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As previous posts have described, AutoFixture creates Anonymous Variables based on the notion of being able to always hit within a well-behaved Equivalence Class for a given type. This works well a lot of the time because AutoFixture has some sensible defaults: numbers are positive integers and strings are GUIDs.

This last part only works as long as strings are nothing but opaque blobs to the consuming class. This is, however, not an unreasonable assumption. Consider classes that implement Entities such as Person or Address. Strings will often take the form of FirstName, LastName, Street, etc. In all such cases, the value of the string usually doesn't matter.

However, there will always be cases where the value of a string has a special meaning of its own. It will often be best to let AutoFixture guide us towards a better API design, but this is not always possible. Sometimes there are rules that constrain the formatting of a string.

As an example, consider a Money class with this constructor:

public Money(decimal amount, string currencyCode)
{
    if (currencyCode == null)
    {
        throw new ArgumentNullException("...");
    }
    if (!CurrencyCodes.IsValid(currencyCode))
    {
        throw new ArgumentException("...");
    }
 
    this.amount = amount;
    this.currencyCode = currencyCode;
}

Notice that the constructor only allows properly formatted currency codes (such as e.g. “DKK”, “USD”, “AUD”, etc.) through, while other strings will throw an exception. AutoFixture's default behavior of creating GUIDs as strings is problematic, as the Money constructor will throw on a GUID.

We could attempt to fix this by changing the way AutoFixture generates strings in general, but that may not be the best solution as it may interfere with other string values. It is, however, easy to do:

fixture.Inject("DKK");

This simply injects the “DKK” string into the fixture, causing all subsequent strings to have the same value. However, a hypothetical Pizza class with Name and Description properties in addition to a Price property of type Money will now look like this:

{
  "Name": "DKK",
  "Price": {
    "Amount": 1.0,
    "CurrencyCode": "DKK"
  },
  "Description": "DKK"
}

What we really want is to customize only the currency code. This is where the extremely customizable architecture of AutoFixture can help us. As the documentation explains, lots of different request will flow through the kernel's Chain of Responsibility to create a Money instance. To populate the two parameters of the Money constructor, two ParameterInfo requests will be issued - one for each parameter. We can take advantage of this to create a custom ISpecimenBuilder that only addresses string parameters with the name currencyCode.

public class CurrencyCodeSpecimenBuilder : 
    ISpecimenBuilder
{
    public object Create(object request,
        ISpecimenContext context)
    {
        var pi = request as ParameterInfo;
        if (pi == null)
        {
            return new NoSpecimen(request);
        }
        if (pi.ParameterType != typeof(string)
            || pi.Name != "currencyCode")
        {
            return new NoSpecimen(request);
        }
 
        return "DKK";
    }
}

It simply examines the request to determine whether this is something that it should address at all. Only if the request is a ParameterInfo representing a string parameter named currencyCode do we deal with it. In any other case we return NoSpecimen, which simply tells AutoFixture that it should ask another ISpecimenBuilder instead.

Here we just return the hard-coded string “DKK”, but we could easily have expanded the example to use a more varied generation algorithm. I will leave that, as well as how to generalize this in other ways, as exercises to the reader.

With CurrencyCodeSpecimenBuilder available, we can add it to the Fixture like this:

fixture.Customizations.Add(
    new CurrencyCodeSpecimenBuilder());

With this customization added, a Pizza instance now looks like this:

{
  "Name": "namec6b7a923-ea78-4817-9e24-a6863a597645",
  "Price": {
    "Amount": 1.0,
    "CurrencyCode": "DKK"
  },
  "Description": "Description63ef17d7-876d-46d8-af73-1ed91f83e699"
}

Notice how only the currency code is affected while all other string values are created by the default algorithm.

In a nutshell, a custom ISpecimenBuilder can be used to implement all sorts of custom conventions for AutoFixture. The one shown here applies the string “DKK” to all string parameters named currencyCode. This mean that the convention isn't necessarily constrained to the Money constructor, but applies to all ParamterInfo instances that fit the specification.

Comments

Recently I attended a workshop where we attempted to migrate an existing web application to Windows Azure. It was a worthwhile workshop that left me with a few key takeaways related to migrating applications to Windows Azure.

The first and most important point is so self-evident that I seriously considered not publishing it. However, apparently it wasn't self-evident to all workshop participants, so someone else might also benefit from this general advice:

Before migrating to Windows Azure, make sure that the application scales to at least two normal servers.

It's as simple as that - still, lots of web developers never consider this aspect of application architecture.

Why is this important in relation to Azure? The Windows Azure SLA only applies if you deploy two or more instances, which makes sense since the hosting servers occasionally need to be patched etc.

Unless you don't care about the SLA, your application must be able to ‘scale' to at least two servers. If it can't, fix this issue first, before attempting to migrate to Windows Azure. You can test this locally by simply installing your application on two different servers and put them behind a load balancer (you can use virtual machines if you don't have the hardware). Only if it works consistently in this configuration should you consider deploying to Azure.

Here are the most common issues that may prevent the application from ‘scaling':

• Keeping state in memory. If you must use session state, use one of the out-of-process session state store providers.
• Using the file system for persistent storage. The file system is local to each server.

Making sure that the application ‘scales' to at least two servers is such a simple sanity check that it should go without saying, but apparently it doesn't.

Please note that I put ‘scaling' in quotes here. An application that runs on only two servers has yet to prove that it's truly scalable, but that's another story.

Also note that this sanity check in no way guarantees that the application will run on Azure. However, if the check fails, it most likely will not.

Comments

It gives me great pleasure to announce the release of AutoFixture 2.0. This release fixes a few minor issues since beta 1, but apart from that there are no significant changes.

This is a major release compared to AutoFixture 1.1, featuring a completely new kernel, as well as new features and extensibility points. The beta 1 announcement sums up the changes. Get it here.

AutoFixture 2.0 comes with a new extension for xUnit.net data theories. For those of us using xUnit.net, it can help make our unit tests more succinct and declarative.

AutoFixture's support for xUnit.net is implemented in a separate assembly. AutoFixture itself has no dependency on xUnit.net, and if you use another unit testing framework, you can just ignore the existence of the Ploeh.AutoFixture.Xunit assembly.

Let's go back and revisit the previous test we wrote using AutoFixture and its auto-mocking extension:

[Fact]
public void AddWillPipeMapCorrectly()
{
    // Fixture setup
    var fixture = new Fixture()
        .Customize(new AutoMoqCustomization());
 
    var basket = fixture.Freeze<Basket>();
    var mapMock = fixture.Freeze<Mock<IPizzaMap>>();
 
    var pizza = fixture.CreateAnonymous<PizzaPresenter>();
 
    var sut = fixture.CreateAnonymous<BasketPresenter>();
    // Exercise system
    sut.Add(pizza);
    // Verify outcome
    mapMock.Verify(m => m.Pipe(pizza, basket.Add));
    // Teardown
}

Notice how all of the Fixture Setup phase is only used to create various objects that will be used in the test. First we create the fixture object, and then we use it to create four other objects. That turns out to be a pretty common idiom when using AutoFixture, so it's worthwhile to reduce the clutter if possible.

With xUnit.net's excellent extensibility features, we can. AutoFixture 2.0 now includes the AutoDataAttribute in a separate assembly. AutoDataAttribute derives from xUnit.net's DataAttribute (just like InlineDataAttribute), and while we can use it as is, it becomes really powerful if we combine it with auto-mocking like this:

public class AutoMoqDataAttribute : AutoDataAttribute
{
    public AutoMoqDataAttribute()
        : base(new Fixture()
            .Customize(new AutoMoqCustomization()))
    {
    }
}

This is a custom attribute that combines AutoFixture's two optional extensions for auto-mocking and xUnit.net support. With the AutoMoqDataAttribute in place, we can now rewrite the above test like this:

[Theory, AutoMoqData]
public void AddWillPipeMapCorrectly([Frozen]Basket basket,
    [Frozen]Mock<IPizzaMap> mapMock, PizzaPresenter pizza,
    BasketPresenter sut)
{
    // Fixture setup
    // Exercise system
    sut.Add(pizza);
    // Verify outcome
    mapMock.Verify(m => m.Pipe(pizza, basket.Add));
    // Teardown
}

The AutoDataAttribute simply uses a Fixture object to create the objects declared in the unit tests parameter list. Notice that the test is no longer a [Fact], but rather a [Theory].

The only slightly tricky thing to notice is that when we declare a parameter object, it will automatically map to a call to the CreateAnonymous method for the parameter type. However, when we need to invoke the Freeze method, we can add the [Frozen] attribute in front of the parameter.

The best part about data theories is that they don't prevent us from writing normal unit tests in the same Test Class, and this carries over to the [AutoData] attribute. We can use it when it's possible, but for those more complex tests where we need to interact with a Fixture instance, we can still write a normal [Fact].

Comments

The subject of Dependency Injection (DI) in general, and DI Containers specifically, suffers from horrible terminology that seems to confuse a lot of people. Newcomers to DI often think about DI Containers as a sort of Abstract Factory on steroids. It's not. Nicholas Blumhardt already realized and described this phenomenon a couple of years ago.

The core of the matter is that as developers, we are extremely accustomed to thinking about components and services in terms of queries instead of commands. However, the Hollywood Principle insists that we should embrace a tell, don't ask philosophy. We can apply this principles to DI Containers as well: Don't call the container; it'l call you.

This leads us to what Krzysztof Koźmic calls the three calls pattern. Basically it states that we should only do three things with a DI Container:

1. Bootstrap the container
2. Resolve root components
3. Dispose this container

This is very sound advice and independently of Krzysztof I've been doing something similar for years - so perhaps the pattern label is actually in order here.

However, I think that the pattern deserves a more catchy name, so in the spirit of the Arrange Act Assert (AAA) pattern for unit testing I propose that we name it the Register Resolve Release (RRR) pattern. The names originate with Castle Windsor terminology, where we:

1. Register components with the container
2. Resolve root components
3. Release components from the container

Other containers also support the RRR pattern, but if we were to pick their terminology, it would rather be the Configure GetInstance Dispose (CGD) pattern (or something similar), and that's just not as catchy.

We can rewrite a previous example with Castle Windsor and annotate it with comments to call out where the three container phases occur:

private static void Main(string[] args)
{
    var container = new WindsorContainer();
    container.Kernel.Resolver.AddSubResolver(
        new CollectionResolver(container.Kernel));
 
    // Register
    container.Register(
        Component.For<IParser>()
            .ImplementedBy<WineInformationParser>(),
        Component.For<IParser>()
            .ImplementedBy<HelpParser>(),
        Component.For<IParseService>()
            .ImplementedBy<CoalescingParserSelector>(),
        Component.For<IWineRepository>()
            .ImplementedBy<SqlWineRepository>(),
        Component.For<IMessageWriter>()
            .ImplementedBy<ConsoleMessageWriter>());
 
    // Resolve
    var ps = container.Resolve<IParseService>();
    ps.Parse(args).CreateCommand().Execute();
 
    // Release
    container.Release(ps);
    container.Dispose();
}

Notice that in most cases, explicitly invoking the Release method isn't necessary, but I included it here to make the pattern stand out.

So there it is: the Register Resolve Release pattern.

Comments

One of my readers recently asked me an interesting question. It relates to my book's chapter about Interception (chapter 9) and Decorators and how they can be used for instrumentation-like purposes.

In an earlier blog post we saw how we can use Decorators to implement Cross-Cutting Concerns, but the question relates to how a set of Decorators can be used to log additional information about code execution, such as the time before and after a method is called, the name of the method and so on.

A Decorator can excellently address such a concern as well, as we will see here. Let us first define an IRegistrar interface and create an implementation like this:

public class ConsoleRegistrar : IRegistrar
{
    public void Register(Guid id, string text)
    {
        var now = DateTimeOffset.Now;
        Console.WriteLine("{0}\t{1:s}.{2}\t{3}",
            id, now, now.Millisecond, text);
    }
}

Although this implementation ‘logs' to the Console, I'm sure you can imagine other implementations. The point is that given this interface, we can add all sorts of ambient information such as the thread ID, the name of the current principal, the current culture and whatnot, while the text string variable still gives us an option to log more information. If we want a more detailed API, we can just make it more detailed - after all, the IRegistrar interface is just an example.

We now know how to register events, but are seemingly no nearer to instrumenting an application. How do we do that? Let us see how we can instrument the OrderProcessor class that I have described several times in past posts.

At the place I left off, the OrderProcessor class uses Constructor Injection all the way down. Although I would normally prefer using a DI Container to auto-wire it, here's a manual composition using Pure DI just to remind you of the general structure of the class and its dependencies:

var sut = new OrderProcessor(
    new OrderValidator(), 
    new OrderShipper(),
    new OrderCollector(
        new AccountsReceivable(),
        new RateExchange(),
        new UserContext()));

All the dependencies injected into the OrderProcessor instance implement interfaces on which OrderProcessor relies. This means that we can decorate each concrete dependency with an implementation that instruments it.

Here's an example that instruments the IOrderProcessor interface itself:

public class InstrumentedOrderProcessor : IOrderProcessor
{
    private readonly IOrderProcessor orderProcessor;
    private readonly IRegistrar registrar;
 
    public InstrumentedOrderProcessor(
        IOrderProcessor processor,
        IRegistrar registrar)
    {
        if (processor == null)
        {
            throw new ArgumentNullException("processor");
        }
        if (registrar == null)
        {
            throw new ArgumentNullException("registrar");
        }
 
        this.orderProcessor = processor;
        this.registrar = registrar;
    }
 
    #region IOrderProcessor Members
 
    public SuccessResult Process(Order order)
    {
        var correlationId = Guid.NewGuid();
        this.registrar.Register(correlationId,
            string.Format("Process begins ({0})",
                this.orderProcessor.GetType().Name));
 
        var result = this.orderProcessor.Process(order);
 
        this.registrar.Register(correlationId,
            string.Format("Process ends   ({0})", 
            this.orderProcessor.GetType().Name));
 
        return result;
    }
 
    #endregion
}

That looks like quite a mouthful, but it's really quite simple - the cyclomatic complexity of the Process method is as low as it can be: 1. We really just register the Process method call before and after invoking the decorated IOrderProcessor.

Without changing anything else than the composition itself, we can now instrument the IOrderProcessor interface:

var registrar = new ConsoleRegistrar();
var sut = new InstrumentedOrderProcessor(
    new OrderProcessor(
        new OrderValidator(),
        new OrderShipper(),
        new OrderCollector(
            new AccountsReceivable(),
            new RateExchange(),
            new UserContext())),
    registrar);

However, imagine implementing an InstrumentedXyz for every IXyz and compose the application with them. It's possible, but it's going to get old really fast - not to mention that it massively violates the DRY principle.

Fortunately we can solve this issue with any DI Container that supports dynamic interception. Castle Windsor does, so let's see how that could work.

Instead of implementing the same code ‘template' over and over again to instrument an interface, we can do it once and for all with an interceptor. Imagine that we delete the InstrumentedOrderProcessor; instead, we create this:

public class InstrumentingInterceptor : IInterceptor
{
    private readonly IRegistrar registrar;
 
    public InstrumentingInterceptor(IRegistrar registrar)
    {
        if (registrar == null)
        {
            throw new ArgumentNullException("registrar");
        }
 
        this.registrar = registrar;
    }
 
    #region IInterceptor Members
 
    public void Intercept(IInvocation invocation)
    {
        var correlationId = Guid.NewGuid();
        this.registrar.Register(correlationId, 
            string.Format("{0} begins ({1})", 
                invocation.Method.Name,
                invocation.TargetType.Name));
 
        invocation.Proceed();
 
        this.registrar.Register(correlationId,
            string.Format("{0} ends   ({1})", 
                invocation.Method.Name, 
                invocation.TargetType.Name));
    }
 
    #endregion
}

If you compare this to the Process method of InstrumentedOrderProcessor (that we don't need anymore), you should be able to see that they are very similar. In this version, we just use the invocation argument to retrieve information about the decorated method.

We can now add InstrumentingInterceptor to a WindsorContainer and enable it for all appropriate components. When we do that and invoke the Process method on the resolved IOrderProcessor, we get a result like this:

bbb9724e-0fad-4b06-9bb0-b8c1c460cded    2010-09-20T21:01:16.744    Process begins (OrderProcessor)
43349d42-a463-463b-8ddf-e569e3170c97    2010-09-20T21:01:16.745    Validate begins (TrueOrderValidator)
43349d42-a463-463b-8ddf-e569e3170c97    2010-09-20T21:01:16.745    Validate ends   (TrueOrderValidator)
44fdccc8-f12d-4057-ae03-791225686504    2010-09-20T21:01:16.746    Collect begins (OrderCollector)
8bbb1a0c-6134-4652-a4af-cd8c0c7184a0    2010-09-20T21:01:16.746    GetCurrentUser begins (UserContext)
8bbb1a0c-6134-4652-a4af-cd8c0c7184a0    2010-09-20T21:01:16.747    GetCurrentUser ends   (UserContext)
d54359ff-8c32-487f-8728-b19ff0bf4942    2010-09-20T21:01:16.747    GetCurrentUser begins (UserContext)
d54359ff-8c32-487f-8728-b19ff0bf4942    2010-09-20T21:01:16.747    GetCurrentUser ends   (UserContext)
c54c4506-23a8-4553-ba9a-066fc64252d2    2010-09-20T21:01:16.748    GetSelectedCurrency begins (UserContext)
c54c4506-23a8-4553-ba9a-066fc64252d2    2010-09-20T21:01:16.748    GetSelectedCurrency ends   (UserContext)
b3dba76b-6b4e-44fa-aca5-52b2d8509db3    2010-09-20T21:01:16.750    Convert begins (RateExchange)
b3dba76b-6b4e-44fa-aca5-52b2d8509db3    2010-09-20T21:01:16.751    Convert ends   (RateExchange)
e07765bd-fe07-4486-96f1-f74d77241343    2010-09-20T21:01:16.751    Collect begins (AccountsReceivable)
e07765bd-fe07-4486-96f1-f74d77241343    2010-09-20T21:01:16.752    Collect ends   (AccountsReceivable)
44fdccc8-f12d-4057-ae03-791225686504    2010-09-20T21:01:16.752    Collect ends   (OrderCollector)
231055d3-4ebb-425d-8d69-fb9c85d9a860    2010-09-20T21:01:16.752    Ship begins (OrderShipper)
231055d3-4ebb-425d-8d69-fb9c85d9a860    2010-09-20T21:01:16.753    Ship ends   (OrderShipper)
bbb9724e-0fad-4b06-9bb0-b8c1c460cded    2010-09-20T21:01:16.753    Process ends   (OrderProcessor)

Notice how we care easily see where and when method calls begin and end using the descriptive text as well as the correlation id. I will leave it as an exercise for the reader to come up with an API that provides better parsing options etc.

As a final note it's worth pointing out that this way of instrumenting an application (or part of it) can be done following the Open/Closed Principle. I never changed the original implementation of any of the components.

As some of my readers may already know, my (previous) employer Safewhere went out of business in August, so I started looking for something new to do. I believe that I have now found it.

October 1st I'll start in my new role as Technical Lead at Commentor, where it will be my responsibility to establish us as the leading Windows Azure center of expertise in Denmark (and beyond?). That's quite a big mouthful for me, but also something that I'm very excited about.

This means that besides working on real Windows Azure projects with Danish customers, I also anticipate doing a lot of blogging, speaking and writing about Azure in the future.

What does this mean for my many other endeavors, like my book, AutoFixture or just blogging and speaking about TDD and DI in general? These things are something that I've always been doing mainly in my spare time, and I intend to keep doing that. Perhaps there are even better opportunities for synergy in my new line of work, but only time will tell.

I'm really thrilled to be given the opportunity to expand in a slightly different direction. It'll be hard, but I'm sure it's also going to be a blast!

Comments

There still seems to be some confusion about what is Dependency Injection (DI) and what is a DI Container, so in this post I will try to sort it out as explicitly as possible.

DI is a set of principles and patterns that enable loose coupling.

That's it; nothing else. Remember that old quote from p. 18 of Design Patterns?

Program to an interface; not an implementation.

This is the concern that DI addresses. The most useful DI pattern is Constructor Injection where we inject dependencies into consumers via their constructors. No container is required to do this.

The easiest way to build a DI-friendly application is to just use Constructor Injection all the way. Conversely, an application does not automatically become loosely coupled when we use a DI Container. Every time application code queries a container we have an instance of the Service Locator anti-pattern. The corollary leads to this variation of the Hollywood Principle:

Don't call the container; it'll call you.

A DI Container is a fantastic tool. It's like a (motorized) mixer: you can whip cream by hand, but it's easier with a mixer. On the other hand, without the cream the mixer is nothing. The same is true for a DI Container: to really be valuable, your code must employ Constructor Injection so that the container can auto-wire dependencies.

A well-designed application adheres to the Hollywood Principle for DI Containers: it doesn't call the container. On the other hand, we can use the container to compose the application - or we can do it the hard way; this is called Poor Man's DI. Here's an example that uses Poor Man's DI to compose a complete application graph in a console application:

private static void Main(string[] args)
{
    var msgWriter = new ConsoleMessageWriter();
    new CoalescingParserSelector(
        new IParser[]
        {
            new HelpParser(msgWriter),
            new WineInformationParser(
                new SqlWineRepository(),
                msgWriter)
        })
        .Parse(args)
        .CreateCommand()
        .Execute();
}

Notice how the nested structure of all the dependencies gives you an almost visual idea about the graph. What we have here is Constructor Injection all the way in.

CoalescingParserSelector's constructor takes an IEnumerable<IParser> as input. Both HelpParser and WineInformationParser requires an IMessageWriter, and WineInformationParser also an IWineRepository. We even pull in types from different assemblies because SqlWineRepository is defined in the SQL Server-based data access assembly.

Another thing to notice is that the msgWriter variable is shared among two consumers. This is what a DI Container normally addresses with its ability to manage component lifetime. Although there's not a DI Container in sight, we could certainly benefit from one. Let's try to wire up the same graph using Unity (just for kicks):

private static void Main(string[] args)
{
    var container = new UnityContainer();
    container.RegisterType<IParser, WineInformationParser>("parser.info");
    container.RegisterType<IParser, HelpParser>("parser.help");
    container.RegisterType<IEnumerable<IParser>, IParser[]>();
 
    container.RegisterType<IParseService, CoalescingParserSelector>();
 
    container.RegisterType<IWineRepository, SqlWineRepository>();
    container.RegisterType<IMessageWriter, ConsoleMessageWriter>(
        new ContainerControlledLifetimeManager());
 
    container.Resolve<IParseService>()
        .Parse(args)
        .CreateCommand()
        .Execute();
    container.Dispose();
}

We are using Constructor Injection throughout, and most DI Containers (even Unity, but not MEF) natively understands that pattern. Consequently, this means that we can mostly just map interfaces to concrete types and the container will figure out the rest for us.

Notice that I'm using the Configure-Resolve-Release pattern described by Krzysztof Koźmic. First I configure the container, then I resolve the entire object graph, and lastly I dispose the container.

The main part of the application's execution time will be spent within the Execute method, which is where all the real application code runs.

In this example I wire up a console application, but it just as well might be any other type of application. In a web application we just do a resolve per web request instead.

But wait! does that mean that we have to resolve the entire object graph of the application, even if we have dependencies that cannot be resolved at run-time? No, but that does not mean that you should pull from the container. Pull from an Abstract Factory instead.

Another question that is likely to arise is: what if I have dependencies that I rarely use? Must I wire these prematurely, even if they are expensive? No, you don't have to do that either.

In conclusion: there is never any reason to query the container. Use a container to compose your object graph, but don't rely on it by querying from it. Constructor Injection all the way enables most containers to auto-wire your application, and an Abstract Factory can be a dependency too.

Comments

One of my Twitter followers who appears to be using AutoFixture recently asked me this:

So with the AutoMoqCustomization I feel like I should get Mocks with concrete types too (except the sut) - why am I wrong?

AutoFixture's extention for auto-mocking with Moq was never meant as a general modification of behavior. Customizations extend the behavior of AutoFixture; they don't change it. There's a subtle difference. In any case, the auto-mocking customization was always meant as a fallback mechanism that would create Mocks for interfaces and abstract types because AutoFixture doesn't know how to deal with those.

Apparently @ZeroBugBounce also want concrete classes to be issued as Mock instances, which is not quite the same thing; AutoFixture already has a strategy for that (it's called ConstructorInvoker).

Nevertheless I decided to spike a little on this to see if I could get it working. It turns out I needed to open some of the auto-mocking classes a bit for extensibility (always a good thing), so the following doesn't work with AutoFixture 2.0 beta 1, but will probably work with the RTW. Also please not that I'm reporting on a spike; I haven't thoroughly tested all edge cases.

That said, the first thing we need to do is to remove AutoFixture's default ConstructorInvoker that invokes the constructor of concrete classes. This is possible with this constructor overload:

public Fixture(DefaultRelays engineParts)

This takes as input a DefaultRelays instance, which is more or less just an IEnumerable<ISpecimenBuilder> (the basic building block of AutoFixture). We need to replace that with a filter that removes the ConstructorInvoker. Here's a derived class that does that:

public class FilteringRelays : DefaultEngineParts
{
    private readonly Func<ISpecimenBuilder, bool> spec;
 
    public FilteringRelays(Func<ISpecimenBuilder, bool> specification)
    {
        if (specification == null)
        {
            throw new ArgumentNullException("specification");
        }
 
        this.spec = specification;
    }
 
    public override IEnumerator<ISpecimenBuilder> GetEnumerator()
    {
        var enumerator = base.GetEnumerator();
        while (enumerator.MoveNext())
        {
            if (this.spec(enumerator.Current))
            {
                yield return enumerator.Current;
            }
        }
    }
}

DefaultEngineParts already derive from DefaultRelays, so this enables us to use the overloaded constructor to remove the ConstructorInvoker by using these filtered relays:

Func<ISpecimenBuilder, bool> concreteFilter
    = sb => !(sb is ConstructorInvoker);
var relays = new FilteringRelays(concreteFilter);

The second thing we need to do is to tell the AutoMoqCustomization that it should Mock all types, not just interfaces and abstract classes. With the new (not in beta 1) overload of the constructor, we can now supply a Specification that determines which types should be mocked.:

Func<Type, bool> mockSpec = t => true;

We can now create the Fixture like this to get auto-mocking of all types:

var fixture = new Fixture(relays).Customize(
    new AutoMoqCustomization(new MockRelay(mockSpec)));

With this Fixture instance, we can now create concrete classes that are mocked. Here's the full test that proves it:

[Fact]
public void CreateAnonymousMockOfConcreteType()
{
    // Fixture setup
    Func<ISpecimenBuilder, bool> concreteFilter
        = sb => !(sb is ConstructorInvoker);
    var relays = new FilteringRelays(concreteFilter);
 
    Func<Type, bool> mockSpec = t => true;
 
    var fixture = new Fixture(relays).Customize(
        new AutoMoqCustomization(new MockRelay(mockSpec)));
    // Exercise system
    var foo = fixture.CreateAnonymous<Foo>();
    foo.DoIt();
    // Verify outcome
    var fooTD = Mock.Get(foo);
    fooTD.Verify(f => f.DoIt());
    // Teardown
}

Foo is this concrete class:

public class Foo
{
    public virtual void DoIt()
    {
    }
}

Finally, a word of caution: this is a spike. It's not fully tested and is bound to fail in certain cases: at least one case is when the type to be created is sealed. Since Moq can't create a Mock of a sealed type, the above code will fail in that case. However, we can address this issue with some more sophisticated filters and Specifications. However, I will leave that up to the interested reader (or a later blog post).

All in all I think this provides an excellent glimpse of the degree of extensibility that is built into AutoFixture 2.0's kernel.

The new internal architecture of AutoFixture 2.0 enables some interesting features. One of these is that it becomes easy to extend AutoFixture to become an auto-mocking container.

Since I personally use Moq, the AutoFixture 2.0 .zip file includes a new assembly called Ploeh.AutoFixture.AutoMoq that includes an auto-mocking extension that uses Moq for Test Doubles.

Please note that AutoFixture in itself has no dependency on Moq. If you don't want to use Moq, you can just ignore the Ploeh.AutoFixture.AutoMoq assembly.

Auto-mocking with AutoFixture does not have to use Moq. Although it only ships with Moq support, it is possible to write an auto-mocking extension for a different dynamic mock library.

To use it, you must first add a reference to Ploeh.AutoFixture.AutoMoq. You can now create your Fixture instance like this:

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

What this does is that it adds a fallback mechanism to the fixture. If a type falls through the normal engine without being handled, the auto-mocking extension will check whether it is a request for an interface or abstract class. If this is so, it will relay the request to a request for a Mock of the same type.

A different part of the extension handles requests for Mocks, which ensures that the Mock will be created and returned.

Splitting up auto-mocking into a relay and a creational strategy for Mock objects proper also means that we can directly request a Mock if we would like that. Even better, we can use the built-in Freeze support to freeze a Mock, and it will also automatically freeze the auto-mocked instance as well (because the relay will ask for a Mock that turns out to be frozen).

Returning to the original frozen pizza example, we can now rewrite it like this:

[Fact]
public void AddWillPipeMapCorrectly()
{
    // Fixture setup
    var fixture = new Fixture()
        .Customize(new AutoMoqCustomization());
 
    var basket = fixture.Freeze<Basket>();
    var mapMock = fixture.Freeze<Mock<IPizzaMap>>();
 
    var pizza = fixture.CreateAnonymous<PizzaPresenter>();
 
    var sut = fixture.CreateAnonymous<BasketPresenter>();
    // Exercise system
    sut.Add(pizza);
    // Verify outcome
    mapMock.Verify(m => m.Pipe(pizza, basket.Add));
    // Teardown
}

Notice that we can simply freeze Mock<IPizzaMap> which also automatically freeze the IPizzaMap instance as well. When we later create the SUT by requesting an anonymous BasketPresenter, IPizzaMap is already frozen in the fixture, so the correct instance will be injected into the SUT.

This is similar to the behavior of the custom FreezeMoq extension method I previously described, but now this feature is baked in.

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