Browsing Posts published in April, 2011

Once upon a time there were four friends – two TextBoxes and two ComboBoxes. They all lived happily on a WPF Window bound together in blissful MVVM harmony. The TextBoxes were bound to the ViewModel’s TextBoxText property and the ComboBoxes to the ComboBoxSelectedItem property.

MVVM ComboBox

<Window x:Class="MvvmComboBox.MainWindow"
        Title="MVVM ComboBox" Width="300" Height="150">
        <mvvm:MainWindowViewModel x:Key="viewModel"/>
    <StackPanel DataContext="{StaticResource viewModel}">
        <TextBox Text="{Binding TextBoxText.Value}"/>
        <TextBox Text="{Binding TextBoxText.Value}"/>
        <ComboBox ItemsSource="{Binding ComboBoxItems}" 
                SelectedItem="{Binding ComboBoxSelectedItem.Value}"/>
        <ComboBox ItemsSource="{Binding ComboBoxItems}" 
                SelectedItem="{Binding ComboBoxSelectedItem.Value}"/>

TextBoxText and ComboBoxItem are both Observable<T> so that INotifyPropertyChanged notifications are broadcast properly to all interested parties.

public class Observable<T> : INotifyPropertyChanged where T:class {
    public event PropertyChangedEventHandler PropertyChanged = (o,e) => { };

    private T value;
    public T Value {
        get { return value; }
        set {
            if(this.value == value) return;
            this.value = value;
            PropertyChanged(this, new PropertyChangedEventArgs("Value"));

ASIDE: Notice the event definition in line 2. By assigned a NOP (no operation) lambda, I don’t have to check whether any listeners are registered for the event. I can simply fire the event and worse case scenario, I execute an empty function body rather than throwing a NullReferenceException. This is tidier in my opinion than the if(PropertyChanged != null) nonsense that we’ve been using for years.

The view model is similarly simple.

public class MainWindowViewModel {
    public MainWindowViewModel() {
        TextBoxText = new Observable<string>();
        ComboBoxSelectedItem = new Observable<string>();

    public Observable<string> TextBoxText { get; set; }
    public Observable<string> ComboBoxSelectedItem { get; set; }

    public IEnumerable<string> ComboBoxItems {
        get {
            yield return "One";
            yield return "Two";
            yield return "Three";
            yield return "Four";
            yield return "Five";

With the INotifyPropetyChanged in place, changing one TextBox will cause its twin to display the same value and same for the two ComboBoxes.

MVVM ComboBox synchronized

We get a strange new requirement that every second change should be ignored. (The actual business requirement was more sensible. I was displaying a list of entities for editing. If the previous entity wasn’t saved, the user would be presented with a save dialog and standard “yes/no/cancel” options. On cancel, I wanted to disregard the ComboBox change and revert back to the previously selected entity.)

public class Observable<T> : INotifyPropertyChanged where T:class {
    public event PropertyChangedEventHandler PropertyChanged = (o,e) => { };

    private T value;
    private int count;
    public T Value {
        get { return value; }
        set {
            if(this.value == value) return;
            if(count++ % 2 == 0) return;
            this.value = value;
            PropertyChanged(this, new PropertyChangedEventArgs("Value"));

So based on business rules, we’ve ignored a change to the view model. The TextBoxes always remain synchronized – reverting back to the previous value if the change is ignored. Unfortunately the changed ComboBoxes becomes unsynchronized with the view model if the view model ignores a change.

MVVM ComboBox unsynchronized

If you examine the view model in the debugger, the TextBoxText is “Hello, world! Hello, again!” as expected. You can change either TextBox as many times as you want. Every other edit is ignored and causes its changes to revert, as expected. Examining the current state of the view model’s ComboBoxSelectedItem, its value is “Four” and not “Two”. I selected “Two” in the first ComboBox, which was the ignored change. The first ComboBox is now unsynchronized with the view model. If you change the selection again, both ComboBoxes have the correct value. The problem is only with changes ignored by the view model.

Let’s create a new type derived from ComboBox to fix this problem.

namespace JamesKovacs.MvvmComboBox {
    public class ComboBox : System.Windows.Controls.ComboBox {
        protected override void OnSelectionChanged(SelectionChangedEventArgs e) {
            var binding = GetBindingExpression(SelectedItemProperty);
            if(binding != null) {

I perform the normal OnSelectionChanged event and then update the ComboBox’s SelectedItemProperty from the bound ViewModel if a binding exists. This ensures that if the view model ignores changes or modifies the selection in some way, the ComboBox displays the correct value. Minor note: I am defining a derived ComboBox in my own namespace, which is also called ComboBox. I do this so that in my XAML, I can apply the fix by prepending <ComboBox/> with my namespace <mvvm:ComboBox/>. With this fix in place, my MVVM ComboBox remains synchronized with its view model.


In most cases, MVVM model binding dramatically simplifies your WPF code. Unfortunately there are some cases where bugs in the framework prevent it from working properly. Fortunately in many cases you can work around these limitations by simply deriving your own custom control from the one supplied with WPF to fix these problems.

For those of you who want to play around with the code, you can find it here. Happy coding!

ADDENDUM: As noted by Michael L. Perry in this comment, WPF behaviours can also be used to inject code without subclassing ComboBox. The code sample now includes his behaviour code in addition to the subclassing option. Also to note is that a bug in .NET 4.0 prevents the fix from working. The ComboBox (regulard, subclassed, or behavioured) becomes unsynchronized from the view model. The subclassing code works correctly in .NET 3.5. If there are any WPF gurus out there who can understand why it works in .NET 3.5, but not .NET 4.0, I would love to hear an explanation.

imageSystem.Type and XML are the nuts and gum of the development world. You can stuff a System.Type into an XML file, but it leaves a bad taste in your mouth. Sometimes you don’t have much choice. For example:

  • Custom configuration sections in App.config/Web.config
  • Handlers and modules in Web.config
  • ProxyFactoryFactory, 2nd-level cache provider, database dialect, and other pieces of configuration in hibernate.cfg.xml*
  • XML-based IoC configuration**

* NHibernate Loquacious configuration is now a better option for configuring NHibernate IMHO and I use it in all my new apps rather than hibernate.cfg.xml.

** For IoC containers, convention-over-configuration is a better choice followed by code-based configuration. I use XML-based configuration only as a last resort, but it can sometimes be useful for configurable overrides.

When you do need to encode a System.Type into a XML file, JetBrains ReSharper makes your life a lot easier. (This is a little-known feature of ReSharper shown to me by Ilya “Orangy” Ryzhenkov, ReSharper Product Manager.) I decided to create a completely silly, contrived example where we need to display a list of hot peppers and their Scoville heat units and the information is stored in App.config. (Yes, I know. A database or flat file would be a better place. I told you that it was a contrived example.) I have created a PeppersConfigurationSection class that is defines the <peppers> configuration section. I need to specify the type name in the <configurationSections> where it says <INSERT_TYPE_NAME_HERE>

<?xml version="1.0" encoding="utf-8" ?>
    <section name="peppers" type="<INSERT_TYPE_NAME_HERE>" />
    <pepper name="Crazy Jerry's Brain Damage" scovilles="11000" type="sauce"/>
    <!-- more peppers -->

Rather than typing out the namespace-qualified type name, I simply type PCS for PeppersConfigurationSection and press ReSharper’s Import Symbol Completion (IDEA: CTRL-ALT-Space; VS: ALT-SHIFT-Space).

Import Type Completion

You can see the normal ReSharper completion pop-up appear. Selecting the first one, I get:

Type Completed

This is fine and dandy if the custom configuration section is defined in your project, but what if you need to specify the assembly name too? ReSharper once again comes to our rescue with a Quick Fix (ALT-Enter).

Add module qualification

Selecting the Add module qualitication Quick Fix and we end up with this. (I added linebreaks between the parts of the fully qualified type name for ease of readability in the screenshot.)


Add module qualification is even handier if the assembly has a public key token as those are a hassle to look up. Once again, ReSharper does the hard work of inserting the type name – I only typed NVSH CTRL-ALT-Space – and then the quick fix to get the qualification with assembly name, version, culture, and public key token:


I hope this little ReSharper gem makes your coding lives just a little bit easier…

N.B. ReSharper 5.1 and 6.0 EAP (up to at least build 2136) has a bug that causes Import Symbol Completion to fail for files named Web.config in a ASP.NET project. If you rename Web.config to Foo.config – no other changes necessary – you can use Import Symbol Completion. Just remember to rename the file back to Web.config when you’re done. Another option is to copy Web.config to another project type (Console and Class Library both work), make your changes, and then copy back. I have reported the issue to JetBrains here and the team is working on a fix.

[Originally published on Telerik blog here. Republished with permission.]

As developers, we like to believe that only raw data matters. And what is more raw than the written word? Does this mean that email is a more efficient communication mechanism than in-person meetings? Email contains just the raw words of our intent, right? The receiver can scan through looking for relevant content. Their computer can index and search our email. Why don’t we communicate everything in email? We’ve all experienced the miscommunication that can happen in email. So what are we missing?

The reality is that humans are social animals. We are attuned to many communication side channels – tone of voice; verbal pauses; facial expressions; body language. These side channels communicate additional information above and beyond the written (or spoken) word. What we are effectively doing with these side channels is increasing our communication bandwidth and when it comes to communication, more bandwidth is better. Just like increasing bandwidth in your computer network means pushing more bits of data per second through the pipe, you can do the same thing with your personal and team communications. Arranging some common forms of communication from lowest to highest bandwidth.

  • One-way text (email/tweet/txt)
  • Two-way text (IM)
  • Voice
  • Video
  • Face-to-Face

As you go from low to high, the amount of information communicated per unit time increases and the likelihood of asking for clarification goes up. You’re much more likely to ask for clarification on an uncertain point when talking to someone than when you’re texting. Having to craft a reply creates a certain amount of impedance and if you’re fairly certain you know what the person is talking about, you won’t send that clarifying text or email. When talking, you’re much more likely to simply say back, “Let me see if I understand what you’re staying” and proceed to paraphrase your understanding of what they said.

Let’s look at daily stand up meetings as an example of high-bandwidth communication. The purpose of the daily stand up meetings is to keep team members apprised of progress on the project. Daily stand up meetings aren’t the only way to do this. Everyone could write daily status reports. Or the project manager could walk around asking everyone for a verbal update. Or team members could update a central progress report wiki. But most successful agile teams use daily stand up meetings. Why are daily stand up meetings more effective than the other techniques? It all has to do with communication bandwidth…

Let’s say a team member emails everyone, “I’m going to finish the enrolment feature on the website today.” Sounds great. No reason to doubt them. Now let’s say that they say exactly the same thing in a daily stand up meeting, but now you notice that they say it while avoiding eye contact with other team members and shuffling their feet? Most people can tell that the developer is quite uncertain about their ability to accomplish the task today. Same raw information, but you’re receiving additional information from the communication sideband of body language. If the developer instead said the same thing, but looks other team members in the eye and says it with confidence in their voice, other team members will feel more certain about the task being completed. (If the person has been saying the same thing for a week straight now, the team has a different issue to deal with – that of honesty and integrity in communication.)

Knowing why agile teams value daily stand up meetings also helps us compensate when face-to-face stand ups aren’t possible or feasible. Let’s say that we have a geographically dispersed team, video calls are going to have higher bandwidth than a voice call or emails. Similarly if the geographically dispersed team is centred in two different locations, it would be better to have two separate face-to-face meetings linked by video rather than everyone calling in separately via video conference. The team’s goal is to maximize efficient information exchange between team members as much as possible.

Daily stand up meetings aren’t the only way that agile teams leverage high bandwidth communications. Other examples include:

  • Pair programming: two developers focused on solving a single problem
  • Team rooms: immersing project members in team-related communications
  • User stories: a reminder to have a conversation with a stakeholder
  • Story walls or Kanban boards: surrounding the team in relevant project data

In each of these cases, successful agile teams use high bandwidth communication to facilitate understanding between team members and with other project stakeholders. Notice that story walls and Kanban boards are a form of non-verbal high bandwidth communication. Team members and stakeholders can quickly glance at the board and see the current status of their project. A well-placed board cannot help but be noticed often and frequently throughout the day. It is a constant visual reminder of what the team is doing and where it is at. It often becomes a focal point of conversation and acts as an information radiator about the project.

This isn’t to say that teams should avoid written communication, but use it when it’s appropriate. Written communication is more easily archived and searchable than verbal communication. Good written communication summarizes and condenses a lot of information into a more useful form. Written communication also decouples the availability of the sender and receiver. If you need a new server set up or need to confirm a meeting time, often a quick email is more efficient than standing outside someone’s office waiting for them to have a moment to talk to you. The lesson here is use the appropriate form of communication for the job at hand. And next time you’re having trouble understanding an email from a team member or project stakeholder, don’t hesitate to increase your communication bandwidth by picking up the phone or inviting them out for coffee…