ploeh blog danish software design

Soon after I posted my post on the AutoFixture Custom Builder's Do method, a much better example occurred to me, so let's revisit this feature in light of a more reasonable context.

When I write WPF code, I always use the MVVM pattern. When I need to create a Master/Detail View, I usually model it so that my View Model has a list of available items, and a property that returns the currently selected item. In this way, I can bind the current Detail View to the currently selected item purely through the View Model.

Such a View Model might look like this:

public class MyViewModel
{
    private readonly List<MyClass> availableItems;
    private MyClass selectedItem;
 
    public MyViewModel()
    {
        this.availableItems = new List<MyClass>();
    }
 
    public ICollection<MyClass> AvailableItems
    {
        get { return this.availableItems; }
    }
 
    public MyClass SelectedItem
    {
        get { return this.selectedItem; }
        set 
        {
            if (!this.availableItems.Contains(value))
            {
                throw new ArgumentException("...");
            }
            this.selectedItem = value;
        }
    }
}

The main point of interest is that if you attempt to set SelectedItem to an instance that's not contained in the list of available items, an exception will be thrown. That's reasonable behavior, since we want the user to select only from the available items.

By default, AutoFixture works by assigning an Anonymous Value to all writable properties. Since these values are auto-generated, the value AutoFixture is going to assign to SelectedItem will be a new instance of MyClass, and thus not one of the available items. In other words, this will throw an exception:

var mvm = fixture.CreateAnonymous<MyViewModel>();

There are several solutions to this situation, depending on the scenario. If you need an instance with SelectedItem correctly set to a non-null value, you can use the Do method like this:

var mc = fixture.CreateAnonymous<MyClass>();
var mvm = fixture.Build<MyViewModel>()
    .Do(x => x.AvailableItems.Add(mc))
    .With(x => x.SelectedItem, mc)
    .CreateAnonymous();

This first creates an anonymous instance of MyClass, adds it to AvailableItems as part of a customized Builder pipeline and subsequently assigns it to SelectedItem.

Another option is to skip assigning only the SelectedItem property. This is a good option if you don't need that value in a particular test. You can use the Without method to do that:

var mvm = fixture.Build<MyViewModel>()
    .Without(s => s.SelectedItem)
    .CreateAnonymous();

This will assign a value to all other writable properties of MyViewModel (if it had had any), except the SelectedItem property. In this case, the value of SelectedItem will be null, since it is being ignored.

Finally you can simply choose to omit all AutoProperties using the OmitAutoProperties method:

var mvm = fixture.Build<MyViewModel>()
    .OmitAutoProperties()
    .CreateAnonymous();

In this scenario, only MyViewModel's constructor is being executed, while all writable properties are being ignored.

As you can see, AutoFixture offers great flexibility in providing specialized custom Builders that fit almost any situation.

Comments

The default behavior of AutoFixture is to create an Anonymous Variable by assigning a value to all writable properties of the created instance. This is great in many scenarios, but not so much in others. You can disable this behavior by using the OmitAutoProperties method, but sometimes, you would like most of the writable properties set, except one or two.

Consider this simple Person class:

public class Person
{
    private Person spouse;
 
    public DateTime BirthDay { get; set; }
 
    public string Name { get; set; }
 
    public Person Spouse 
    {
        get { return this.spouse; }
        set
        {
            this.spouse = value;
            if (value != null)
            {
                value.spouse = this;
            }
        }
    }
}

The main trouble with this class, seen from AutoFixture's perspective, is the circular reference exposed by the Spouse property. When AutoFixture attempts to create an anonymous instance of Person, it will create anonymous values for all writable properties, and that includes the Spouse property, so it attempts to create a new instance of the Person class and assign values to all public properties, including the Spouse property, etc.

In other words, this line of code throws a StackOverflowException:

var person = fixture.CreateAnonymous<Person>();

If you would still like to have anonymous values assigned to Name and BirthDay, you can use the Without method:

var person = fixture.Build<Person>()
    .Without(p => p.Spouse)
    .CreateAnonymous();

This will give you an anonymous instance of the Person class, but with the Spouse property still with its default value of null.

Several calls to Without can be chained if you want to skip more than one property.

If you are doing Rich UI, INotifyPropertyChanged is a pretty important interface. This is as true for WPF as it was for Windows Forms. Consisting solely of an event, it's not any harder to unit test than other events.

You can certainly write each test manually like the following.

[TestMethod]
public void ChangingMyPropertyWillRaiseNotifyEvent_Classic()
{
    // Fixture setup
    bool eventWasRaised = false;
    var sut = new MyClass();
    sut.PropertyChanged += (sender, e) =>
        {
            if (e.PropertyName == "MyProperty")
            {
                eventWasRaised = true;
            }
        };
    // Exercise system
    sut.MyProperty = "Some new value";
    // Verify outcome
    Assert.IsTrue(eventWasRaised, "Event was raised");
    // Teardown
}

Even for a one-off test, this one has a few problems. From an xUnit Test Patterns point of view, there's the issue that the test contains conditional logic, but that aside, the main problem is that if you have a lot of properties, writing all these very similar tests become old hat very soon.

To make testing INotifyPropertyChanged events easier, I created a simple fluent interface that allows me to write the same test like this:

[TestMethod]
public void ChangingMyPropertyWillRaiseNotifyEvent_Fluent()
{
    // Fixture setup
    var sut = new MyClass();
    // Exercise system and verify outcome
    sut.ShouldNotifyOn(s => s.MyProperty)
        .When(s => s.MyProperty = "Some new value");
    // Teardown
}

You simply state for which property you want to verify the event when a certain operation is invoked. This is certainly more concise and intention-revealing than the previous test.

If you have interdependent properties, you can specify than an event was raised when another property was modified.

[TestMethod]
public void ChangingMyPropertyWillRaiseNotifyForDerived()
{
    // Fixture setup
    var sut = new MyClass();
    // Exercise system and verify outcome
    sut.ShouldNotifyOn(s => s.MyDerivedProperty)
        .When(s => s.MyProperty = "Some new value");
    // Teardown
}

The When method takes any Action<T>, so you can also invoke methods, use Closures and what not.

There's also a ShouldNotNotifyOn method to verify that an event was not raised when a particular operation was invoked.

This fluent interface is implemented with an extension method on INotifyPropertyChanged, combined with a custom class that performs the verification. Here are the extension methods:

public static class NotifyPropertyChanged
{
    public static NotifyExpectation<T>
        ShouldNotifyOn<T, TProperty>(this T owner,
        Expression<Func<T, TProperty>> propertyPicker) 
        where T : INotifyPropertyChanged
    {
        return NotifyPropertyChanged.CreateExpectation(owner,
            propertyPicker, true);
    }
 
    public static NotifyExpectation<T> 
        ShouldNotNotifyOn<T, TProperty>(this T owner,
        Expression<Func<T, TProperty>> propertyPicker)
        where T : INotifyPropertyChanged
    {
        return NotifyPropertyChanged.CreateExpectation(owner,
            propertyPicker, false);
    }
 
    private static NotifyExpectation<T>
        CreateExpectation<T, TProperty>(T owner,
        Expression<Func<T, TProperty>> pickProperty,
        bool eventExpected) where T : INotifyPropertyChanged
    {
        string propertyName =
            ((MemberExpression)pickProperty.Body).Member.Name;
        return new NotifyExpectation<T>(owner,
            propertyName, eventExpected);
    }
}

And here's the NotifyExpectation class returned by both extension methods:

public class NotifyExpectation<T>
    where T : INotifyPropertyChanged
{
    private readonly T owner;
    private readonly string propertyName;
    private readonly bool eventExpected;
 
    public NotifyExpectation(T owner,
        string propertyName, bool eventExpected)
    {
        this.owner = owner;
        this.propertyName = propertyName;
        this.eventExpected = eventExpected;
    }
 
    public void When(Action<T> action)
    {
        bool eventWasRaised = false;
        this.owner.PropertyChanged += (sender, e) =>
        {
            if (e.PropertyName == this.propertyName)
            {
                eventWasRaised = true;
            }
        };
        action(this.owner);
 
        Assert.AreEqual<bool>(this.eventExpected,
            eventWasRaised,
            "PropertyChanged on {0}", this.propertyName);
    }
}

You can replace the Assertion with one that matches your test framework of choice (this one was written for MSTest).

Comments

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.

Comments

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.

About a year ago, one of my readers asked me about how to make custom tokens work over TPC in WCF. Here's the question again:

"i'm trying to implement the CustomToken over tcp. The original used the SymmetricSecurityBindingElement and the transport http, this works fine, but when i change URI's and and the transport, it gives an error saying:

"Binding 'CustomBinding' doesn't support creating any channel types. This often indicates that the BindingElements in a CustomBinding have been stacked incorrectly or in the wrong order. A Transport is required at the bottom of the stack. The recommended order for BindingElements is: TransactionFlow, ReliableSession, Security, CompositeDuplex, OneWay, StreamSecurity, MessageEncoding, Transport."

As it turns out, this seems to be a general issue with more transports than just TCP - at least, I've seen the exact same behavior for the Named Pipes transport.

When I originally received the question, it seemed that no-one knew the answer, and neither did I. Now, about a year later, I've managed to find a solution, and it's really simple.

If you build up your CustomBinding in code, all you need to do is set the TransferMode property to Streamed:

var pipeTransport = new NamedPipeTransportBindingElement();
pipeTransport.TransferMode = TransferMode.Streamed;

In this example, I'm setting the property on a Named Pipe transport, but you can do exactly the same with a TCP transport.

Although I wasn't able to find any documentation to that effect, experimentation seems to indicate that you can also set the property in a .config file (at least, it works on my computer):

<namedPipeTransport transferMode="Streamed" />

I will not claim that I fully understand this fix/workaround, or that it applies in every situation, but I hope that it might prove helpful to some of my readers some day.

It was only earlier this week that I released AutoFixture .8.2, but now I'm releasing version .8.3 - not that there was anything wrong with .8.2 (that I know of), but I had some time to implement new features, and I wanted to properly release those.

In the future I will blog about these new features (along with all the other AutoFixture features I haven't introduced yet).

Get it at the AutoFixture CodePlex site as usual.

Yesterday I created a new release (.8.2) of AutoFixture; this time with a new feature that I recently discovered that I needed, and about which I will blog later.

There are no breaking changes and no known bugs.