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MVVM with WPF, Silverlight, and… Windows Forms

In my post-NTK09 conference blog post I mentioned one of my MVVM demos was about sharing a ViewModel between WPF, Silverlight and even Windows Forms application. I got a lot of requests for posting sample code online since then, which I intended to do earlier, but because the sample application code was closely related to the conference, I wanted to create a whole new sample application first and share that. Of course time passed and other things kept me busy from actually doing it. With Silverlight 4 Beta coming out recently, things progressed even further in so I decided to keep things as they were, and post the original sample code. I’ll explain it in few lines (and pictures) here, and the source code can be found here.

What’s it all about?

The purpose of the sample (and my NTK09 talk in general) was about the MVVM pattern principle and how it can be used to totally separate the User Interface from the business logic and data. The application I built was a conference session schedule viewer / editor. Technically, there are three application flavors: WPF, Silverlight and WinForms. They share the same data access (WCF service) and ViewModel. The only thing different between the three is their UI implementation.

imageimageimage

Notice the small arrow overlay over the C# icon on Silverlight and WinForms PageViewModel.cs? That’s because both project are linking the original file from the WPF project, which I created first. That means that ViewModel is exactly the same across all three projects when they are compiled. And with all three project, this is the ViewModel that serves up the main page.

The WPF project

I created the WPF project to ‘just work’. I didn’t bother with styling and the looks, I intentionally left it ugly. I used Prism for commanding and WCF based web service for communicating the data.

image

The ‘Dan’ (Day) and ‘Kategorija’ (Category) serve as the filter for the session list on the left. Each combo box change results in a server request which returns new data for filling the session list. Other fields are editable session details fields. Three buttons on the top left correspond to New, Edit and Delete, in that order.

The Silverlight project

The Silverlight project differ from WPF in that that it’s in color :)

Silverlight

Prism is still used for commanding, but the UI is quite different. I even provided the View (above) and Edit (bottom) screens.

Silverlight Edit

As said, the difference between the WPF and Silverlight applications is just and only different XAML. And both have zero code-behind code. Moving on to…

The Windows Forms project

I  didn’t bother much with making Windows Forms project to work. I last coded anything in WinForms 2+ years ago so I suspect I won’t have to go there again… However, this is the UI I put together:

Windows Forms

It’s very similar to WPF, but data bindings perform much slower. Data bindings? There’s data bindings in WinForms too?

LOL. Sometimes I really do get questions like that. Anyway, let’s see the ‘code behind’. [There’s actually quite a few lines of code behind in Windows Forms because it’s used to instantiate and initialize all controls on the screen. That code is stored in the .designer file and I’m ignoring it as the code behind]

private void MainForm_Load(object sender, EventArgs e)
{
    PageViewModel viewModel = new PageViewModel();

    dayBox.DataBindings.Add("DataSource", viewModel, "Days");
    categoryBox.DataBindings.Add("DataSource", viewModel, "Categories");
    categoryBox.DisplayMember = "Name";
    speakerBox.DataBindings.Add("DataSource", viewModel, "Speakers");
    speakerBox.DisplayMember = "FullName";
    speakerBox.ValueMember = "Id";
    startTimeBox.DataBindings.Add("DataSource", viewModel, "TimeValues");
    endTimeBox.DataBindings.Add("DataSource", viewModel, "TimeValues");
    sessionList.DataBindings.Add("DataSource", viewModel, "Sessions");
    sessionList.DisplayMember = "Title";

    dayBox.DataBindings.Add("SelectedValue", 
        viewModel, "SelectedDay", true, DataSourceUpdateMode.OnPropertyChanged);
    categoryBox.DataBindings.Add("SelectedValue", 
        viewModel, "SelectedCategory", true, DataSourceUpdateMode.OnPropertyChanged);
    sessionList.DataBindings.Add("SelectedValue", 
        viewModel, "SelectedSession", true, DataSourceUpdateMode.OnPropertyChanged);
    speakerBox.DataBindings.Add("SelectedValue", 
        viewModel, "SelectedSession.SpeakerId", true, DataSourceUpdateMode.OnPropertyChanged);

    startTimeBox.DataBindings.Add("Text", 
        viewModel, "SelectedSession.StartTime", true, DataSourceUpdateMode.OnValidation);
    endTimeBox.DataBindings.Add("Text", 
        viewModel, "SelectedSession.EndTime", true, DataSourceUpdateMode.OnValidation);
    titleBox.DataBindings.Add("Text", 
        viewModel, "SelectedSession.Title", true, DataSourceUpdateMode.OnValidation);
    descriptionBox.DataBindings.Add("Text", 
        viewModel, "SelectedSession.Description", true, DataSourceUpdateMode.OnValidation);

    newButton.Click += (s, ev) => 
        { viewModel.NewSessionCommand.Execute(null); };
    editButton.Click += (s, ev) => 
        { viewModel.EditSessionCommand.Execute(viewModel.SelectedSession); };
    deleteButton.Click += (s, ev) => 
        { viewModel.DeleteSessionCommand.Execute(viewModel.SelectedSession); };
    saveButton.Click += (s, ev) => 
        { viewModel.EndEditSessionCommand.Execute(viewModel.SelectedSession); };
    cancelButton.Click += (s, ev) => 
        { viewModel.CancelEditSessionCommand.Execute(viewModel.SelectedSession); };
}

This is all the code for the main form. You’ll notice it’s all used for binding controls to various properties on the ViewModel. I could also get rid of some of the lines by setting the properties on the WinForms designer (thus moving them to the .designer file). The last section wires button clicks to execute commands in the ViewModel.

There is absolutely no business logic in this code which makes it very similar to declarative XAML in WPF and Silverlight projects. I was toying with the idea of making an Extender Provider to get rid of the above lines completely and do it all just with the Windows Forms designer – I made a working prototype, but wasn’t generic enough to include it in the sample.

Source code

Bellow is the full source for all three main and related projects. Also included is the Test project with a few Silverlight Unit Test Framework tests. Database is not included so you’ll not be able to actually run any of the applications. Sorry. I’ll post the Silverlight version online when time permits.



Silverlight LOB: Validation (#2 – Annotations and shared classes)

In my previous post, I wrote about “the first line of defense” against inputting invalid data in Silverlight applications (or any kind of application, for that matter) – preventing the user from entering invalid data through some an input form. Input form fields are commonly directly bound to the underlying object’s properties – either directly or through a of value converter – in both cases the entered value gets assigned to the bound object’s property; and this is exactly the place where we would want to put our second line of defense.

Silverlight 3 supports data validation. The majority of controls now feature new visual states for presenting themselves whenever they are associated with invalid data. Putting a control into the invalid state is as easy as throwing an exception in its bound property’s setter code:

public DateTime BirthDate
{
    get { return birthDate; }
    set
    {
        if (birthDate == value)
        {
            return;
        }

        if (birthDate > DateTime.Today)
        {
            throw new ArgumentException("The birth date is invalid.");
        }

        birthDate = value;
        OnPropertyChanged("BirthDate");
    }
}

When the property is set to an invalid date, the exception gets thrown. This exception is then caught by the data binding engine, which in turn triggers the BindingValidationError event, prompting the control to go to invalid state:

 image

This kind of notification can be enabled by setting binding’s ValidatesOnExceptions and NotifyOnValidationError properties to true.

System.ComponentModel.DataAnnotation

There is another way to perform validation over entity’s properties:  by using validators from the System.ComponentModel.DataAnnotation assembly. What you do is put an appropriate attribute above the property definition, describing the kind of validation to performed over property value.

[Required(ErrorMessage = "The date of birth is required.")]
public DateTime BirthDate
{
    get { return birthDate; }
    set
    {
        if (birthDate == value)
        {
            return;
        }

        birthDate = value;
        OnPropertyChanged("BirthDate");
    }
}

You can even write your own validation attributes by deriving from the ValidationAttribute class and overriding the validation method. Here’s an example to match my first method of validation (throwing an exception in property setter):

public class BirthDateValidationAttribute : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, 
ValidationContext validationContext) { if (value == null) { return ValidationResult.Success; } DateTime date = (DateTime)value; if (date > DateTime.Today) { return new ValidationResult("The birth date is invalid."); } return ValidationResult.Success; } }

Let’s put this to work. Say we’re using shared entity classes in our project; a kind of a real world scenario. When projects are set up as described in my previous blog entry, we immediately bump into a problem – decorating the Person’s BirthDate property with the new BirthDateValidationAttribute results with the compilation error: it’s just that full .NET framework’s ValidationAttribute implementation differs form how Silverlight’s version is implemented and therefore above BirthDateValidationAttribute can’t be shared between both entity projects:

Client side: protected abstract ValidationResult IsValid(object value, ValidationContext validationContext)
Server side: public abstract bool IsValid(object value)

To work around this, we need to create a dummy validator attribute class in the server side entity project, having the same name as its client counterpart, except always returning true as the validation result:

public class BirthDateValidationAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        return true;
    }
}

Validation

Instead of throwing an exception from the property trigger as in the previous example, we can now simply call the validation method, which would validate against every ValidationAttribute defined for the property.

Modified Property definition (Validate() method is called instead of throwing an exception):

[DataMember]
[BirthDateValidation]
public DateTime BirthDate
{
    get { return birthDate; }
    set
    {
        if (birthDate == value)
        {
            return;
        }

        Validate("BirthDate", value);
        birthDate = value;
        OnPropertyChanged("BirthDate");
    }
}

Because we only want to validate properties on the client, we need to implement the Validate() method just on the client side. But since the Person class is shared, we really have to include this method on both sides, except we would implement it differently.

In the end, Validate() method on the client is defined as:

private void Validate(string propertyName, object value)
{
    Validator.ValidateProperty(value, new ValidationContext(this, null, null)
    {
        MemberName = propertyName,
    });
}

… while the server side method can be left empty:

private void Validate(string propertyName, object value)
{
}

Single base class for entities?

To avoid having duplicate validation methods for all entities, it’s best to put it in a single base page, where you would put all the common entity code (like INotifyPropertyChanged etc.). In this case, partial methods are going to prove themselves very useful:

Main base class part:

partial void ValidateProperty(string propertyName, object value);
protected void Validate(string propertyName, object value)
{
    ValidateProperty(propertyName, value);
}

Client side base class part:

public partial class EntityBase
{
    partial void ValidateProperty(string propertyName, object value)
    {
        Validator.ValidateProperty(value, new ValidationContext(this, null, null)
        {
            MemberName = propertyName,
        });
    }
}

We can just leave the server side base class part out and forget about it. The best thing with with partial methods is that you don’t have to provide any implementation if you don’t want to – in which case the method simply won’t be called.

Source code for this project is available:




Detecting duplicate instances of a running Silverlight application

This is a short tip for constraining your Silverlight applications to a single instance, something that’s quite popular in the desktop applications world. What you want to do is allow only the first instance of your application while rejecting all subsequent instances.

Silverlight 3 introduced a way for applications to communicate between each other, either on the same page or instantiated on a different browser instances (works even with Installed/OOB apps). The communication is performed by sender and receiver classes, which exchange messages through named channels. Each receiver must register a unique name for the channel or an exception will be thrown.

And that’s the trick. Listening on a specific named channel will act like a mutex. By catching the ListenFailed exception you get an option to display appropriate message or launch a different version of the application. All the work is done in App.Xaml.cs file:

private void Application_Startup(object sender, StartupEventArgs e)
{
    try
    {
        receiver = new LocalMessageReceiver("singleinstance");
        receiver.Listen();
        this.RootVisual = new MainPage();
    }
    catch (ListenFailedException)
    {
        this.RootVisual = new DuplicateInstancePage();
    }
}

The test project is available through here.