About me

Michael L Perry

Improving Enterprises

Principal Consultant


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Correspondence presentation from Dallas .NET User Group

The recording of my Correspondence presentation is up on UserGroup.tv. This one covers Silverlight out-of-browser applications. I used Thought Cloud as a demo, and opened it up for the audience to participate. It makes for some interesting on-screen shenanigans.

DevLink app results

DevLink 2011 was a fantastic conference. I met some well-known members of the development community. And I met some brilliant people. Occasionally, these were one-and-the-same!

I presented two sessions at DevLink. The first was on the CAP Theorem. I’ve posted the slides and demo for your perusal. The second was on Correspondence, the same session I gave at Dallas TechFest the week prior. Both sessions were well attended, though I did have problems with my live coding demo in the Correspondence session. Fortunately, the first demo had already gotten the point across.

Like I did for Dallas TechFest, I created a conference application using Correspondence. I pushed this app to the Marketplace for attendees to use. This time, I got 30 downloads out of about 650 attendees, and only one review. So the overall percentage was lower this time around, but still useful.

The one reviewer gave the app 4 out of 5 stars, with the following comment:

So far I really like it. It seems a little slow but I like the format a lot better than the eventboard program.

The EventBoard application that he references was the official app available on the Windows Phone and iPhone platforms. It’s really cool to be favorably compared with a professional app. I think the primary differentiator between the two is that, where EventBoard is all about the event, my app was all about the attendee. The first page of my app shows your schedule. The first page of EventBoard shows a list of events.

Performance enhacements

The reviewer noted that the application seems a little slow. This is because of performance problems in Correspondence. I made some improvements, but was unable to get them into the Marketplace in time. More performance enhancements are on the way.

The first enhancement was to reuse the IsolatedStorageFile object rather than recreating it. Every example of using isolated storage that I’ve seen follows the same pattern:

public class IsolatedStorageStorageStrategy : IStorageStrategy
    public static IsolatedStorageStorageStrategy Load()
        var result = new IsolatedStorageStorageStrategy();

        using (IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication())
            using (BinaryReader input = new BinaryReader(store.OpenFile(ClientGuidFileName, FileMode.Open)))
                result._clientGuid = new Guid(input.ReadBytes(16));

        return result;

Isolated storage is notoriously slow. It was easy to isolate the slowness to the creation of the IsolatedStorageFile object. So I just changed the pattern:

public class IsolatedStorageStorageStrategy : IStorageStrategy
    private IsolatedStorageFile _store;

    private IsolatedStorageStorageStrategy(IsolatedStorageFile store)
        _store = store;

    public static IsolatedStorageStorageStrategy Load()
        IsolatedStorageFile store = IsolatedStorageFile.GetUserStoreForApplication();
        var result = new IsolatedStorageStorageStrategy(store);

        using (BinaryReader input = new BinaryReader(store.OpenFile(ClientGuidFileName, FileMode.Open)))
            result._clientGuid = new Guid(input.ReadBytes(16));

        return result;

This was low-hanging fruit. Other performance enhancements that I have in the queue are:

  • Pooling the Stream objects instead of opening and closing files.
  • Switching from XML to binary serialization of facts for HTTP communications.
  • Pulling more facts at a time from the server.
  • Evaluating SQL CE as an alternate storage strategy.

The CAP Theorem and its Consequences

Here you will find the slides and code from my talk “The CAP Theorem and its Consequences”. The slides are in Silverlight, so use these controls to navigate:

  • Page down/up to go forward and back by slide.
  • Arrow down/up to go forward and back within a slide (Definitions, Proof, Choices, and Event Sourcing).
  • Scroll wheel to zoom in.
  • Drag to pan.

You can also download the source code and run the examples yourself. Run databases.sql to create the three databases. Populate the databases with test data in accounts “12345”, “23456”, and “34567”. The application transfers money among these accounts.

Get Microsoft Silverlight

For more information, please read the summary article.

Dallas TechFest 2011

I presented two sessions at Dallas TechFest this year. I also attended some excellent sessions, hung out with old friends, and made some new contacts. Thanks to Tim Rayburn, Teresa Burger, Shane Holder, and other organizers, this was a fantastic event. They’ve really set the bar high for next year.

My sessions

My first session, Provable APIs, was very well attended. This talk presents techniques that you can use to ensure that people consuming your API do so correctly. These techniques take advantage of the compiler, and the type system in particular. I present samples in C#, but the techniques work well in any statically typed language.

The session was very well attended. We had people using not only .NET, but also Java, and C++. We even had some dynamically-typed languages represented including JavaScript and Ruby. Although they don’t have a compiler and static type system to leverage, they can still use some of these techniques.

The audience was extremely interactive. They posed some excellent questions. They were particularly interested to see a refactoring of the .NET Socket class, with its notoriously error-prone API.

My second session, Correspondence: Occasionally-Connected Windows Phone Apps, was not as well attended. Those that did participate were rather quiet during the theory portion, but started to interact during the demo. Based on their feedback, I will move the demo to the beginning of the talk, and save the theory only for after I get questions about how it works.

The app

The demo for the Correspondence session was actually the Dallas TechFest 2011 Windows Phone 7 app. I got some great feedback from people who were using the app during the conference. I also saw a few bugs that others reported and that I experienced myself. I’ll be rolling out bug fixes for upcoming conferences.

All told, I had 22 downloads of the app. Out of those, I received 2 ratings with feedback, for an average of 4.5 stars. 22 downloads out of a possible set of about 200 attendees, only about 30% of which carry Windows Phones, is pretty decent reach. And to get 10% of the downloads giving feedback is excellent. Sure, the absolute numbers are too low to make any generalizations, but I’ll take what I can get.

The DevLink 2011 app is in the Marketplace now. If you are attending DevLink and you carry a Windows Phone 7, please give it a try.

Old friends and new contacts

Tim, Teresa, and Shane are all old friends, so to see them pull off such an outstanding conference was a real treat. As an added bonus, I got to see a good friend from work present in his first conference. Girish Gangadharan presented on JQuery, and packed the room. Actually, I didn’t see him present because my Correspondence session was at the same time. But I saw pictures, so I know it was a full house.

I also got to meet some new folks. Devlin Liles from Improving Enterprises and Jay Smith from Tyson Foods are active members of the open source community. Their tools help out user groups and event organizers across the country and the technology spectrum. I look forward to working with them on community-centered open source projects.

I had a great time at Dallas TechFest this year. I’m looking forward to following up on all the contacts I’ve made, to applying all the techniques that I’ve learned, and most importantly to participating next year.

Now on to DevLink.


Correspondence is a collaboration framework for occasionally-connected clients. Express your model once, and it gives you local storage, synchronization, and push notification across devices. We currently support Silverlight, Windows Phone, WPF, and MVC 3. Android is coming soon.

Collaborative framework

LogoPeople don’t just own one computer anymore. Now they have a desktop, a laptop, and a phone. They want their data to seamlessly flow across all of their devices.

People take their devices with them. These devices aren’t always connected. And even when they are, people don’t want to wait for them to connect to a central server. Everyone should have their own data on their own device for immediate access.

People use software to collaborate with each other. Some domains are overtly collaborative, such as social networking and gaming. Others are more discreet, like customer relationship management and project planning. Actions performed by collaborators affect the user experience.

Systems built with Correspondence

We have built a number of systems using Correspondence.

  • HoneyDo List: Todo lists that you can share with your family.
  • Commuter: A continuous podcast playlist for iTunes on Windows.
  • Faceted Reversi: Head-to-head reversi game for Windows Phone.
  • Dallas TechFest 2011: Personal conference schedule for Windows Phone.
  • Thought Cloud: Collaborative mind mapper (demo).


The slides for the presentation are rendered in Silverlight. Once the page loads, click on it to give it focus. Then use Page Down to progress through the presentation. Hit F11 to enter full-screen mode. The sample code used in this presentation is on GitHub.


Get started

These are the resources you will need to build an occasionally connected Windows Phone 7 or Silverlight application using Correspondence.

  1. Install the NuGet Package Manager through the Visual Studio 2010 Extension Manager. Detailed instructions on the NuGet project site.
  2. Add the Correspondence.WindowsPhone.AllInOne or Correspondence.Silverlight.AllInOne package to a Windows Phone 7 or Silverlight 4 application.
  3. Follow the walkthrough on the Correspondence project site to learn how to build a Correspondence model.
  4. Sign up for a synchronization server API Key. Put the API key in your POXConfigurationProvider.

Speaking at Dallas TechFest

I’ll be presenting next week at Dallas TechFest. I’m doing two sessions: Provable APIs and Correspondence for Windows Phone 7.

Provable APIs is a session that I originally created for the Q.E.D. lunches at work. It shows how you can use regular features of your compiler – not even the new contract validation stuff – to verify that people are using your classes correctly. If you create your APIs provably, then your users will find it a pleasure to work with your code.

Correspondence is a library for building occasionally connected clients. I used it to build the Dallas TechFest conference scheduling app for Windows Phone 7. In this session, I take apart the conference app and show you how you can build your own great user experiences.

As an added bonus, I will reveal the Correspondence logo at the conference. Come out and see it before I post it on the project site.

Microsoft MVP in Client Application Development


I have received the Microsoft MVP award in Client Application Development. While this award is intended to recognize community leadership, I feel that it is only possible because of the support I get from the community. Thank you for attending my talks, asking great questions, and encouraging me to learn and to teach. This recognition will open even more doors for us to converse. And for that I am grateful.

Special thanks go out to Chris Koenig, who gave me little pushes, key opportunities, and finally the nomination that lead to this award.

Thought Cloud

Capture thoughts and share them with others.

Get Microsoft Silverlight

Right-click and install on this computer for off-line use.

Approaching an ideal from two directions

The ideal specification is unambiguous. After reading such a specification, the consumer should have no question as to the correct behavior of the system described.

The ideal code is not extraneous. There are no implementation details written in the code that were not part of the problem domain. Such details would not have been described in the specification, and could therefore not be verified as correct. If they can’t be verified as correct, then they must be arbitrary. If more than one arbitrary implementation choice could lead to a correct program, then why must the programmer make the decision? And why must he express that decision in code? The computer could figure it out.

As specifications approach their ideal lack of ambiguity, they remain firmly in the state of having no extraneous parts. The specification doesn’t contain implementation details. The customer doesn’t care how it is implemented. They just want it to be correct, and optimized for cost and speed.

As code approaches its ideal brevity, it remains firmly unambiguous. Since code is executed by a deterministic computer, the computer knows at any point what the program will do. There is no question.

Specification, in trying to become unambiguous, is approaching code. Code, in removing extraneous details, is trying to become specification.

Example: a checkbook ledger

If we were to write ideal specifications for a checkbook ledger, we would use only terms from the problem domain. We would define a transaction. We would say what a user could do with a transaction. And we would describe the reaction of the system to those operations. These statements would leave no doubt as to the correct behavior of the system. For example, we could write:

  • A transaction is a named, dated event that either increases or decreases the balance of an account by some positive amount.
  • A user can create a transaction, giving the name, date, and amount of increase or decrease.
  • A user can change the name, date, or amount of a transaction.
  • A user can void a transaction.
  • The system displays transactions in chronological order.
  • The system displays the running account balance of each transaction, that being the previous transaction’s running account balance plus the current transaction’s increase or minus its decrease.
    • The chronologically first transaction, having no previous transaction, assumes a prior balance of zero.
    • A voided transaction does not increase or decrease the prior balance.

If we were to write ideal code for a checkbook ledger, we would only solve the business problem. Storage, communications, and representation details would be handled by our chosen frameworks. We wouldn’t have to describe how a transaction is turned into bits on the network, nor would we have to generate SQL statements to persist or query them. For example, we might write this code:

Transaction(BelongingTo: Account) {
    Name: string
    Effective: date
    Change: [Increase, Decrease]
    Amount: decimal {Amount > 0.0}

Void(Voided: Transaction) { }

Account {
    Transactions {
        Transaction t : t.BelongingTo = this
        where not exists Void v : v.Voided = t
        order by t.Effective
        select {
            RunningBalance = (prior.RunningBalance ?? 0.0) +
                (t.Change = Increase) ? t.Amount : -t.Amount

The code verifiably satisfies the specifications, because it says no more and no less than the specifications do. In fact, besides the syntax, the code is the specification. The two artifacts, each evolving toward its own ideal, have ended up in the same place.

Programming languages of the future

No programming language yet developed is as concise as the above code. But we are getting closer. Modern frameworks raise the level of abstraction to remove implementation details from application code. Languages are moving away from the imperative style and adopting declarative and functional styles.

Today, programs are be specified by a Business Analyst, who is skilled in decomposing and unambiguously describing business processes. And programs are written by developers, who are skilled in constructing solutions from technical components at all levels of abstraction. But when programming languages finally do reach this level of brevity, a new discipline will emerge. Neither skill set will be sufficient to solve business problems with software. One person will create this artifact, an executable specification, using a mix of these two skill sets. This person will possess the mind of a mathematician.

Line of business applications in XAML

I spoke at the Dallas XAML User Group on Tuesday about building line-of-business applications in WPF and Silverlight. This was a hands-on event, so the participants followed along with their own laptops. Please download the source code and try it yourself.

The idea behind the talk was to exercise some of the most common features of XAML, Visual Studio, and Blend that are used in a line-of-business application. There are more features in XAML than one person can master. This is the subset that will give you the best return on your learning investment.

Unit test your view models

One of the benefits of separation patterns like MVVM is that you can unit test more of your code. Take advantage of it.

You can only unit test code that is in a class library. So it follows that view models should be in class libraries.

View models need to call services to access data and make changes. At run time, these can be WCF, RIA Services, OData feeds, or any other kind of service. But at unit test time, these have to be mocks. There are two reasons for mocking a service in a unit test:

  • Unit tests should run in isolation; they should not pass or fail depending upon the environment.
  • You need to test that the service gets called appropriately.

To mock a service at unit test time, we inject our dependencies. For example:

public interface ICatalogService
    List<Product> LoadProducts();
public class CatalogViewModel
    private ICatalogService _catalogService;

    public CatalogViewModel(ICatalogService catalogService)
        _catalogService = catalogService;

By injecting the service into the constructor via an interface, we can provide a real implementation at run time, and a mock implementation at unit test time.

public class MockCatalogService : ICatalogService
    public List<Product> LoadProducts()
        return new List<Product>()
            new Product() { Name = "Widget" },
            new Product() { Name = "Gadget" }
public class CatalogViewModelTest
    private CatalogViewModel _catalogViewModel;

    public void Initialize()
        MockCatalogService mockService = new MockCatalogService();
        _catalogViewModel = new CatalogViewModel(

    public void CanGetProductListFromService()
        var products = _catalogViewModel.Products;
        Assert.AreEqual(2, products.Count());

Put all views in UserControls

Don’t use the MainWindow as a view. Don’t drop controls directly into it. Instead, create a new UserControl and build your view in there. Doing so will have several benefits:

  • You can reuse and compose UserControls, but not Windows.
  • You can switch from one UserControl to another within a Window for simple navigation.
  • UserControls can be used within a DataTemplate to polymorphically select the view based on the view model.
  • The container can inject the DataContext into a UserControl, rather than relying upon the view model locator pattern.

That last point requires further explanation. We’ve already uses dependency injection to get the service into the view model. We could have used a pattern called “service locator” instead to let the view model find the service itself. Service locator is well documented to have disadvantages when compared to dependency injection.

The view model locator pattern is just another service locator. As such, it suffers from many of the same drawbacks. For example, when the user navigates to a view, they have already selected a specific object. The view model locator must somehow locate that selected object. We end up with a Hail Mary pass, where the selection is stored in one view and retrieved in the locator for another view. If the object was instead injected into the view, it becomes much easier to manage.

To inject the first view model at run time, set the DataContext inside of the main window’s Loaded event.

private void Window_Loaded(object sender, RoutedEventArgs e)
    CatalogViewModel viewModel = new CatalogViewModel(
        new RealCatalogService());
    this.DataContext = viewModel;

From that point forward, you can use properties of the main view model to declaratively inject child view models. DataTemplates come in handy for this.

Design time data

There are two ways to use design-time data in Blend with the MVVM pattern. The easy way is to create sample data from a class.


Select the view model class. Then layout your controls and drag properties onto them to databind.

The second way is to create an object data source.


First create a class with one property: the view model. Initialize this view model with a mock service. This mock data will appear at design time.

public class CatalogDesignerData
    public CatalogViewModel ViewModel
            CatalogViewModel viewModel = new CatalogViewModel(
                new MockCatalogService());
            return viewModel;

Drag and drop to create list boxes

If you create sample data from a class, you will be able to drag and drop collections onto the art board. This will automatically generate a ListBox and give it an ItemTemplate. Then you can customize this ItemTemplate using Edit Additional Templates: Edit Generated Items: Edit Current.


By default, all properties are added to a StackPanel. If you want a horizontal layout:

  • Set the StackPanel’s Orientation property to Horizontal.
  • Right-click the StackPanel in the Objects and Timeline view and select Change Layout Type: Grid.
  • Click the grid ruler to create columns.
  • Right-click each property and select Auto Size: Fill.
  • Set all but the last column to Pixel sized. Set the last column to Auto sized.

If you want a vertical layout, adjust the Margin of the lower properties to indent them and add spacing between items. Also, adjust the colors and fonts to enhance the contrast of the first property.

If you use the object data source method, Blend will not create a ListBox when you drag and drop. I recommend starting with the simple sample data, then creating an object data source later if you find you need to.

Use ValueConverters for application-specific view logic

You might need to represent data using different styles based on its value. For example, positive numbers appear black while negative numbers appear red. Create ValueConverters for these cases.

public class RedWhenNegativeValueConverter : IValueConverter
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        if (targetType != typeof(Brush))
            throw new ApplicationException("You must bind to a brush.");

        decimal decimalValue = (decimal)value;
        if (decimalValue < 0.00m)
            return new SolidColorBrush(Colors.Black);
            return new SolidColorBrush(Colors.Red);

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        throw new NotImplementedException();

Data bind the element’s color to the value. Apply value converters in Blend using the advanced properties pane in the binding dialog. Click the ellipsis to create a new instance of the value converter class.

These are the techniques that I find myself using most frequently when building line-of-business applications in WPF and Silverlight. I don’t know everything there is to know about XAML, but by focusing on the features that I find most useful, I’m able to build apps quickly.