Since AutoFixture is a Test Data Builder, one of its most important tasks is to build up graphs of fully populated, yet semantically correct, strongly typed objects. As such, its default behavior is to assign a value to every writable property in the object graph.

While this is sometimes the desired behavior, at other times it is not.

This is particularly the case when you want to test that a newly created object has a property of a particular value. When you want to test the default value of a writable property, AutoFixture's AutoProperty feature is very much in the way.

Let's consider as an example a piece of software that deals with vehicle registration. By default, a vehicle should have four wheels, since this is the most common occurrence. Although I always practice TDD, I'll start by showing you the Vehicle class to illustrate what I mean.

public class Vehicle

{

    public Vehicle()

    {

        this.Wheels = 4;

    }

    public int Wheels { get; set; }

}

Here's a test that ensures that the default number of wheels is 4 – or does it?

In fact the assertion fails because the actual value is 1, not 4.

[TestMethod]

public void AnonymousVehicleHasWheelsAssignedByFixture()

{

    // Fixture setup

    var fixture = new Fixture();

    var sut = fixture.CreateAnonymous<Vehicle>();

    // Exercise system

    var result = sut.Wheels;

    // Verify outcome

    Assert.AreEqual<int>(4, result, "Wheels");

    // Teardown

}

Why does the test fail when the value of Wheels is set to 4 in the constructor? It fails because AutoFixture is designed to create populated test data, so it assigns a value to every writable property. Wheels is a writable property, so AutoFixture assigns an integer value to it using its default algorithm for creating anonymous numbers. Since no other numbers are being created during this test, the number assigned to Wheels is 1. This is AutoFixture's AutoProperties feature in effect.

When you want to test constructor logic, or otherwise wish to disable the AutoProperties feature, you can use a customized Builder with the OmitAutoProperties method:

[TestMethod]

public void VehicleWithoutAutoPropertiesWillHaveFourWheels()

{

    // Fixture setup

    var fixture = new Fixture();

    var sut = fixture.Build<Vehicle>()

        .OmitAutoProperties()

        .CreateAnonymous();

    // Exercise system

    var result = sut.Wheels;

    // Verify outcome

    Assert.AreEqual<int>(4, result, "Wheels");

    // Teardown

}

The OmitAutoProperties method instructs AutoFixture to skip assigning automatic Anonymous Values to all writable properties in the object graph. Any properties specifically assigned by the With method will still be assigned.

The test using OmitAutoProperties succeeds.

The expressiveness of WPF is amazing. I particularly like the databinding and templating features. The ability to selectively render an object based on its type is very strong.

When I recently began working with ASP.NET MVC (which I like so far), I quickly ran into a scenario where I would have liked to have WPF's DataTemplates at my disposal. Maybe it's just because I've become used to WPF, but I missed the feature and set out to find out if something similar is possible in ASP.NET MVC.

Before we dive into that, I'd like to present the 'problem' in WPF terms, but the underlying View Model that I want to expose will be shared between both solutions.

The main point is that the Items property exposes a polymorphic list. While all items in this list share a common property (Name), they are otherwise different; one contains a piece of Text, one contains a Color, and one is a complex item that contains child items.

When I render this list, I want each item to render according to its type.

In WPF, this is fairly easy to accomplish with DataTemplates:

<ListBox.Resources>

    <DataTemplate DataType="{x:Type pm:NamedTextItem}">

        <StackPanel>

            <TextBlock Text="{Binding Path=Name}"

                       FontWeight="bold" />

            <TextBlock Text="{Binding Path=Text}" />

        </StackPanel>

    </DataTemplate>

    <DataTemplate DataType="{x:Type pm:NamedColorItem}">

        <StackPanel>

            <TextBlock Text="{Binding Path=Name}"

                       FontWeight="bold" />

            <Ellipse Height="25" Width="25"

                     HorizontalAlignment="Left"

                     Fill="{Binding Path=Brush}"

                     Stroke="DarkGray" />

        </StackPanel>

    </DataTemplate>

    <DataTemplate DataType="{x:Type pm:NamedComplexItem}">

        <StackPanel>

            <TextBlock Text="{Binding Path=Name}"

                       FontWeight="bold" />

            <ListBox ItemsSource="{Binding Path=Children}"

                     BorderThickness="0">

                <ListBox.Resources>

                    <DataTemplate

                        DataType="{x:Type pm:ChildItem}">

                        <TextBlock

                            Text="{Binding Path=Text}" />

                    </DataTemplate>

                </ListBox.Resources>

            </ListBox>

        </StackPanel>

    </DataTemplate>

</ListBox.Resources>

Each DataTemplate is contained within a ListBox. When the ListBox binds to each item in the Items list, it automatically picks the correct template for the item.

The result is something like this:

The NamedTextItem is rendered as a box containing the Name and the Text on two separate lines; the NamedColorItem is rendered as a box containing the Name and a circle filled with the Color defined by the item; and the NamedComplexItem is rendered as a box with the Name and each child of the Children list.

This is all implemented declaratively without a single line of imperative UI code.

Is it possible to do the same in ASP.NET MVC?

To my knowledge (but please correct me if I'm wrong), ASP.NET MVC has no explicit concept of a DataTemplate, so we will have to mimic it. The following describes the best I've been able to come up with so far.

In ASP.NET MVC, there's no declarative databinding, so we will need to loop through the list of items. My View page derives from ViewPage<MyViewModel>, so I can write

<% foreach (var item in this.Model.Items)

   { %>

   <div class="ploeh">

   <% // Render each item %>

   </div>

<% } %>

The challenge is to figure out how to render each item according to its own template.

To define the templates, I create a UserControl for each item. The NamedTextItemUserControl derives from ViewUserControl<NamedTextItem>, which gives me a strongly typed Model:

<div><strong><%= this.Model.Name %></strong></div>

<div><%= this.Model.Text %></div>

The other two UserControls are implemented similarly.

A UserControl can be rendered using the RenderPartial extension method, so the only thing left is to select the correct UserControl name for each item. It would be nice to be able to do this in markup, like WPF, but I'm not aware of any way that is possible.

I will have to resort to code, but we can at least strive for code that is as declarative in style as possible.

First, I need to define the map from type to UserControl:

<%

    var dataTemplates = new Dictionary<Type, string>();

    dataTemplates[typeof(NamedTextItem)] =

        "NamedTextItemUserControl";

    dataTemplates[typeof(NamedColorItem)] =

        "NamedColorItemUserControl";

    dataTemplates[typeof(NamedComplexItem)] =

        "NamedComplexItemUserControl";

%>

Next, I can use this map to render each item correctly:

<% foreach (var item in this.Model.Items)

   { %>

   <div class="ploeh">

   <% // Render each item %>

   <% this.Html.RenderPartial(dataTemplates[item.GetType()],

                            item); %>

   </div>

<% } %>

This is definitely less pretty than with WPF, but if you overlook the aesthetics and focus on the structure of the code, it's darn close to markup. The Cyclomatic Complexity of the page is only 2, and that's even because of the foreach statement that we need in any case.

The resulting page looks like this:

My HTML skills aren't good enough to draw circles with markup, so I had to replace them with blocks, but apart from that, the result is pretty much the same.

A potential improvement on this technique could be to embed the knowledge of the UserControl into each item. ASP.NET MVC Controllers already know of Views in an abstract sense, so letting the View Model know about a UserControl (identified as a string) may be conceptually sound.

The advantage would be that we could get rid of the Dictionary in the ViewPage and instead let the item itself tell us the name of the UserControl that should be used to render it.

The disadvantage would be that we lose some flexibility. It would then require a recompilation of the application if we wanted to render an item using a different UserControl.

The technique outlined here represents an explorative work in progress, so comments are welcome.

A couple of days ago I released AutoFixture .8.4. It contains a few small feature additions and a bug fix. You can get it at the AutoFixture CodePlex site as usual.

You may have noticed that I'm frequently releasing incremental versions these days. This is because we have begun using AutoFixture for writing production software at Safewhere. So far, it has been a pleasure to use AutoFixture in my daily work, and watch colleagues pick it up and run with it.

Such intensive usage obviously uncovers missing features as well as a few bugs, which is the driving force behind the frequent releases. I've been happy to observe that so far, there have been only a few bugs, and the general API is very expressive and useful.

After we ship the first product written with AutoFixture, I plan to upgrade it to version .9 (beta). That should hopefully happen in the autumn of 2009.

Some time ago, my good friend Martin Gildenpfennig from Ative made me aware that AutoFixture (among other things) is an generic implementation of the Test Data Builder pattern, and indeed it is.

In the original Gang of Four definition of a Design Pattern, several people must independently have arrived at the same general solution to a given problem before we can call a coding idiom a true Pattern. In this spirit, the Test Data Builder pattern is on the verge of becoming a true Design Pattern, since I came up with AutoFixture without having heard about Test Data Builder :)

The problem is the same: How do we create semantically correct test objects in a manner that is flexible and yet hides away irrelevant complexity?

Like others before me, I tried the Object Mother (anti?)pattern and found it lacking. To me the answer was AutoFixture, a library that heuristically builds object graphs using Reflection and built-in rules for specific types.

Although my original approach was different, you can certainly use AutoFixture as a generic Test Data Builder.

To demonstrate this, let's work with this (oversimplified) Order object model:

Assuming that we have an instance of the Fixture class (called fixture), we can create a new instance of the Order class with a ShippingAddress in Denmark:

var order = fixture.Build<Order>()

    .With(o => o.ShippingAddress, 

        fixture.Build<Address>()

        .With(a => a.Country, "Denmark")

        .CreateAnonymous())

    .CreateAnonymous();

While this works and follows the Test Data Builder pattern, I find this more concise and readable:

var order = fixture.CreateAnonymous<Order>();

order.ShippingAddress.Country = "Denmark";

The result is the same.

We can also add anonymous order lines to the order using this fluent interface:

var order = fixture.Build<Order>()

    .Do(o => fixture.AddManyTo(o.OrderLines))

    .CreateAnonymous();

but again, I find it easier to simply let AutoFixture create a fully anonymous Order instance, and then afterwards modify the relevant parts:

var order = fixture.CreateAnonymous<Order>();

fixture.AddManyTo(order.OrderLines);

Whether you prefer one style over the other, AutoFixture supports them both.