’s what I’m talking about.

Now, in theory, this shouldn’t be considered a breaking change. I say this because AutoFixture is all about anonymous variables, which, by definition, can’t be expected to have specific values during a test run. So, as long as you’ve played by this rule, the new behavior shouldn’t impact any of your existing tests.

However, if this does turn out to be a problem or you simply prefer the old way of doing things, you shouldn’t feel left out in the cold. The previous behavior is still in the box, packaged up in a nice customization unambiguously named NumericSequencePerTypeCustomization. The simple act of adding it to a Fixture instance will restore things the way they were:

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

If you wish to try this out today, I encourage you to go head and grab the latest build off of AutoFixture’s project page on Team City. Enjoy.

If you aren’t familiar with AutoFixture, let me give you the pitch:

Does this sound interesting to you? In that case head over to the AutoFixture CodePlex site and find out more. You’ll be glad you did.

For those of you already familiar with AutoFixture, the newly added support for delegates means that every time AutoFixture is asked to create an anonymous instance of a delegate type (or more precisely a delegate specimen), it will actually return one, instead of throwing an exception.

So, you’ll be able to say things like:

public delegate void MyDelegate();

var fixture = new Fixture();
var delegateSpecimen = fixture.CreateAnonymous<MyDelegate>();

and get back a delegate pointing to a dynamically generated method, whose signature matches the one of the requested delegate type. In other words AutoFixture will satisfy the requests for delegates by providing a method specimen.

That’s cool, but it may leave you wondering: what on Earth does a method specimen do when it gets invoked? Well, in order to answer that question, we need to look at the signature of the delegate that was requested in the first place. The rule basically says:

This principle is best illustrated by examples. Consider the following code snippet:

var fixture = new Fixture();
var funcSpecimen = fixture.CreateAnonymous<Func<string>>();
var result = funcSpecimen();

// result = "fd95320f-0a37-42be-bd49-3afbbe089d9d"

In this example, since the signature of the requested delegate has a return value of type String, the result variable will contain an anonymous string value, which in AutoFixture usually translates into a GUID.
On the other hand, if requested delegate didn’t have a return value, invoking the anonymous delegate would do just about nothing:

var fixture = new Fixture();
var actionSpecimen = fixture.CreateAnonymous<Action<string>>();
actionSpecimen("whatever"); // no-op

Note that in both cases any input arguments passed to the anonymous delegate will be ignored, since they don’t have any impact on the generated method specimen.

Now, if you’re using AutoFixture from its NuGet package (which, by the way, you should) you’ll have to wait until the next release to get this feature. However, taking advantage of it with the current version of AutoFixture requires a minimal amount of effort. Just grab the DelegateGenerator.cs class from AutoFixture’s main trunk on CodePlex and include it in your project. You’ll then be able to add support for delegates to your Fixture instance by simply saying:

var fixture = new Fixture();
fixture.Customizations.Add(new DelegateGenerator());

You can even wrap that up in a Customization to make it more centralized and keep your test library DRY:

public class DelegateCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        if (fixture == null)
        {
            throw new ArgumentNullException("fixture");
        }

        fixture.Customizations.Add(new DelegateGenerator());
    }
}

Before finishing this off, let me give you a more concrete example that shows how this is useful in a real world scenario. Keeping in mind that delegates offer a pretty terse way to implement the Strategy Design Pattern in .NET, consider this implementation of the IEqualityComparer<T> interface:

public class EqualityComparer<T> : IEqualityComparer<T>
{
    private readonly Func<T, T, bool> equalityStrategy;
    private readonly Func<T, int> hashCodeStrategy;

    public EqualityComparer(Func<T, T, bool> equalityStrategy, Func<T, int> hashCodeStrategy)
    {
        if (equalityStrategy == null)
        {
            throw new ArgumentNullException("equalityStrategy");
        }

        if (hashCodeStrategy == null)
        {
            throw new ArgumentNullException("hashCodeStrategy");
        }

        this.equalityStrategy = equalityStrategy;
        this.hashCodeStrategy = hashCodeStrategy;
    }

    public bool Equals(T x, T y)
    {
        return equalityStrategy(x, y);
    }

    public int GetHashCode(T obj)
    {
        return hashCodeStrategy(obj);
    }
}

That’s a nice flexible class that, by allowing to specify the comparison logic in the form of delegates, is suitable in different scenarios. Before the support for delegates was added, however, having AutoFixture play along with this class in the context of unit testing would be quite problematic. The tests would, in fact, fail consistently with a NotSupportedException, since the constructor of the EqualityComparer class requires the creation of two delegates.
Luckily, this is not a problem anymore.