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Service announcement about syndication feed addresses for ploeh blog.

Now that Google Reader is closing down and a lot of my readers may want to migrate their ploeh blog subscription to another service, I think it's the right time for a quick post about ploeh blog's syndication feed addresses.

When I migrated my blog to Jekyll I made it a priority to ensure a certain level of backwards compatibility. The permalinks for the old posts are still served, although they are now redirects. For that reason, I also left the old syndication feed addresses in place. Specifically, I left this old RSS feed address in place: /SyndicationService.asmx/GetRss. This feed address sort of works, but has issues.

There are several problems with the 'legacy' feed address:

• Due to the way Jekyll works, the address actually points to an index.html file. Since a Jekyll-powered site is a static site, I don't control the server, and thus I can't manipulate the HTTP headers for individual resources. HTML files are served with the "text/html" Content-Type, which doesn't fit the XML content. Some clients seem to dislike this.
• Google Reader has created some 'ghost' entries based on the feed. I wonder if it has anything to do with the faulty Content-Type.
• Other users have reported that their clients (e.g. Outlook) don't like the feed. Again, I suspect it's because of the faulty Content-Type. While I think those clients should follow Postel's law, they apparently don't.
• The address looks very implementation-specific.

However, ploeh blog has new, 'proper' syndication feed addresses, so if you're already in the process of migrating your subscriptions away from Google Reader, please take a moment to update your subscription. That 'legacy' RSS address may not stick around forever.

The proper syndication feed addresses for ploeh blog are:

• ATOM: /atom.xml
• RSS: /rss.xml

This post explains how to automatically populate properties when using AutoFixture.AutoMoq.

In a previous blog post I described how to configure AutoFixture.AutoMoq to set up all mock instances to have 'normal' property behavior. This enables you to assign and retrieve values from properties defined by interfaces, but still doesn't fill those properties with values.

Apparently, people want to do that, so here's how to do it with the AutoMoq glue library.

This solution builds upon the PropertiesPostprocessor described in my previous blog post. All you have to do is define a different Customization for AutoFixture so that, instead of using the AutoMoqPropertiesCustomization described in the previous post, you'll need a variation:

public class AutoPopulatedMoqPropertiesCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        fixture.Customizations.Add(
            new PropertiesPostprocessor(
                new MockPostprocessor(
                    new MethodInvoker(
                        new MockConstructorQuery()))));
        fixture.ResidueCollectors.Add(
            new Postprocessor(
                new MockRelay(),
                new AutoPropertiesCommand(
                    new PropertiesOnlySpecification())));
    }
 
    private class PropertiesOnlySpecification : IRequestSpecification
    {
        public bool IsSatisfiedBy(object request)
        {
            return request is PropertyInfo;
        }
    }
}

The PropertiesPostprocessor assigned to the Fixture's Customizations has the same configuration as shown in AutoMoqPropertiesCustomization, but the object graph passed to the Fixture's ResidueCollectors is different. It's still a MockRelay, but now decorated with a Postprocessor instance, configured with an AutoPropertiesCommand instance, which is the class in AutoFixture responsible for implementing the AutoProperties feature.

The only thing special about this configuration is that you need to pass a PropertiesOnlySpecification to the AutoPropertiesCommand instance. This is because, by default, AutoPropertiesCommand attempts to fill both properties and fields of a generated instance (we call that a specimen), but it turns out that when you ask Moq to generate an instance of an interface, the generated type has a lot of public fields that you don't want to mess with. The PropertiesOnlySpecification class filters the population algorithm so that it only attempts to populate public properties.

This test passes:

[Fact]
public void AutoPopulatedProperties()
{
    var fixture = new Fixture().Customize(
        new AutoPopulatedMoqPropertiesCustomization());
    var h = fixture.Create<IHasProperties>();
 
    Assert.NotEqual(default(string), h.Text);
    Assert.NotEqual(default(int), h.Number);
}

With the described AutoPopulatedMoqPropertiesCustomization, AutoFixture will populate all writable properties on interfaces generated by Moq. I still don't think this is a good idea, which is why it isn't the default behavior for AutoFixture, but as you can tell, it's not too hard to do.

This post explains how to configure AutoFixture.AutoMoq to setup all interface properties to behave like normal properties.

From time to time, people want the AutoFixture.AutoMoq Auto-Mocking Container to set up all mock instances to have 'normal' property behavior.

By default, Moq doesn't implement any members of an interface. You have to explicitly configure the desired behavior using the Setup methods. This is also true for properties (which are really only special methods).

Consider, as an example, this interface:

public interface IHasProperties
{
    string Text { get; set; }
 
    int Number { get; set; }
}

Personally, I think such an interface design is a design smell, but that's not the issue here.

Moq behavior #

The issue is that by default, Moq behaves like this:

[Fact]
public void DefaultMoqDoesNotSetupProperties()
{
    var h = new Mock<IHasProperties>().Object;
 
    h.Text = "foo";
    h.Number = 42;
 
    Assert.Equal(default(string), h.Text);
    Assert.Equal(default(int), h.Number);
}

As you can infer from this test, the IHasProperties instance completely ignores any attempt at assigning values to the properties. When you attempt to read the property values, Moq returns the default value for the property type - often null.

You can configure each property individually, but you can also configure the Mock instance to implement all properties as thought they are normal, well-behaved properties:

[Fact]
public void ManualSetupProperties()
{
    var td = new Mock<IHasProperties>();
    td.SetupAllProperties();
    var h = td.Object;
 
    h.Text = "foo";
    h.Number = 42;
 
    Assert.Equal("foo", h.Text);
    Assert.Equal(42, h.Number);
}

Notice the use of the SetupAllProperties method. So far, this is all about Moq, and really has nothing to do with AutoFixture.

AutoFixture.AutoMoq and interface property behavior #

Adhering to the Principle of least surprise, the AutoFixture.AutoMoq glue library doesn't change this behavior:

[Fact]
public void DefaultAutoMoqDoesNotSetupProperties()
{
    var fixture = new Fixture().Customize(new AutoMoqCustomization());
    var h = fixture.Create<IHasProperties>();
 
    h.Text = "foo";
    h.Number = 42;
 
    Assert.Equal(default(string), h.Text);
    Assert.Equal(default(int), h.Number);
}

However, it's easy to change the behavior in an ad hoc manner:

[Fact]
public void ManualSetupPropertiesOnAutoMoq()
{
    var fixture = new Fixture().Customize(new AutoMoqCustomization());
    var h = fixture.Create<IHasProperties>();
    Mock.Get(h).SetupAllProperties();
 
    h.Text = "foo";
    h.Number = 42;
 
    Assert.Equal("foo", h.Text);
    Assert.Equal(42, h.Number);
}

As you can see, you can use the static Mock.Get method to get the underlying Mock<IHasProperties> and explicitly invoke the SetupAllProperties method.

If you prefer AutoFixture to automate this for you, it's fairly easy to do. First, you'll need to write an ISpecimenBuilder that hooks into the Mock creation process and invokes the SetupAllProperties method:

public class PropertiesPostprocessor : ISpecimenBuilder
{
    private readonly ISpecimenBuilder builder;
 
    public PropertiesPostprocessor(ISpecimenBuilder builder)
    {
        this.builder = builder;
    }
 
    public object Create(object request, ISpecimenContext context)
    {
        dynamic s = this.builder.Create(request, context);
        if (s is NoSpecimen)
            return s;
 
        s.SetupAllProperties();
        return s;
    }
}

This class must decorate another ISpecimenBuilder, whose responsibility it is to create Mock<T> instances. The example code shown here is only a proof of concept, so it only does the bare minimum of defensive coding, assuming that if the object returned from the decorated ISpecimenBuilder is not a NoSpecimen, then it must be a Mock instance. In that case, it invokes SetupAllProperties on it.

You'll need to compose this PropertiesPostprocessor class together with all the other building blocks of the AutoMoq glue library. Currently, there's no specific hook into AutoMoqCustomization that enables you to do that, but if you decompose AutoMoqCustomization, you'll realize how easy it is to replicate the behavior while adding your new PropertiesPostprocessor class:

public class AutoMoqPropertiesCustomization : ICustomization
{
    public void Customize(IFixture fixture)
    {
        fixture.Customizations.Add(
            new PropertiesPostprocessor(
                new MockPostprocessor(
                    new MethodInvoker(
                        new MockConstructorQuery()))));
        fixture.ResidueCollectors.Add(new MockRelay());
    }
}

The only difference from the AutoMoqCustomization implementation is the addition of PropertiesPostprocessor decorating MockPostprocessor.

This test now passes:

[Fact]
public void AutoSetupProperties()
{
    var fixture = new Fixture().Customize(
        new AutoMoqPropertiesCustomization());
    var h = fixture.Create<IHasProperties>();
 
    h.Text = "foo";
    h.Number = 42;
 
    Assert.Equal("foo", h.Text);
    Assert.Equal(42, h.Number);
}

As you can see, the Mock object created by AutoFixture now implements properties as normal, well-behaved properties.

One way to unit test a complex system is to test the behavior of each part and then verify that the structure of the composition is correct.

How do you unit test complex systems? How do you apply TDD to a complex system? This seems to be one of the most difficult questions related to unit testing.

First, a little language lesson. Complex means that something is, by its very nature, composed of parts.

"Complexity is intrinsic. Something is complex if it involves a lot of [metaphorical] moving parts even when considered as a Platonic ideal."

"Complication is extrinsic. Something is complicated by external influences, or because of external influences."

Traditional introductions to TDD tend to focus on simple systems, such as

• Stack
• Fibonacci
• Prime factors
• Bowling game
• Word wrap

A partial answer is provided by one of my favorite quotes from Design Patterns:

Favor object composition over class inheritance.

Before I go on, I think it's important to point out that Structural Inspection is one among several approaches you can take to unit testing and TDD. Personally, I've had quite a few successes with this technique, but it tends to require a rather meticulous modus operandi, so it's not suitable in all cases. It works best against complex systems where you can't afford mistakes. The disadvantage is that you'll have to write a lot of tests and at times you may feel that you are moving very slowly, but the advantage is that the final system is not only likely to be correct, but it's proven to be correct.

A note about the following example: it describes a way to unit test a shopping basket, including discounts, VAT, totals, etc. You may think that there are easier ways, and that the technique I describe is overkill. However, I've attempted to strike a balance between something that is sufficiently complex to make the examples realistic, yet still not so complex that it can fit in a single (although long) article.

API design #

Structural Inspection is not only a unit testing technique; it's also an API design philosophy:

What you compose, you can also expose.

public class Discount : IBasketElement
{
    public Discount(decimal amount)
 
    public decimal Amount { get; }
 
    public IBasketVisitor Accept(IBasketVisitor visitor)
}

In the above example, the Discount class contains an amount (a decimal value), and implements the IBasketElement interface. The Accept method is the only member defined by that interface. The design philosophy of Structural Inspection is very simple: because it accepts an amount in its constructor, it should also expose that value as a public Amount property (or field).

People unused to TDD often react in one of two ways:

• "I don't like adding members only for testing"
• "It breaks encapsulation"

Well, if you follow TDD, all production code you write, you write as a response to a test you just wrote. The unit tests are the first client of the production API. The important part is whether or not adding a member breaks encapsulation. Adding a property that exposes something that was passed into the constructor by a client, can hardly be said to break encapsulation. Exposing properties doesn't necessarily break encapsulation. Even if you believe that encapsulation is 'information hiding', it makes no sense to hide data that was passed into the object instance by a third party. The object simply isn't the exclusive owner of that data.

However, since the object (like the Discount class in the above example) already holds an instance of the data as a field, it might as well be courteous and share that information with its client. Otherwise, that would just put the onus on the client to remember the value it passed via the constructor.

Ultimately, adding a property to a concrete class has no impact on the exposed interface. Since a constructor is an implementation detail, by corollary Inspection Properties are also implementation details. While they don't break encapsulation, they are only part of the concrete class. At the interface level, they are invisible. Interfaces are general, but concrete types are specific.

Unit testing #

The API design philosophy is easy to understand, but what does that mean for unit testing? The great thing is that it's possible to test each part of the whole in isolation, and then subsequently prove that parts correctly interact.

Consider the Discount class. It's easy to prove that the injected amount is properly exposed as a property:

[Theory]
[InlineData(1)]
[InlineData(2)]
public void AmountIsCorrect(int expected)
{
    var sut = new Discount(expected);
    var actual = sut.Amount;
    Assert.Equal(expected, actual);
}

Now, that's not particularly interesting in itself, but it's a small part of a larger whole. In the big picture, the concrete class is invisible, but the interfaces it implements are important. In this example, it must implement the IBasketElement interface. That's also easy to prove:

[Fact]
public void SutIsBasketElement()
{
    var sut = new Discount();
    Assert.IsAssignableFrom<IBasketElement>(sut);
}

The above fact only states that Discount implements IBasketElement, but not that it correctly implements it. One last test verifies that this is, indeed, the case:

[Fact]
public void AcceptReturnsCorrectResponse()
{
    var expected = new Mock<IBasketVisitor>().Object;
    var sut = new Discount();
 
    var visitorStub = new Mock<IBasketVisitor>();
    visitorStub.Setup(v => v.Visit(sut)).Returns(expected);
    var actual = sut.Accept(visitorStub.Object);
 
    Assert.Same(expected, actual);
}

This test verifies that the Accept method is correctly implemented. It sets up visitorStub in such a way that it'll only return expected if sut is passed to the Visit method. The actual value returned by the Accept method must be the same as the expected value.

Given these tests, the implementation must look something like this:

public class Discount : IBasketElement
{
    private readonly decimal amount;
 
    public Discount(decimal amount)
    {
        this.amount = amount;
    }
 
    public IBasketVisitor Accept(IBasketVisitor visitor)
    {
        return visitor.Visit(this);
    }
 
    public decimal Amount 
    {
        get { return this.amount; }
    }
}

With these tests in place, you can pretty much forget about the Discount class. It has appropriate concrete behavior and correctly implements the IBasketElement protocol. You can write similar tests for all other classes that implements IBasketElement.

That may still seem trivial, but it begins to become interesting when you consider that a Basket is simply a collection of IBasketElement instances. That enables you to write this (Behavior Verification) test:

[Fact]
public void AcceptReturnsCorrectResult()
{
    // Fixture setup
    var v1 = new Mock<IBasketVisitor>().Object;
    var v2 = new Mock<IBasketVisitor>().Object;
    var v3 = new Mock<IBasketVisitor>().Object;
    var e1Stub = new Mock<IBasketElement>();
    var e2Stub = new Mock<IBasketElement>();
    e1Stub.Setup(e => e.Accept(v1)).Returns(v2);
    e2Stub.Setup(e => e.Accept(v2)).Returns(v3);
 
    var sut = new Basket(e1Stub.Object, e2Stub.Object);
    // Exercise system
    var actual = sut.Accept(v1);
    // Verify outcome
    Assert.Same(v3, actual);
    // Teardown
}

That may look difficult, but it simply states that when the Accept method is invoked with v1, it will pass v1 to the first IBasketElement, and the return value will be v2, which is then passed to the second IBasketElement, which returns v3, which is the last result and thus expected to be returned by the Basket instance itself. This works for all Mock<IBasketVisitor>.Object instances, so according to the Liskov Substitution Principle, it will work for any IBasketVisitor that conforms to the protocol.

In other words: because you know that Discount correctly implements IBasketElement, you are guaranteed that if there's a Discount element in a basket, it will call any visitor's Visit(Discount) method. This puts you in a position to implement an IBasketVisitor to calculate the total for the basket.

A BasketTotalVisitor must correctly implement all Visit methods defined by the IBasketVisitor interface and accumulate a total. However, whenever it encounters a Discount, it must subtract the discount from the total. Because BasketTotalVisitor is only a small part of a complex whole, this is easy to test:

[Theory]
[InlineData(1, 1)]
[InlineData(2, 1)]
[InlineData(3, 2)]
public void VisitDiscountReturnsCorrectResult(
    int initialTotal,
    int discount)
{
    var sut = new BasketTotalVisitor(initialTotal);
 
    var actual = sut.Visit(new Discount(discount));
 
    var btv = Assert.IsAssignableFrom<BasketTotalVisitor>(actual);
    Assert.Equal(initialTotal - discount, btv.Total);
}

Notice how this test uses Structural Inspection by verifying that the returned IBasketVisitor is, indeed, a BasketTotalVisitor instance, which has a Total property that the test verifies matches the expected value. It can only do that because the concrete BasketTotalVisitor class exposes the Total property.

Verifying the whole #

You can keep on working like this: establishing that the behavior of each small part follows the correct protocols. Once all fine-grained building blocks are in place, the only thing left to do is to verify that they are correctly composed. This is where Structural Inspection is very powerful. This is how you can verify that a BasketPipeline will have the correct behavior:

[Fact]
public void SutCorrectlyConvertsToPipe()
{
    CompositePipe<Basket> sut = new BasketPipeline();
 
    var visitors = sut
        .Cast<BasketVisitorPipe>()
        .Select(bvp => bvp.Visitor);
 
    var dv = Assert.IsAssignableFrom<VolumeDiscountVisitor>(visitors.First());
    Assert.Equal(500, dv.Threshold);
    Assert.Equal(.05m, dv.Rate);
 
    var vv = Assert.IsAssignableFrom<VatVisitor>(visitors.ElementAt(1));
    Assert.Equal(.25m, vv.Rate);
 
    var btv = Assert.IsAssignableFrom<BasketTotalVisitor>(visitors.Last());
}

This may look like a verbose test, but it's actually quite information-dense.

First, the BasketPipeline is a CompositePipe<Basket>, and while I haven't shown that part in this article, I know from other unit tests that this class adheres to the Liskov Substitution Principle, so if the test can verify that its content is correct, the composed behavior must also be correct.

In this test case, you can project all the contained pipes into the visitors they adapt. This is only possible because the concrete class BasketVisitorPipe exposes the concrete Visitor property. This is Structural Inspection in action.

The BasketPipeline should be able to apply a volume discount, calculate VAT and the total. Because all the small parts have the correct behavior, you only need to verify that the BasketPipeline has the correct structure.

A business rule states that a volume discount of five percent should be applied if the total value of all basket items is 500 or more. This means that the first visitor should be a VolumeDiscountVisitor, and its threshold should be 500 and the rate .05. You can use Structural Inspection to verify that this is the case.

A legal requirement is that a VAT rate of 25 percent (real Danish VAT rate) should be applied to the value of the basket. This means that the next visitor should be a VatVisitor, and its rate should be .25 .

Finally, the total price for the basket, including discounts and VAT should be calculated. This means that the last visitor should be a BasketTotalVisitor.

The test verifies the structure of the API's Facade, and because the Facade has the correct structure and previous tests have proven that all the parts have the correct behavior, the Facade must have the desired higher-order behavior.

In order to write this article, I TDD'ed all the code presented here (and more), and I deliberately didn't try to apply the BasketPipeline to some Basket instances until I had all the structure in place. Once I had that, I wrote a few Facade Tests as sanity checks, and they all passed in the first attempt.

Conclusion #

Structural Inspection is a useful technique when you need to unit test a complex system, and you need a high degree of confidence that everything works correctly. It's not the only option available to you. Another option is to use Triangulation with a large set of Facade Tests, but there may come a time where the number of test cases required to exercise all possible combinations of business rules becomes so large that Structural Inspection may be a better alternative.

The code I wrote for this article contains 136 test cases, distributed over 88 test methods. It has 100 percent code coverage, and it covers 67 members over 11 classes and 3 interfaces. That's probably too enterprisey for a simple shopping basket calculation that only does volume discounts, VAT and total calculation. However, the way I've modeled this Basket API is quite extensible, so it may be quite well-suited for complex shopping baskets that might include various sorts of inclusive and exclusive discounts, VAT rates that are dependent on the category of goods in the basket, or that may be dependent on the shipping destination. Add to that calculation of shipping rates based on shipping origin and destination, as well as various characteristics (weight, volume, etc.) of the items in the basket, and Structural Inspection starts to look attractive.

Resources #

For more details:

• My Pluralsight course Advanced Unit Testing contains an entire 35 minute module on Structural Inspection. The course also comes with full source code.
• My NDC 2013 talk Faking Homoiconicity in C# with graphs approaches the same subject from a different angle.

Comments

Tests are trustworthy if they are simple and you've seen them fail.

When I started out with TDD some ten years ago, it was really hard to convince programmers that unit testing was valuable. It's better today, but much software is still produced without test coverage. One of the most common arguments for unit tests is to ask the question: "How do you know that your code works?"

That's a darn good question, and one of the best answers is that you should cover the code with automated tests (unit tests).

Okay, then: How do you know that your unit tests work?

A test watches the correctness of the SUT, but who watches the watchmen? Should you write some more unit tests to test your unit tests? Is it going to be turtles all the way down?

Obviously, that's not what we do. Then why do we trust unit tests?

I think that there's two different reasons for that:

• They are easy to review
• We've seen them fail

Review #

The reason that a lot of code fails to do what you think it does is because it's complicated. Sometimes there's also going to be subtle bugs, but I think that the most common problem is that as the code becomes more involved, it becomes more difficult for you to hold it in your brain. A simple Hello World application is easy to understand. Typical software isn't.

Unit tests tend to be a lot simpler. First of all, unit tests should be deterministic. This means that there should be a clear path through each test case. In other words, a unit test should have a Cyclomatic Complexity of 1.

If you follow the AAA or Four-Phase Test patterns, keep the Cyclomatic Complexity at 1, keep the total line count down, and use good names, you should have a readable test case. Such test code is easy to review for correctness. If you are Pair Programming, you and your partner review the unit test code as you write it. If you use other review mechanisms (e.g. pull requests), the reviewer can review your unit test code. Even if you have no other person reviewing your code, keeping it simple helps you understand what it is that you do. Ultimately, it can't be more error prone than single-handedly writing production code without tests.

Part of the reason why we trust unit tests is because, if written well, they are easy to review for correctness.

See the test fail #

Deeply ingrained into proper TDD is the Red/Green/Refactor cycle. It's extremely important to see the test fail. Personally, I'm saved by this rule about once a week. I write a test and run it, and much to my surprise it turns out to be a false negative. Although I've been doing TDD for some ten years, this still happens to me regularly, so I think it's one of those rigid rules you should always follow. Seeing the test fail verifies that the test indeed tests something.

This is also the reason why, if you don't do TDD, but rather write tests after the fact, it becomes extremely important to follow a proper procedure for writing Characterization Tests.

Part of the reason that we trust unit tests is that we've seen them fail. It's like double-entry bookkeeping. The tests keep the SUT in place, and the SUT makes the tests pass.

Append-only tests #

Both of the reasons I've provided are focused on the test as it's being created. At that time, you see it fail and you review it for correctness. As time goes by, you should end up with a test suite with lots of tests. Do you ever look at those tests?

You trust them because they were correct when they were created. Are they still correct today?

Well, they probably are if you never again touched them, but every time you modify a test case, you make it just a little less trustworthy. Similar in spirit to the Open/Closed Principle, a test suite should be open for extension, but closed for modification. Trustworthy test suites are append-only.

Listen to your tests. If it's too much bother writing a test for a property, perhaps it's telling you that you don't need a property there.

A couple of days ago I published a blog post that caused some people to react, telling me that I'm wrong. That's most certainly a possibility, but what if I'm not? In this post I'd like to provide a bit of background on why I wrote that previous post, and where it may lead us.

Craftmanship #

My blog post was a reaction to Robert C. Martin's post on The Pragmatics of TDD. One of the qualities I've always appreciated of people like Robert C. Martin and Martin Fowler is that they consistently explain the reasoning behind their actions. The part of The Pragmatics of TDD that caused me to react was that it contains practices that aren't explained.

Notice that Robert C. Martin takes time to explain why he doesn't unit test UI code (too much fiddling is involved). To me, that's a fully satisfactory explanation, so I didn't react against that at all. On the other hand, he states that he doesn't write tests against obviously trivial members, but no satisfactory explanation is given.

Robert C. Martin has been programming for more than 40 years, and been doing TDD longer than me, so I have no doubt that this pragmatic approach works for him (as well as for many of the people who disagree with my previous post). However, if, for the sake of argument, we accept the Dreyfus model of skill acquisition, Robert C. Martin is clearly an Expert. He uses intuition and tacit knowledge to make appropriate decisions. Normally, as in Clean Code, he does a great job of making this tacit knowledge explicit, but in this case I found the explanation was missing, so I wanted to explore that question.

Perhaps I was a bit high on laying out a logical argument. Yesterday, my wife (who's not at all a programmer) kindly read through the article and let me know that my language could have been less aggressive. It could, and I apologize if I offended anyone - particularly Robert C. Martin from whom I've learned so much.

Most of all, though, I think I could have stated my motivation and context clearer.

As I have previously stated on Twitter, I have come to the realization that I'm a Programmer more than I'm a Developer. While I love software, I love code more. If you ask my past employers and customers, they will tell you that I can get stuff done, too. However, I often code just to learn. This is an aspect of Software Craftmanship that is sometimes overlooked: Yes, we want to ship working software. However, we must always strive to become better at our craft, and that means practice. That means writing code for code's sake. That means obsessing over details.

It also means thinking about aspects of programming that many people take for granted, such as properties.

Tests provide feedback #

Several people attacked my previous post because they think it's not practical and that it puts TDD in a bad light. Actually, I think it puts TDD in a splendid light because it made me explicitly realize something that I've implicitly 'known' for a long time: we use way too many properties in our code.

As always, we should take to heart the advice from GOOS: Listen to your tests.

If you think that it's ridiculous to write tests for automatic properties, then what is the feedback really about?

Most of my critics present the choices like this:

However, I think the choice is more like this:

In his post on the Pragmatics of TDD, Robert C. Martin states that getters and setters (properties) will be indirectly covered by other tests. This would also be true for public fields. This is even more pronounced in C# where the syntax for accessing a property is identical to the syntax for accessing a field.

I agree with both Robert C. Martin and Mark Rendle that in a lot of cases, we don't really care about anything as nitty-gritty as a single property. What we often care about is the overall behavior of a system. This is also the approach I teach in my Pluralsight course on Outside-In Test Driven Development. You don't see me write tests of properties there.

However, I made a mistake in that course, and another mistake in my previous post, and that was to design View Models with properties. Why are we even adding properties to View Models? The purpose of a View Model is to render in a particular way, either as UI, XML, JSON or something else. We care about the end result, not what the View Model class looks like. At the boundaries, applications aren't object-oriented.

The JournalEntryModel from my Pluralsight course should really have look like this:

public class JournalEntryModel
{
    public DateTimeOffset Time;
 
    public int Distance;
 
    public TimeSpan Duration;
}

Notice that the class simply holds three public fields. This produces the same JSON as the version where those three members are properties.

Summary #

There may still be cases where properties are appropriate. If, for example, instead of writing an end-to-end application, you are writing a reusable library, properties provide encapsulation. In such cases, the property has a particular purpose, and it still makes sense to me to capture that behavior (yes: behavior, not data) in a regression test.

However, most of the times properties aren't warranted, and you might as well use a public field - this is the YAGNI principle applied. You don't have to test public fields because they can't possibly have any behavior.

Comments

A unit test pattern

This article describes a unit test pattern called an Auto-mocking Container. It can be used as a solution to the problem:

How can unit tests be decoupled from Dependency Injection mechanics?

By using a heuristic composition engine to compose dynamic Test Doubles into the System Under Test

A major problem with unit tests is to make sure that they are robust in the face of a changing system. One of the most common problems programmers have with unit tests is the so-called Fragile Test smell. Every time you attempt to refactor your code, tests break.

There are various reasons why that happens and the Auto-mocking Container pattern does not help in all cases, but a common reason that tests break is when you change the constructor of your System Under Test (SUT). In such cases, the use of an Auto-mocking Container can help.

How it works #

In order to decouple a unit test from the mechanics of creating an instance of the SUT, the test code can repurpose a Dependency Injection (DI) Container to compose it. The DI Container must be customizable to the extent that it can automatically compose dynamic Test Doubles (mocks/stubs) into the SUT.

It's important to notice that this technique is a pure unit testing concern. Even if you use an Auto-mocking Container in your unit test, you don't have to use a DI Container in your production code - or you can use a different DI Container than the one you've chosen to repurpose as an Auto-mocking Container.

When a test case wants to exercise the SUT, it needs an instance of the SUT. Instead of creating the SUT directly by invoking its constructor, the test case uses an Auto-mocking Container to create the SUT instance. The Auto-mocking Container automatically supplies dynamic mocks in place of all the SUT's dependencies, freeing the test writer from explicitly dealing with this concern.

This decouples each test case from the mechanics of how the SUT is created, making the test more robust. Even if the SUT's constructor is refactored, the test case remains unimpacted because the Auto-mocking Container dynamically deals with the changed constructor signature.

When to use it #

Use an Auto-mocking Container when you are unit testing classes which utilize DI (particularly Constructor Injection). In a fully loosely coupled system, constructors are implementation details, which means that you may decide to change constructor signatures as part of refactoring.

Particularly early in a code base's lifetime, the system and its internal design may be in flux. In order to enable refactoring, it's important to be able to change constructor signatures without worrying about breaking tests. In such cases, an Auto-mocking Container can be of great help.

In well-established code bases, introducing an Auto-mocking Container is unlikely to be of much benefit, as the code is assumed to be more stable.

Auto-mocking Containers are also less ideally suited for code-bases which rely more on a functional programming style with Value Objects and data flow algorithms, and less on DI.

Implementation details #

Use an existing DI Container, but repurpose it as an Auto-mocking Container. Normal DI Containers don't serve dynamic mock objects by default, so you'll need to pick a DI Container which is sufficiently extensible to enable you to change its behavior as desired.

In order to extend the DI Container with the ability to serve dynamic mock objects, you must also pick a suitable dynamic mock library. The Auto-mocking Container is nothing more than a 'Glue Library' that connects the behavior of the DI Container with the behavior of the dynamic mock library.

Some open source projects exist that provide pre-packaged Glue Libraries as Auto-mocking Containers by combining two other libraries, but with good DI Containers and dynamic mock libraries, it's a trivial effort to produce the Auto-mocking Container as part of the unit test infrastructure.

Motivating example #

In order to understand how unit tests can become tightly coupled to the construction mechanics of SUTs, imagine that you are developing a simple shopping basket web service using Test-Driven Development (TDD).

To keep the example simple, imagine that the shopping basket is going to be a CRUD service exposed over HTTP. The framework you've chosen to use is based on the concept of a Controller that handles incoming requests and serves responses. In order to get started, you write the first (ice breaker) unit test:

[Fact]
public void SutIsController()
{
    var sut = new BasketController();
    Assert.IsAssignableFrom<IHttpController>(sut);
}

This is a straight-forward unit test which prompts you to create the BasketController class.

The next thing you want to do is to enable clients to add new items to the basket. In order to do so, you write the next test:

[Fact]
public void PostSendsCorrectEvent()
{
    var channelMock = new Mock<ICommandChannel>();
    var sut = new BasketController(channelMock.Object);
 
    var item = new BasketItemModel { ProductId = 1234, Quantity = 3 };
    sut.Post(item);
 
    var expected = item.AddToBasket();
    channelMock.Verify(c => c.Send(expected));
}

In order to make this test compile you need to add this constructor to the BasketController class:

private ICommandChannel channel;
 
public BasketController(ICommandChannel channel)
{
    this.channel = channel;
}

However, this breaks the first unit test, and you have to go back and fix the first unit test in order to make the test suite compile:

[Fact]
public void SutIsController()
{
    var channelDummy = new Mock<ICommandChannel>();
    var sut = new BasketController(channelDummy.Object);
    Assert.IsAssignableFrom<IHttpController>(sut);
}

In this example only a single unit test broke, but it gets progressively worse as you go along.

Satisfied with your implementation so far, you now decide to implement a feature where the client of the service can read the basket. This is supported by the HTTP GET method, so you write this unit test to drive the feature into existence:

[Fact]
public void GetReturnsCorrectResult()
{
    // Arrange            
    var readerStub = new Mock<IBasketReader>();
    var expected = new BasketModel();
    readerStub.Setup(r => r.GetBasket()).Returns(expected);
 
    var channelDummy = new Mock<ICommandChannel>().Object;
    var sut = new BasketController(channelDummy, readerStub.Object);
 
    // Act
    var response = sut.Get();
    var actual = response.Content.ReadAsAsync<BasketModel>().Result;
 
    // Assert
    Assert.Equal(expected, actual);
}

This test introduces yet another dependency into the SUT, forcing you to change the BasketController constructor to this:

private ICommandChannel channel;
private IBasketReader reader;
 
public BasketController(ICommandChannel channel, IBasketReader reader)
{
    this.channel = channel;
    this.reader = reader;
}

Alas, this breaks both previous unit tests and you have to revisit them and fix them before you can proceed.

In this simple example, fixing a couple of unit tests in order to introduce a new dependency doesn't sound like much, but if you already have hundreds of tests, the prospect of breaking dozens of tests each time you wish to refactor by moving dependencies around can seriously hamper your productivity.

Refactoring notes #

The problem is that the tests are too tightly coupled to the mechanics of how the SUT is constructed. The irony is that although you somehow need to create instances of the SUT, you shouldn't really care about how it happens.

If you closely examine the tests in the motivating example, you will notice that the SUT is created in the Arrange phase of the test. This phase of the test is also called the Fixture Setup phase; it's where you put all the initialization code which is required before you can interact with the SUT. To be brutally honest, the code that goes into the Arrange phase is just a necessary evil. You should care only about the Act and Assert phases - after all, these tests don't test the constructors. In other words, what happens in the Arrange phase is mostly incidental, so it's unfortunate if that part of a test is holding you back from refactoring. You need a way to decouple the tests from the constructor signature, while still being able to manipulate the injected dynamic mock objects.

There are various ways to achieve that goal. A common approach is to declare the SUT and its dependencies as class fields and compose them all as part of a test class' Implicit Setup . This can be an easy way to address the issue, but carries with it all the disadvantages of the Implicit Setup pattern. Additionally, it can lead to an explosion of fields and low cohesion of the test class itself.

Another approach is to build the SUT with a helper method. However, if the SUT has more than one dependency, you may need to create a lot of overloads of such helper methods, in order to manipulate only the dynamic mocks you may care about in a given test case. This tends to lead towards the Object Mother (anti-)pattern.

A good alternative is an Auto-mocking Container to decouple the tests from the constructor signature of the SUT.

Example: Castle Windsor as an Auto-mocking Container #

In this example you repurpose Castle Windsor as an Auto-mocking Container. Castle Windsor is one among many DI Containers for .NET with a pretty good extensibility model. This can be used to turn the standard WindsorContainer into an Auto-mocking Container. In this case you will combine it with Moq in order to automatically create dynamic mocks every time you need an instance of an interface.

It only takes two small classes to make this happen. The first class is a so-called SubDependencyResolver that translates requests for an interface into a request for a mock of that interface:

public class AutoMoqResolver : ISubDependencyResolver
{
    private readonly IKernel kernel;
 
    public AutoMoqResolver(IKernel kernel)
    {
        this.kernel = kernel;
    }
 
    public bool CanResolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        return dependency.TargetType.IsInterface;
    }
 
    public object Resolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        var mockType = typeof(Mock<>).MakeGenericType(dependency.TargetType);
        return ((Mock)this.kernel.Resolve(mockType)).Object;
    }
}

The ISubDependencyResolver interface is a Castle Windsor extensibility point. It follows the Tester-Doer pattern, which means that the WindsorContainer will first call the CanResolve method, and subsequently only call the Resolve method if the return value from CanResolve was true.

In this implementation you return true if and only if the requested dependency is an interface. When this is the case, you construct a generic type of Moq's Mock<T> class. For example, in the code above, if dependency.TargetType is the interface ICommandChannel, the mockType variable becomes the type Mock<ICommandChannel>.

The next line of code asks the kernel to resolve that type (e.g. Mock<ICommandChannel>). The resolved instance is cast to Mock in order to access and return the value of its Object property. If you ask the kernel for an instance of Mock<ICommandChannel>, it'll return an instance of that type, and its Object property will then be an instance of ICommandChannel. This is the dynamic mock object you need in order to compose a SUT with automatic Test Doubles.

The other class wires everything together:

public class ShopFixture : IWindsorInstaller
{
    public void Install(
        IWindsorContainer container,
        IConfigurationStore store)
    {
        container.Kernel.Resolver.AddSubResolver(
            new AutoMoqResolver(
                container.Kernel));
        container.Register(Component
            .For(typeof(Mock<>)));
 
        container.Register(Classes
            .FromAssemblyContaining<Shop.MvcApplication>()
            .Pick()
            .WithServiceSelf()
            .LifestyleTransient());
    }
}

This is an IWindsorInstaller, which is simply a way of packaging Castle Windsor configuration together in a class. In the first line you add the AutoMoqResolver. In the next line you register the open generic type Mock<>. This means that the WindsorContainer knows about any generic variation of Mock<T> you might want to create. Recall that AutoMockResolver asks the kernel to resolve an instance of Mock<T> (e.g. Mock<ICommandChannel>). This registration makes this possible.

Finally, the ShopFixture scans through all public types in the SUT assembly and registers their concrete types. This is all it takes to turn Castle Windsor into an Auto-mocking Container.

Once you've done that, you can start TDD'ing, and you should rarely (if ever) have to go back to the ShopFixture to tweak anything.

The first test you write is equivalent to the first test in the previous motivating example:

[Fact]
public void SutIsController()
{
    var container = new WindsorContainer().Install(new ShopFixture());
    var sut = container.Resolve<BasketController>();
    Assert.IsAssignableFrom<IHttpController>(sut);
}

First you create the container and install the ShopFixture. This is going to be the first line of code in all of your tests.

Next, you ask the container to resolve an instance of BasketController. At this point there's only a default constructor, so Auto-mocking hasn't even kicked in yet, but you get an instance of the SUT against which you can make your assertion.

Now it's time to write the next test, equivalent to the second test in the motivating example:

[Fact]
public void PostSendsCorrectEvent()
{
    var container = new WindsorContainer().Install(new ShopFixture());
    var sut = container.Resolve<BasketController>();
 
    var item = new BasketItemModel { ProductId = 1234, Quantity = 3 };
    sut.Post(item);
 
    var expected = item.AddToBasket();
    container
        .Resolve<Mock<ICommandChannel>>()
        .Verify(c => c.Send(expected));
}

Here an interesting thing happens. Notice that the first two lines of code are the same as in the previous test. You don't need to define the mock object before creating the SUT. In fact, you can exercise the SUT without ever referencing the mock.

In the last part of the test you need to verify that the SUT interacted with the mock as expected. At that time you can ask the container to resolve the mock and verify against it. This can be done in a single method call chain, so you don't even have to declare a variable for the mock.

This is possible because the ShopFixture registers all Mock<T> instances with the so-called Singleton lifestyle. The Singleton lifestyle shouldn't be confused with the Singleton design pattern. It means that every time you ask a container instance for an instance of a type, you will get back the same instance. In Castle Windsor, this is the default lifestyle, so this code:

container.Register(Component
    .For(typeof(Mock<>)));

is equivalent to this:

container.Register(Component
    .For(typeof(Mock<>))
    .LifestyleSingleton());

When you implement the Post method on BasketController by injecting ICommandChannel into its constructor, the test is going to pass. This is because when you ask the container to resolve an instance of BasketController, it will need to first resolve an instance of ICommandChannel.

This is done by the AutoMoqResolver, which in turn asks the kernel for an instance of Mock<ICommandChannel>. The Object property is the dynamically generated instance of ICommandChannel, which is injected into the BasketController instance ultimately returned by the container.

When the test case subsequently asks the container for an instance of Mock<ICommandChannel> the same instance is reused because it's configured with the Singleton lifestyle.

More importantly, while the second test prompted you to change the constructor signature of BasketController, this change didn't break the existing test.

You can now write the third test, equivalent to the third test in the motivating example:

[Fact]
public void GetReturnsCorrectResult()
{
    var container = new WindsorContainer().Install(new ShopFixture());
    var sut = container.Resolve<BasketController>();
    var expected = new BasketModel();
    container.Resolve<Mock<IBasketReader>>()
        .Setup(r => r.GetBasket())
        .Returns(expected);            
 
    var response = sut.Get();
    var actual = response.Content.ReadAsAsync<BasketModel>().Result;
 
    Assert.Equal(expected, actual);
}

Once more you change the constructor of BasketController, this time to inject an IBasketReader into it, but none of the existing tests break.

This example demonstrated how to repurpose Castle Windsor as an Auto-mocking Container. It's important to realize that this is still a pure unit testing concern. It doesn't require you to use Castle Windsor in your production code.

Other DI Containers can be repurposed as Auto-mocking Containers as well. One option is Autofac, and while not strictly a DI Container, another option is AutoFixture.

This article first appeared in the Open Source Journal 2012 issue 4. It is reprinted here with kind permission.

Comments

public class When_sending_an_http_get_request : WithSubject<BasketController>
{
    private static BasketModel expectedContent;
    private static HttpResponseMessage response;

    Establish context = () =>
    {
        expected = new BasketModel();
        The<IBasketReader>()
            .WhenToldTo(r => r.GetBasket())
            .Return(expectedContent);
    }

    Because of = () =>
        response = Subject.Get();

    It should_return_a_successful_http_response = () =>
        response.IsSuccessfulStatusCode.ShouldBeTrue();

    It should_return_the_contents_of_the_basket = () =>
        response.ReadContentAs<BasketModel>().ShouldEqual(expectedContent);
}

Even if code is trivial you should still test it.

A few days ago, Robert C. Martin posted a blog post on The Pragmatics of TDD, where he explains that he doesn't test-drive everything. Some of the exceptions he give, such as not test-driving GUI code and true one-shot code, make sense to me, but there are two exceptions I think are inconsistent.

Robert C. Martin states that he doesn't test-drive

• getters and setters
• one-line functions
• functions that are obviously trivial

There are several problems with this exception that I'd like to point out:

• It confuses cause and effect
• Trivial code may not stay trivial
• It's horrible advice for novices

Causality #

The whole point of Test-Driven Development is that the tests drive the implementation. The test is the cause and the implementation is the effect. If you accept this premise, then how on Earth can you decide not to write a test because the implementation is going to be trivial? You don't know that yet. It's logically impossible.

Robert C. Martin himself proposed the Transformation Priority Premise (TPP), and one of the points here is that as you start out test-driving new code, you should strive to do so in small, formalized steps. The first step is extremely likely to leave you with a 'trivial' implementation, such as returning a constant.

With the TPP, the only difference between trivial and non-trivial implementation code is how far into the TDD process you've come. So if 'trivial' is all you need, the only proper course is to write that single test that demonstrates that the trivial behavior works as expected. Then, according to the TPP, you'd be done.

Encapsulation #

Robert C. Martin's exception about getters and setters is particularly confounding. If you consider a getter/setter (or .NET property) trivial, why even have it? Why not expose a public class field instead?

There are good reasons why you shouldn't expose public class fields, and they are all releated to encapsulation. Data should be exposed via getters and setters because it gives you the option of changing the implementation in the future.

What do we call the process of changing the implementation code without changing the behavior?

This is called refactoring. How do we know that when we change the implementation code, we don't change the behavior?

As Martin Fowler states in Refactoring, you must have a good test suite in place as a safety net, because otherwise you don't know if you broke something.

Successful code lives a long time and evolves. What started out being trivial may not remain trivial, and you can't predict which trivial members will remain trivial years in the future. It's important to make sure that the trivial behavior remains correct when you start to add more complexity. A regression suite addresses this problem, but only if you actually test the trivial features.

Robert C. Martin argues that the getters and setters are indirectly tested by other test cases, but while that may be true when you introduce the member, it may not stay true. Months later, those tests may be gone, leaving the member uncovered by tests.

You can look at it like this: with TDD you may be applying the TPP, but for trivial members, the time span between the first and second transformation may be measured in months instead of minutes.

Learning #

It's fine to be pragmatic, but I think that this 'rule' that you don't have to test-drive 'trivial' code is horrible advice for novices. If you give someone learning TDD a 'way out', they will take it every time things become difficult. If you provide a 'way out', at least make the condition explicit and measurable.

A fluffy condition that "you may be able to predict that the implementation will be trivial" isn't at all measurable. It's exactly this way of thinking that TDD attempts to address: you may think that you already know how the implementation is going to look, but letting tests drive the implementation, it often turns out that you'll be suprised. What you originally thought would work doesn't.

Addressing the root cause #

Am I insisting that you should test-drive all getters and setters? Yes, indeed I am.

But, you may say, it takes too much time? Ironically, Robert C. Martin has much to say on exactly that subject:

The only way to go fast, is to go well

Even so, let's see what it would be like to apply the TPP to a property (Java programmers can keep on reading. A C# property is just syntactic suger for a getter and setter).

Let's say that I have a DateViewModel class and I'd like it to have a Year property of the int type. The first test is this:

[Fact]
public void GetYearReturnsAssignedValue()
{
    var sut = new DateViewModel();
    sut.Year = 2013;
    Assert.Equal(2013, sut.Year);
}

However, applying the Devil's Advocate technique, the correct implementation is this:

public int Year
{
    get { return 2013; }
    set { }
}

That's just by the TPP book, so I have to write another test:

[Fact]
public void GetYearReturnsAssignedValue2()
{
    var sut = new DateViewModel();
    sut.Year = 2010;
    Assert.Equal(2010, sut.Year);
}

Together, these two tests prompt me to correctly implement the property:

public int Year { get; set; }

While those two tests could be refactored into a single Parameterized Test, that's still a lot of work. Not only do you need to write two test cases for a single property, but you'd have to do exactly the same for every other property!

Exactly the same, you say? And what are you?

Ah, you're a programmer. What do programmers do when they have to do exactly the same over and over again?

Well, if you make up inconsistent rules that enable you to skip out of doing things that hurt, then that's the wrong answer. If it hurts, do it more often.

Programmers automate repeated tasks. You can do that when testing properties too. Here's how I've done that, using AutoFixture:

[Theory, AutoWebData]
public void YearIsWritable(WritablePropertyAssertion a)
{
    a.Verify(Reflect<DateViewModel>.GetProperty<int>(sut => sut.Year));
}

This is a declarative way of checking exactly the same behavior as the previous two tests, but in a single line of code.

Root cause analysis is in order here. It seems as if the cost/benefit ratio of test-driving getters and setters is too high. However, I think that when Robert C. Martin stops there, it's because he considers the cost fixed, and the benefit is then too low. However, while the benefit may seem low, the cost doesn't have to be fixed. Lower the cost, and the cost/benefit ratio improves. This is why you should also test-drive getters and setters.

Update 2014.01.07: As a follow-up to this article, Jon Galloway interviewed me about this, and other subject, for a Herding Code podcast.

Update, 2018.11.12: A new article reflects on this one, and gives a more nuanced picture of my current thinking.

Comments

This post proposes a scientific experiment about code readability

Once in a while someone tries to drag me into one of many so-called 'religious' debates over code readability:

• Tabs versus spaces?
• Where should the curly braces go?
• Should class members be prefixed (e.g. with an underscore)?
• Is the this C# keyword redundant?
• etc.

In most cases, the argument revolves around what is most readable.

Let's look at the C# keyword this as an example. Many programmers feel that it's redundant because they can omit it without changing the program. The compiler doesn't care. However, as Martin Fowler wrote in Refactoring: "Any fool can write code that a computer can understand. Good programmers write code that humans can understand." Code is read much more than written, so I don't buy the argument about redundancy.

There are lots of circumstances when code is read. Often, programmers read code from inside their IDE. This has been used as an argument against Hungarian notation. However, at other times, you may be reading code as part of a review process - e.g. on GitHub or CodePlex. Or you may be reading code on a blog post, or in a book.

Often I feel that the this keyword helps me because it provides information about the scope of a variable. However, I also preach that code should consist of small classes with small methods, and if you follow that principle, variable scoping should be easily visible on a single screen.

Another argument against using the this keyword is that it's simply noise and that it makes it harder to read the program, simply because there's more text to read. Such a statement is impossible to refute with logic, because it depends on the person reading the code.

Ultimately, the readability of code depends on circumstances and is highly subjective. For that reason, we can't arrive at a conclusion to any of those 'religious' debates by force of logic. However, what we could do, on the other hand, is to perform a set of scientific experiments to figure out what is most readable.

A science experiment idea for future computer scientists #

Here's an idea for a computer science experiment:

1. Pick a 'religious' programming debate (e.g. whether or not the this keyword enhances or reduces readability).
2. Form a hypothesis (e.g. predict that the this keyword enhances readability).
3. Prepare a set of simple code listings, each with two alternatives (with and without this).
4. Randomly select two groubs of test subjects.
5. Ask each group to explain to you what the code does (what is the result of calling a method, etc.). One group gets one alternative set, and the other group gets the other set.
6. Measure how quickly members of each group arrives at a correct conclusion.
7. Compare results against original hypothesis.

Comments

Is there a conflict between Outside-In TDD and DDD? I don't think so.

A reader of my book recently asked me a question:

"you are a DDD and TDD fan and at the same time you like to design your User Interfaces first. I have been reading about DDD and TDD for some time now and your approach confuses me, TDD is about writing tests first to lead the design, DDD is focusing on the domain yet you start with the UI."

In my opinion there's no conflict between Outside-In TDD and DDD, but I must admit that I was a bit surprised by the question. It never occurred to me that there's an apparent conflict between these two approaches, but now that I was asked the question, I understand why one would think so. In other words, I think it's a good question that warrants a proper answer.

Before discussing the synthesis of Outside-In TDD and DDD I think we need to examine each concept individually.

DDD #

As far as I can tell, Domain-Driven Design is a horribly misunderstood book. By corollary, so is the DDD concept itself. The problem with the book is that it provides value and guidance along two axes:

• Patterns and other coding advice
• Principles for analysis and modeling

Don't let the fact that all these patterns are described in a book called Domain-Driven Design trick you into thinking that they are intrinsically bound to DDD. Using the patterns don't make your code Domain-Driven, and you can do DDD without them. Obviously, you can also combine them.

This provides a partial answer to the original question. Outside-In TDD doesn't conflict with the patterns described in Domain-Driven Design, just as it doesn't conflict with most other design patterns.

That still leaves the 'true' DDD: the principles for analyzing and modeling the subject matter for a piece of software. If that is a design technique, and Outside-In TDD is also a design technique, isn't there a conflict?

Outside-In TDD #

Outside-In TDD, as I describe it in my Pluralsight course, isn't a design technique. TDD isn't a design technique. Granted, TDD provides fast feedback about the design you are implementing, but it's not a blind design technique.

Outside-In TDD and DDD #

Once you realize that a major reason that Outside-In TDD and DDD seems to be at odds, is because of the false premise that TDD is a design technique, you should also realize that there isn't, after all, any conflict. There's no reason that you can't do both, but it doesn't mean you always should.

• You can do Outside-In TDD without DDD. If you are building a CRUD application, DDD is probably going to be overkill. If so, skip it.
• You can do DDD without Outside-In TDD. If you are being paid to build or extract a true Domain Model, it makes sense to do so decoupled from any sort of application boundaries.
• You can combine Outside-In TDD with DDD, but I think this will often make best sense as an iterative approach. First you build the application. Then you realize that the business logic within the application is so complex that you'll need to extract a proper Domain Model in order to keep it maintainable.

In my Pluralsight course, I also discuss the Test Pyramid. While it makes sense to begin development at the boundary (e.g. UI) of an application, you shouldn't stay at the boundary. Most tests should still be unit tests. Once you reach the phase where you realize that you'll need to evolve a proper Domain Model, you should do that with unit tests, not boundary tests. Domain Models should be implemented decoupled from all boundaries (UI, I/O and persistence), so you can only directly test a Domain Model with unit tests.

In the end, it's all a question of perspective. Outside-In TDD states that you should begin your TDD process at the boundary. In the beginning, it will focus on the externally visible behavior of the system, but as development progresses, other parts of the system must be dealt with. This is where DDD can be valuable. The lack of conflict eventually stems from the fact that Outside-In TDD and DDD operate on different levels of application architecture, with little overlap.

In my experience, a lot of applications can be written with Outside-In TDD without a Domain Model ever having to evolve - and still be maintainable. These applications may still employ the 'Supple Design Patterns', but that doesn't mean that they are Domain-Driven.

Comments