JAPF

Introduction: In this article, I’m describing a technique which leverage the expression trees of C# 3.0 in order to facilitate the unit testing of ViewModel’s properties. My final goal is to be able to unit test a ViewModel property in 1 line.

Without any doubt MVVM is now the most used framework to leverage WPF and Silverlight functionalities in the best way ! During the last Mix, 3 sessions were dedicated to this methodology (you can watch the videos online here).

As you already know one of the key advantage of the MVVM methodology is to improve the testability of the overall application by reducing the amount of code in the code-behind and producing ViewModel classes which are testable. We use to say that ViewModel classes are testable because:

• they are not coupled to UI concepts (controls, focus, keyboard input…)
• they can wrap model objects using interfaces (for instance a PersonViewModel wraps a IPerson object)
• they are not subclassing a UI control (such as Button or Window)

Today I’d like to share a technique I’m using to facilitate the unit tests of some properties of my ViewModel classes.

Let’s use a very simple ViewModel class as example:

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public class PersonViewModel : ViewModelBase
{
  private IPerson person;
  private bool isSelected;
 
  public string Name 
  {
     get
     {
        return this.person.Name;
     }
     set
     {
        this.person.Name = value;
        this.OnPropertyChanged("Name");
     }
  }
 
  public bool IsSelected
  {
    get
    {
      return this.isSelected;
    }
    set
    {
      this.isSelected = value;
      this.OnPropertyChanged("IsSelected");
    }
  }
 
  // rest of the code omitted for simplicity
}

The Name property, as usually with the MVVM pattern gets its value from the wrapped model object. The easiest way to unit test this property is to use a mocking library. Here is a example using MOQ (my favourite mocking library):

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[Test]
public void TestName()
{
  var mockPerson = new Mock<IPerson>();
 
  var vm = new PersonViewModel(mockPerson.Object);
 
  vm.Name = "Jeremy";
 
  // verify that the Name property of the IPerson interface has been set
  mockPerson.VerifySet(p => p.Name = "Jeremy");
}

The Selected property is different because it doesn’t wrap a model property. It’s an information that is added to the ViewModel layer in order to control a UI-related property (for example the IsSelected property of a ListBoxItem). This technique is heavily used to have ViewModel classes interact with the WPF or Silverlight TreeView or ListBox control (you can check out this excellent article of Josh Smith for more detail).

In order to unit test this property, we must:
1/ ensure the PropertyChanged event of the INotifyPropertyChanged is raised properly
2/ ensure we can write a value and read back the correct value

Here is a sample code which does this unit test:

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[Test]
public void TestName()
{
var vm = new  PersonViewModel();
bool propertyChanged = false;
 
vm.PropertyChanged += (s, e) => propertyChanged = e.PropertyName ==  "Name";
vm.Name = "newName";
 
Assert.IsTrue(propertyChanged);
Assert.AreEqual("newName", vm.Name);
}

It quickly become cumbersome to copy/paste this unit test for all the ViewModel properties we have. That’s the reason I started thinking about another way to do it…

Here is the feature I’m proposing:

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[Test]
public void TestName()
{
var vm = new PersonViewModel();
TestHelper.TestProperty(vm, v => v.IsSelected);
}

In this sample, I’m telling I want to test the IsSelected property of the PersonViewModel type. The advantages are:
1/ less code involved : 1 line to test 1 property
2/ intellisense support in order to prevent typing error and no more “magic” string to give the name of the property
3/ refactoring the name of the property will refactor this sample code too
4/ automatic generation of default test values behind the scene

How does it works ?

• TestProperty treats the second parameter as an Expression<Func> and not as a Func directly
• Using expression tree (the “v => v.IsSelected” part),  I’m able to retrieve the name of the property and its type
• Using reflection, I’m able to get and set the value
• Depending on the type of the property (string, bool, int, double), I have default values write and read back (with a test to ensure that the PropertyChanged event has been raised properly).

Here is the code of the TestPropertyMethod:

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public static void TestProperty<T, U>(T viewmodel, Expression<Func<T, U>> expression)
    where T : INotifyPropertyChanged
{
    if(expression.Body is MemberExpression)
    {
        MemberExpression memberExpression = (MemberExpression) expression.Body;
 
        if (expression.Body.Type == typeof(bool))
        {
            TestViewModelProperty(viewmodel, memberExpression.Member.Name, true, false);
        }
        else if (expression.Body.Type == typeof(string))
        {
            TestViewModelProperty(viewmodel, memberExpression.Member.Name, "value1", "value2");
        } 
        else if (expression.Body.Type == typeof(int))
        {
            TestViewModelProperty(viewmodel, memberExpression.Member.Name, 1, 99);
        }
        else if (expression.Body.Type == typeof(double))
        {
            TestViewModelProperty(viewmodel, memberExpression.Member.Name, 1.0, 99.0);
        }
        else
        {
            throw new NotSupportedException("Type is not supported");
        }
   }
}

And the TestViewModelProperty:

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private static void TestViewModelProperty<T, U>(T viewModel, string propertyName, U value1, U value2)
    where T : INotifyPropertyChanged
{
    bool propertyChanged;
    viewModel.PropertyChanged += (s, e) => propertyChanged = e.PropertyName == propertyName;
 
    propertyChanged = false;
    viewModel.SetValue(propertyName, value1);
    Assert.IsTrue(propertyChanged);
    Assert.IsTrue(viewModel.GetValue<U>(propertyName).Equals(value1));
 
    propertyChanged = false;
    viewModel.SetValue(propertyName, value2);
    Assert.IsTrue(propertyChanged);
    Assert.IsTrue(viewModel.GetValue<U>(propertyName).Equals(value2));
}

I’m using 2 extensions methods in order to get and set value from the ViewModel object using reflection. Here they are:

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private static T GetValue<T>(this object obj, string propertyName)
{
    var propertyInfo = obj.GetType().GetProperty(propertyName);
    return (T)propertyInfo.GetValue(obj, null);
}
 
private static void SetValue<T>(this object obj, string propertyName, T value)
{
    var propertyInfo = obj.GetType().GetProperty(propertyName);
    propertyInfo.SetValue(obj, value, null);
}

Please feel free to download the source code of the ViewModelTestHelper class.

Mix10 is starting today and we can expect many cool announcements during the keynote. You can watch the keynote online at live.visitmix.com. I’ll try to give feedback as soon as possible. Because I’m not lucky enough to be in Vegas, I’ll watch the keynote tonight (French time !) at home with some coworkers.

Last information before the keynote, it looks like we’ll have some announcements about Silverlight running on Symbian devices…

A couple of weeks ago, I started a series of articles about WPF internals organization. In the first article I did a tour of the core WPF classes. In this second part, I’m reviewing the organization of the various controls that exist in the framework.

Because the image of the diagram is pretty big, I decided to use Silverlight DeepZoom and the result is just below this text You can download the full image resolution here. Please use the full screen button in the upper right corner of the viewer for the best browsing experience.

Here are general remarks that might help you get information from those diagrams.

The top level Control class:

• Defines general UI properties such as Background, Foreground, BorderBrush and BorderThickness
• Defines a set of properties to control font rendering: FontFamily, FontSize, FontStyle…
• Has a DoubleClick event (other mouse events such as MouseUp/MouseDown comes from the UIElement class)

Below the Control class, we have (this list is not complete):

• The ContentControl class: those controls have… a single Content
• The ItemsControls class which handles a collection of child controls
• The TextBoxBase class which is the parent of TextBox and RichTextBox
• The RangerBase class which handles a range (minimum, maximum) and which is the parent of the ScrollBar, Slider and ProgressBar classes
• The Thumb class which has the ability to be dragged over a surface

Below the ContentControl class we can find many existing WPF controls:

• ButtonBase and its child: Button, ToggleButton and RepeatButton
• Label (see this post for an explanation of the differences between a Label and a TextBlock)
• ListBoxItem and its children: ListViewItem, ComboBoxItem
• ScrollViewer
• HeaderedContentControl, a ContentControl that has both a Content and a Header. This is the case of the Expander, GroupBox and TabItem controls
• Tooltip
• Window
• UserControl

General other remarks:

• It’s funny to see that both Window and UserControl inherits from ContentControl. Before doing the diagram I though that Window came from somewhere else
• Having those diagrams in mind (or on a screen !) is very useful when you need to create your own custom control
• We can see the differences between creating a custom control (inherit from Control or derived class) and a UserControl (inherit from UserControl)
• .Net4 will introduce new controls (not in this diagram) in the WPF framework such: DataGrid, Calendar, TimePicker

A couple of weeks ago, I wrote a blog post where I compared the existing MVVM frameworks. This post became a bit famous in the WPF/Silverlight blog world and I received a lot of feedback to update the list, fix information, etc. I also got a request from Erik suggesting me to put all the datas in a matrix.

Today I’m proud to announce the MVVM frameworks Silverlight application (click the image to open the Silverlight3 page).

A couple of observation:

• please contact me via this blog or twitter if you find incorrect information
• I’m not judging anybody’s work by giving rating, it’s just my personal feeling to have an easiest way to sort the data

Hope you’ll like it

In my last blog post, I did a review of a PDC09 Session “Advanced performance tuning with WPF”. Today, I’m doing a review for another very interesting session “How VS2010 was built with WPF ?”. The video is available here.

Why did Microsoft choose WPF for VS2010 ?

• Technological: prove the capabilities of WPF4
• Architectural
  • Separated presentation
  • Support for long range road map (+10 years)
• Key VS2010 features need it: Editor, Architecture Editor, Parallel tools debugging
• Take an opportunity to give feedback for WPF4: when VS2010 development started, .Net 3.5 SP1 was just shipped and it was the right time to give feedback for the new features and fixes for WPF4.

It is worth noting that VS2010 is a project that is being watched carefully inside Microsoft. We can expect that more Microsoft applications will move to WPF in the next coming years.

What WPF features are used ?

• Declarative UI using XAML
• Databinding
• Styles and templates
• Application Resources
• Interop Win32 (because they did not have the time to rewrite everything using WPF or because features doest not need to use WPF for example de WinForms designer…)
• Integration with the message loop (to deal with particular focus issues)
• New text stack (part of WPF4) based on DirectWrite

Staged approach

• Define data models: a huge diagramming and architectural exercice
• Replace the main window with WPF (only the window not its content !) to start the mix approach (managed/unmanaged, WPF/Hwnds)
• Write new managed components: Window manager, command bar presentation
• Scout with other VS teams
• Test, Test, Test…

What were the challenges ?

• Mixed mode application: native and managed code; WPF and HWNDs (Win32 or WinForms)
• Keep existing extensions working and allow new extensions to take advantage of WPF
• Don’t “stop the train” other teams were working at the same time on the product to add new functionalities
• Text clarity
• Performance
• Focus management

You can watch the session here if you want more detail and demos of what I mention in this post.

Update 18th january 2011: as Cory pointed out, the latest version of Snoop is now available on CodePlex.

If like me you’re curious about how big WPF applications are made, you probably tried to use Snoop to inspect the visual tree of Visual Studio 2010 Beta2. And you probably had a problem because Snoop wasn’t able to detect the devenv process of VS2010:

I wasn’t sure about the cause of this problem, but I grab the source of Snoop and took a look at the code that list the available window:

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public void Refresh() {
	this.windows.Clear();
 
	foreach (IntPtr windowHandle in NativeMethods.ToplevelWindows) {
		WindowInfo window = new WindowInfo(windowHandle, this);
		if (window.IsValidProcess && !this.HasProcess(window.OwningProcess))
			this.windows.Add(window);
	}
 
	if (this.windows.Count > 0)
		this.windowsView.MoveCurrentTo(this.windows[0]);
}

I setup a breakpoint to find out what was wrong and discover that the window.IsValidProcess failed with the VS2010 process. Because I just wanted to check if it was the only reason why Snoop wasn’t able to inspect it, I hack the code to remove this test:

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public void Refresh() {
	this.windows.Clear();
 
	foreach (IntPtr windowHandle in NativeMethods.ToplevelWindows) {
		WindowInfo window = new WindowInfo(windowHandle, this);
		if (/*window.IsValidProcess &&*/ !this.HasProcess(window.OwningProcess))
			this.windows.Add(window);
	}
 
	if (this.windows.Count > 0)
		this.windowsView.MoveCurrentTo(this.windows[0]);
}

And it’s working:

Happy exploration with Snoop

In case you don’t have the info yet, VS2010 Beta2 is coming today October 19th for MSDN subscribers. It will be released to public on Wednesday (source).

Also, Visual Studio 2010 RTM will officially launch on March 22, 2010

I’ll try to give some feedback as soon as I’ll be able to grab my copy and install it on ma workstation !

In this first article, I’d like to make a tour of the core WPF classes and how they are related.

Knowing the organization of WPF classes is important when we create a new control because we have to determine which base class we’re going to use. It is also interesting to know how the framework has been designed. That’s the goal of this first article.

Here is an image of the core WPF classes and how they are related (click for larger resolution):

The diagram is voluntary simple above the Control classes because that will be targeted by another article.

As you might already know, even if the .Net framework has a garbage collector, you can easily create memory leaks in your application.

The most common way to create a leak is to register to an event handler on a object that has a longer lifetime than the object where the handler is defined. The problem can also occurs by using static class such as EventManager (for more information see this blog post). Some .Net developers have been working on a way to go round the problem using Reflection, Weak Reference and other cool stuff. You can check out for example this excellent article on Code Project.

However, if you cannot change the way your declare events (because of internal policies in the company or because you don’t have the source code), you must be very carefull about the way you manage your events.

I’ve been working lately on a Resharper plugin that helps detecting events that are never unsubscribed. Basically, what is does is the following:

I’d like to have feedback from you .Net developpers, about whether you find such a plugin useful or not.

• How do you deal with the event memory leak problem ?
• Would you like to use my plugin ?
• Would you like me to release it on a open source platform ?
• What other kind of possible leaks are you thinking about to enhance the plugin ?

Please write a comment to let my know what you think. Thank you for your feedback !

I’m starting a new series of article I’m calling “WPF internals” where I’ll be reviewing how things works <inside> WPF. I haven’t the entire content yet, but here is a summary of what I’m planning to do:

• Part 1 of N: what are the core WPF classes ?
• Part 2 of N: how the WPF controls are organised ?
• Part 3 of N: how ZIndex works ?
• Part 4 of N: how InputGestures works ?
• Part 5 of N: how databinding with DataContext works ?
• Part 6 of N: how databinding with an ItemsControl works ?
• Part 7 of N: what is the difference between a ContentControl and a ContentPresenter ?

I setup a new page in the header so that links are easy to access.

All the information that I’m going to give here is based on my PERSONAL understanding using tools like Reflector and VS Class Diagram Designer that help me figure out how things fit together. I’m neither a Microsoft WPF Architect nor a Program Manager on the .Net framework !

Please let me know if you’re interested in special topics, I’ll do my best to try to include them in the series. Please also feel free to leave comments on the articles so that if I’m misunderstanding something I can fix it.

I hope to post the first article this week (before October, 16th)