The TestDataBuilder pattern is used in unit testing to create collaborators. Its advantages versus using constructors:

  • Changes to constructors are localized.
  • Test code becomes clearer, because you don't have to specify all the required parameters to the constructor.

A disadvantage is that the usage is somewhat lengthy which decreases readability:

var service = serviceBuilder.WithRepo(
      new [] { entityBuilder.WithName("name") }).Build())
  • The word "With" is typically repeated many times.
  • Same for the call to Build().
  • Same for the word Builder.

Although all of them can be mitigated, I guess the solutions themselves have drawbacks. For instance, we could rename serviceBuilder to service, but this could give the wrong impression about the type of the variable. We could implement an implicit conversion from a builder class to the object it builds, but it is more code in the builder in addition to already abundant With* methods.

Is there an alternative to TestDataBuilder that doesn't have these drawbacks?

  • If you follow principle "One statement per line" your builder logic will look clear and readable. And easy to understand for other readers without digging to factory methods – Fabio Sep 14 '17 at 4:31

While I hope I can answer parts of this question in a useful fashion, I do feel the need to question parts of the premise.

  • First, the repetition of With[...] and Build() doesn't have to be a problem. After all, software development productivity is hardly measured by how quickly you can type code, and from a perspective af reading, repetition can sometimes aid in understanding, because it adds structure to code.
  • Second, while I realise that the above example is only that: an example, I don't think it's bad to have a couple of Build steps. If, on the other hand, you have a lot of such code, it may not indicate a problem with the Test Data Builder pattern itself, but rather it may be a smell that the test in question is too coarse-grained, or attempting to test too much.

Depending on your language, you can use various tricks. In C#, for example, you can avoid the need for the Build() method by adding explicit or implicit conversions between the Builders and their target classes.

Still, I think a better SUT API can address the problems in a better way. For instance, considering the above example, you could design the SUT so that you could simply write the code like this:

var service = defaultService
        new [] { defaultEntity.WithName("name") })

That is, the SUT API itself has With[...] methods, while the default[...] objects are test-specific default objects.

Do notice, however, that this particular example says WithMailSender(defaultMailSender), which I'd expect to be redundant, so that the example could probably be reduced even further:

var service = defaultService
        new [] { defaultEntity.WithName("name") });

I've outlined various options in an article series about the topic.

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  • You can change name for With... methods with different arguments to just With(argument) (use overload methods), then code will looks like service.With(repository).With(mailSender) – Fabio Sep 14 '17 at 4:29
  • There's another problem with the builder API. The compiler won't prevent you from writing WithName twice. What's the expected behavior of this? Intuitively, the "last call wins" seems appropriate, but do we need this possibility at all? With MotherFactory you can only specify the property once, otherwise you will get a compile error. – Gebb Sep 14 '17 at 7:40
  • @Gebb, that problem can be "trasformed" in a benefit where you can create builder with "default" values once and reuse it by changing only one value for example in parameterized tests. Where with factory you need create new instance. Programming is all about context - choose solution fitted your requirements and use it. – Fabio Sep 14 '17 at 8:44

My suggestion is to use the Factory pattern with a twist, taking advantage of default parameters. The implementation is more concise than that of a builder, the test code too.

I call it MotherFactory. Here is the code:

// ==== In a common test library ===

public abstract class MotherFactory

// ==== In a test project ===

public static class MotherFactoryEntityExtensions
    public static Entity Entity(
        this MotherFactory a,
        int? year = 2014,
        string name = "name",
        Dependency dependency = null,
        withoutDependency = false)
        if (withoutDependency && dependency != null)
            throw new ArgumentException(
                "The parameter 'dependency' cannot be used when true is specified for 'withoutDependency'.",

        if(dependency == null && !withoutDependency)
            dependency = a.Dependency();
        return new Entity(startYear, endYear, name, summary, dependency);

public class EntityTest
    private static readonly MotherFactory an = null;

    public void Ctor_GivenNullDependency_Throws()
        Exception e = Record.Exception(() =>
            Entity systemUnderTest = an.Entity(withoutDependency: true);


public class ServiceTest
    private static readonly MotherFactory a = null;
    private static readonly MotherFactory an = null;

    public void Find_GivenCorrectRequest_FindsEntity()
        var service = a.Service(repo: a.Repository(dbEntities: new [] { an.Entity(name: "XXX") });
        var result = service.Find();
        Assert.Equal("XXX", result.Name);


  • Put the MotherFactory class in a library and in test projects create a class with extension methods for each collaborator class.
  • Parameters that have a prefix without (like withoutDependency) are used only in cases when we want to disable creating the default instance of a dependency and pass null instead.
  • A MotherFactory*Extensions class can also be an ObjectMother by providing methods like UserWithCompleteProfile().
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  • Any if ... else in the test code makes feeling "dirty". With if in test configuration it is a question of time when you get "bizarre" behaviour of your tests – Fabio Sep 14 '17 at 4:25
  • 1
    @Fabio The if statements are in the infrastructure code, and are only used to configure the default behavior. There are no conditions in tests, which, I agree, would be a cause for concern. I've been using this pattern for over 2 years, and haven't had any problems. – Gebb Sep 14 '17 at 7:43

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