One of the great things about WPF is that it encourages you to use a sensible pattern for writing software - separating content from design, keeping the business logic away from the UI. If you are completely unfamiliar with WPF and Expression Blend, now is a good time to have a play. Delete any controls that you have added - we're going to create a functional application and will need a clean slate.
We've decided to write a simple address book application, to allow us to manage some information about our contacts. Find the 'Text Block' control in the Assets panel (there's a handy search box if you can't find it). Go to selection mode by selecting the darker arrow from the top of the toolbar on the left. Drag a 'List Box' control onto the left hand side of the window - this is where we'll show the list of contacts.
Create another text block on the right hand side of the window and set the text to 'Contact Details'. Finally, drag a 'Button' control onto the design surface and set its content to 'Create a New Contact'. From the specification, we have a fairly good idea of what sort of data objects we'll need.
We're going to want some data to make sure our UI is working, so add a few contacts in the constructor of the MainWindow object. We can bind to the 'Contacts' property of the MainWindow object by selecting 'Element' at the top and finding the 'Contacts' property.
Delete the binding we set before by clicking on the 'Reset' advanced properties popup on the list box 'ItemsSource' property. In the 'Name' text box, bind the 'Text' property to the 'SelectedContact > Name' property ('SelectedContact' can be expanded to show its members). In the 'Email' text box, bind the 'Text' property to the 'SelectedContact > Email' property. In the 'Phone' text box, bind the 'Text' property to the 'SelectedContact > PhoneNumber' property.
In a WinForms or Win32 application, we'd need to tell the UI that the object has changed; we might call Invalidate, Refresh or something along those lines.
Now that we have the name set as the display member, we can see that we still have a limitation. After the initial complexity of setting the data context and creating some view model entities, we can implement business logic very rapidly now.
As an aside, it is worth pointing out that now that we've got to this stage, we're even closer to an ideal MVVM pattern. Rather than starting out with the MVVM pattern, we've moved towards it because we've needed to.
A new window, a blank space for us to arrange content, and the typical features found in a Windows application (close button and so on). Typically, this is where we'd start to think about things like the information we want to present, how the UI might look, functional requirements and so on that we need to implement.
Expression Blend has a powerful UI prototyping tool called 'SketchFlow' that makes it easier than ever to mock up UI designs, as well as add animation, gather feedback from clients, and more. If you have already played around with Model-View-ViewModel, then don't worry - we'll be coming to that soon. In our case, the ViewModel would contain the list of contacts we're showing, the current contact, and anything else that we might need the view to connect to.
If you were to continue building the app with this approach, you'd soon notice something that is lacking - when we change ViewModel properties programmatically, the UI doesn't update.
In the 'Objects and Timeline' panel, double click on the 'Window' item at the top of the tree to select the main window. The code you've written is being called, but the UI doesn't know that the SelectedContact has changed.

This is is not particularly good; it means that our small chunk of business logic needs to know about the UI, it has to know what to update. An ObservableCollection is a collection that sends notifications when items are added or removed. For one thing, the list box on the left shows the name of the object it's displaying, but we want the contact name.
Now when you change the name and tab out of the control or select another, this list is updated. In the code above, we don't tell the UI to update, we don't care whether the view is showing a list or how it displays itself.
Expression Blend is great for layout, design, and styling, but Visual Studio is required for anything relating to code that is non-trivial.
It has a slightly less powerful data binding editor, and does things like creating data contexts visually less well, but when you're a bit more familiar with XAML, you'll probably end up writing a lot of it by hand. Data binding, MVVM, XAML, Pixel Shaders, Control Templates, Styles, Resources, and Triggers are all terms that may well be unfamiliar. By the end of the article, you will be comfortable with some of the core concepts of WPF development, and you'll be well on the way to being able to harness the power of WPF to create stunning applications. We'll start simple and build a skeleton application, then we can take our basic application and refine it. It's not the most original idea, but is simple enough for us to look at some of the features we'll be interested in. This is a large topic in itself, but if there is enough interest, then I'll write an article in this series focused on SketchFlow. We know from the above points that a ViewModel is an object that ties the content to the presentation. Does the View ask for a ViewModel, does the application create the ViewModel and set it into the View? Select the MainWindow in the designer by double clicking on the MainWindow.xaml file in the projects panel.
The DataContext can be thought of as the object or data that we're going to be binding UI elements to. The three text boxes are bound to various properties of the 'SelectedObject' - so when we change, the text boxes are updated. As a demonstration of this, we're going to update the program so that when the window has loaded, the first contact is automatically selected. We can ignore the Contact object for now, because we don't change any of its properties programmatically. We know why now - the property is being changed by the list box, but the property doesn't send a notification when it is changed. Contact data will probably be stored in a file or database - these will be Contact Model entities - from these, we create Contact ViewModel entities - which might be quite different.
Expression Blend is the choice for designing your UIs, but Visual Studio is generally needed for the code behind. Rather than being bombarded with technical details and abstracts, we're going for a very hands on approach. In the world of HTML, we rarely know how big a user screen will be - or how big their browser window will be. The Expression Blend design window is geared towards UI design - visual hints are provided as items are moved and resized. Remember, at the bottom of each panel in the Properties window is a small bar - press it for even more advanced properties. Depending on how you laid out your controls, you'll get a variety of behaviour for how controls are repositioned and re-sized. In a typical WinForms application, we'd probably go through the list of contacts, create a list view item for each one, set the tag to the data object, and insert it into a list box. It is hierarchical, so by setting it in the window, we set the data context for all of the controls - we can set it once and forget about it.

Now that we've set the data context, select the list box on the left hand side and go back to the 'Items Source' property. Why don't we need to call the 'NotifyPropertyChanged' function in the 'set' part of the 'Contacts' property? We could create an 'ItemTemplate' in the XAML that does much more - show the name, the email address below as a hyperlink, and so on, but that is a more advanced topic. We need to change the Contact object so that it derives from ViewModelEntity and sends notifications. For example, the ContactViewModel entity might want to store other data - such as whether it has been changed (allowing the view to show a 'Save Changes' button perhaps).
XAML is quite similar to HTML; it allows us to lay out windows, arrange controls, and more. Because of this, we rarely rely on absolute positioning, such as 'this button is 30 pixels from the left'. Get used to using the search box too - it'll come in handy as you get more familiar with Blend and WPF.
This is the sort of question that causes confusion - you may have heard that the place to put data is in the model and present it with the model view - but what are these entities? The view must show the contacts and the details for the selected contact, so we'll put that data in a model view class.
The short answer is that we're going to have a single instance of the ViewModel in the window object. The plan is to set the data context of the window to an instance of the ViewModel - this will greatly simplify things. Our changes have persisted - the selected object is simply a reference to one of the items in our 'Contacts' list, so everything ties in together perfectly. This interface allows us to send a notification when the property changes; anything that has registered to receive this notification will be updated. Because we don't change the reference to the collection - we change the items in the collection.
We can keep the business logic and data layer separate to the presentation of the data; when you get used to this pattern, it makes more and more sense. This is the first in a series of articles I am planning - any feedback would be great - let me know what you like or dislike and anything that's unclear. Remember - the only code we've done to get the UI of the app working is setting some simple contacts in the constructor - we've actually tied in the text boxes and list control without ever leaving the designer. If we implement this interface in our ViewModel, then the UI will be notified that the property has changed and updates itself accordingly. One of the nice things about WPF is that it really forces you into this approach to get the most out of it - it'll help you design and write better code. WPF can do this very well - we're going to make sure that our app works whether the window is small or large. We're going to come back to this shortly; for now, we'll go for what seems to be the quickest solution - then, we'll look at how to do it the WPF way, which it turns out is even quicker!
The next few steps are really important - you'll probably use them for every window in your app. If we were switching between lots of different collections, we'd need to call the function just like we do for the 'SelectedContact' property. Open the advanced properties by pressing the little square next to it, and choose 'Data Binding'.

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