Azure Inter-role communication using callback instead of queues

I’m currently playing with Azure and the Azure training kit, and I learned something cool today. When you work with Azure you can setup multiple worker roles for your Azure application. If you want to make these roles talk to one another you can use the queuing mechanism which is part of Azure. But you can also use WCF dual interface mechanism. Imagine you want to build a chat application using Azure and WCF. In this case you define a worker role that exposes a dual interface like this: Code Snippet [ServiceContract(     Namespace = "urn:WindowsAzurePlatformKit:Labs:AzureTalk:2009:10",     CallbackContract = typeof(IClientNotification),     SessionMode = SessionMode.Required)] public interface IChatService {   /// <summary>   /// Called by client to announce they are connected at this chat endpoint.   /// </summary>   /// <param name="userName">The user name of the client.</param>   /// <returns>The ClientInformation object for the new session.</returns>   [OperationContract(IsInitiating = true)]   ClientInformation Register(string userName);     /// <summary>   /// Sends a message to a user.   /// </summary>   /// <param name="message">The message to send.</param>   /// <param name="sessionId">The recipient's session ID.</param>   [OperationContract(IsInitiating = false)]   void SendMessage(string message, string sessionId);     /// <summary>   /// Returns a list of connected clients.   /// </summary>   /// <returns>The list of active sessions.</returns>   [OperationContract(IsInitiating = false)]   IEnumerable<ClientInformation> GetConnectedClients(); } The IClientNotification interface is the call-back interface which is implemented by the client of the service. The client calls the Register method on the server, which then can keep track of the client by using the callback interface: Code Snippet IClientNotification callback =   OperationContext.Current.GetCallbackChannel<IClientNotification>();   If you host the service as a single instance, all clients will register to the same service, so this service can easily track each client. But if you grow and start using multiple instances of your service in Azure, each client will register to a single instance, so each instance will know only about its own clients. If two clients, registered to different instances, want to communicate, the services will have to handle the communication for this. The solution is easy, make them also expose the IClientNotification callback service interface using internal endpoints, so they can communicate to each other: Code Snippet public class ChatService : IChatService, IClientNotification   Of course each service will have to be able to find the other service instances. This you can do with the RoleEnvironment class, which you can use in your worker role class: Code Snippet var current = RoleEnvironment.CurrentRoleInstance; var endPoints = current.Role.Instances                 .Where(instance => instance != current)                 .Select(instance => instance.InstanceEndpoints["NotificationService"]);   This does require the worker role to define an internal endpoint. You can do this in Visual Studio 2010. Select the worker role’s properties, go to the endpoint tab and enter an internal endpoint: The rest of the code is straightforward (of your comfortable with WCF that is) and can be found in the Azure Training Kit (look for the Worker Role Communication lab).

Looking at generated queries for LINQ 2 SQL and EF with Visual Studio 2010 Intelli-Trace

When you use LINQ to SQL or Entity Framework you might wonder from time to time what the actual SQL is that was generated by the runtime. Both frameworks have their specific way to allow you to look at this, but require some extra code. Of course you shouldn’t forget to remove this code. Visual Studio 2010 Ultimate gives you another way to look at the generated SQL, without having to change to code. When you turn on intelli-trace, the intelli-trace log will show you the SQL. For example running some LINQ to SQL code will show up as: The ADO.NET entries show the actual query being executed. Note that older queries will also show in here, so if you forgot to look at the query at time of execution, you can scroll back to it later!

How to implement INotifyPropertyChanged without strings

INotifyPropertyChanged is an interface which is very important to do proper databinding and is heavily used in the MVVM pattern. However, to implement it you have to raise the PropertyChanged event whenever a data-bound property changes value, and you have to pass the name of the property to it. This results in string based programming and this generally is not good for maintenance. Some people create a RaisePropertyChanged method in the base class and then invoke this method in each property setter. Again this is not ideal because you cannot always derive from this base class, especially if you already have a base class. In this post I want to show you an alternative way of implementing INotifyPropertyChanged that doesn’t require a base class (you simple implement the INotifyPropertyChanged on each class) with a nice, string-less way of raising the PropertyChanged event. For example, look at this class: 1: public class Customer : INotifyPropertyChanged 2: { 3: private string firstName; 4:  5: public string FirstName 6: { 7: get { return firstName; } 8: set 9: { 10: if (!object.Equals(value, firstName)) 11: { 12: firstName = value; 13: PropertyChanged.Raise(this, o => o.FirstName); 14: } 15: } 16: } 17: } As you can see, the PropertyChanged.Raise does all the work, and you pass in the sender and the property using a Lambda expression. Simple and no strings. To make things even simpler, I use a code snippet that implements the property directly for me: 1: <?xml version="1.0" encoding="utf-8" ?> 2: <CodeSnippet Format="1.0.0" 3: xmlns=""> 4: <Header> 5: <Title>NPC</Title> 6: <Author>Peter</Author> 7: <Shortcut>npc</Shortcut> 8: <Description>NotifyPropertyChangedProperty</Description> 9: <SnippetTypes> 10: <SnippetType>SurroundsWith</SnippetType> 11: <SnippetType>Expansion</SnippetType> 12: </SnippetTypes> 13: </Header> 14: <Snippet> 15: <Declarations> 16: <Literal> 17: <ID>type</ID> 18: <Default>int</Default> 19: </Literal> 20: <Literal> 21: <ID>field</ID> 22: <Default>prop</Default> 23: </Literal> 24: <Literal> 25: <ID>name</ID> 26: <Default>Prop</Default> 27: </Literal> 28: </Declarations> 29: <Code Language="CSharp"> 30: <![CDATA[ 31: [System.Diagnostics.DebuggerBrowsable(System.Diagnostics.DebuggerBrowsableState.Never)] 32: private $type$ $field$; 33: 34: public $type$ $name$ { 35: get { return $field$; } 36: set { 37: if( ! object.Equals( value, $field$ ) ) 38: { 39: $field$ = value; 40: PropertyChanged.Raise( this, o => o.$name$ ); 41: } 42: } 43: } 44: ]]> 45: </Code> 46: </Snippet> 47: </CodeSnippet> So how does it work? Here is the definition of the Raise Extension method: 1: public static void Raise<T, P>( 2: this PropertyChangedEventHandler pc 3: , T source 4: , Expression<Func<T, P>> pe) 5: { 6: if (pc != null) 7: { 8: pc.Invoke(source, 9: new PropertyChangedEventArgs(((MemberExpression)pe.Body).Member.Name)); 10: } 11: } This extension method has three arguments, the first two should be clear. The last is a LINQ expression. When you invoke it the compiler converts this into a tree of objects, which represents the lambda expression o => o.FirstName, and passes it as an argument. This tree of objects is then traversed to find the MemberExpression which contains the name of the property. So the overhead is the creation and traversal of this small tree of objects. So it is a little less fast then passing a string, but the code is compile-time safe and supported by Visual Studio refactoring. This technique is generally known as static reflection.

Pex and Code Contracts

I’m currently experimenting with Pex, Moles and Code Contracts, and I wondered what effect code contracts have on Pex tests. So I build this simple piece of code: 1: public class Demo 2: { 3: public int Foo(int a, int b) 4: { 5: if (a < 0) 6: return a; 7: else if (a > 0) 8: return b; 9: else 10: return a + b; 11: } 12: } Then I make Pex generate its Parameterized Unit Tests (PUTs). This generates the following test: 1: [PexClass(typeof(Demo))] 2: [PexAllowedExceptionFromTypeUnderTest(typeof(InvalidOperationException))] 3: [PexAllowedExceptionFromTypeUnderTest(typeof(ArgumentException), AcceptExceptionSubtypes = true)] 4: [TestClass] 5: public partial class DemoTests 6: { 7: /// <summary>Test stub for Foo(Int32, Int32)</summary> 8: [PexMethod] 9: public int Foo( 10: [PexAssumeUnderTest]Demo target, 11: int a, 12: int b 13: ) 14: { 15: int result = target.Foo(a, b); 16: return result; 17: // TODO: add assertions to method DemoTests.Foo(Demo, Int32, Int32) 18: } 19: } I just leave the code, right-click on The Foo method of the DemoTests class, choose “Run Pex explorations” and I get this: As you can see the exploration calls my code with negative, zero and positive values. What happens when I add a contract stating that a should be positive? 1: public class Demo 2: { 3: public int Foo(int a, int b) 4: { 5: Contract.Requires(a > 0); 6: if (a < 0) 7: return a; 8: else if (a > 0) 9: return b; 10: else 11: return a + b; 12: } 13: } Only line 5 was added, but if I run the pex explorations again I get:   The effect is that Pex now doesn’t explore negative numbers, because it can deduce from the contract not to even try. What if I use a contract to state that b should be negative? 1: Contract.Requires(b < 0); Again Pex sees this and explores my code with negative b: One more. When I change my code to do a little more with b, like this: 1: public int Foo(int a, int b) 2: { 3: Contract.Requires(a > 0); 4: Contract.Requires(b < 0 || b > 10); 5: if (a < 0) 6: return a; 7: else if (a > 0) 8: { 9: if (b > a) 10: return a; 11: else 12: return b; 13: } 14: else 15: return a + b; 16: } and when I run the explorations again: So, you can guide Pex by supplying contracts on the arguments of your methods.

Overloading & Co/Contra-variance could break your code!

Co and contra-variance were introduced to VB.NET and C# to make working with certain classes more natural (“because it should work”). But beware, I was experimenting a bit with this and found following possible breaking change. I started with these two classes: C# 1: public class Person 2: { 3: // ... 4: } 5:  6: public class Vip : Person 7: { 8: // ... 9: } VB.NET 1: Public Class Person 2: ' 3: End Class 4:  5: Public Class Vip 6: Inherits Person 7: ' 8: End Class Then I added a collection of people: C# 1: public class PersonList 2: { 3: public void Add(object obj) { 4: // ... 5: } 6:  7: public void Add(Person p) 8: { 9: // ... 10: } 11:  12: public void Add(IEnumerable<Person> people) 13: { 14: foreach (Person p in people) 15: { 16: // ... 17: } 18: } 19: } VB.NET 1: Public Class PersonList 2: Public Sub Add(ByVal obj As Object) 3: ' 4: End Sub 5:  6: Public Sub Add(ByVal person As Person) 7: ' 8: End Sub 9:  10: Public Sub Add(ByVal list As IEnumerable(Of Person)) 11: ' 12: End Sub 13: End Class Next I create a PersonList collection and then add a list of Vip’s: C# 1: class Program 2: { 3: static void Main(string[] args) 4: { 5: PersonList people = new PersonList(); 6:  7: List<Vip> others = new List<Vip> { 8: new Vip(), new Vip() 9: }; 10:  11: people.Add(others); 12: } 13: } VB.NET 1: Sub Main() 2: Dim people As New PersonList 3: Dim others As New List(Of Vip)(New Vip() {New Vip(), New Vip()}) 4: people.Add(others) 5: End Sub When I compile this in Visual Studio 2008 the first Add method, taking the object argument, gets called. But with Visual Studio 2010 (using the .NET 4 target) the third Add method gets called. This is because now IEnumerable<T> is co-variant and will now convert the List<Vip> to an IEnumerable<Person>. Ok, granted. This is NOT something you will see every day, but if you ever encounter it, this might be very confusing. This kind of problem can also occur with extension methods and user-defined conversions…

Building a declarative WCF service using Workflow Foundation 4 and Content Based Correlation

This blog post accompanies my session on Workflow Foundation 4 programming during the Belgian Tech Days (actually developers and IT-pro days :)). During this session I built a WCF service using Workflow Foundation 4, and I want to show you how to do this on your own… In the first part you’ll learn how to create a simple FlowChart workflow and test it, and then in the second part you’ll learn how to setup correlation so multiple players can play the game… Preparing the lab This lab starts with a new project, so start Visual Studio 2010 and create a new workflow service project (call it NumberGuessingGame): This creates a new solution with a workflow project. Remove Service1.xamlx, we’re going to add a new service with a better name. Right-click the project and add a new item. Select the WCF Workflow Service template and name it GuessIt.xamlx. Creating the initial FlowChart This adds a workflow service with a sequential activity containing a Receive and Send activity. Delete the sequential activity leaving the workflow empty. Open the toolbox and drag a FlowChart activity onto the workflow designer. This should look like this now: Next drag a Receive activity onto the designer, below Start. Name it Start A New Game. Select the receive activity and enter following the OperationName, ServiceContractName and CanCreateInstance properties in the properties window: Next right-click on the receive activity and select the Create SendReply option from the drop-down menu. This add a Send activity to the workflow. The Send is coupled to the receive activity through its Request property: Now connect the activities: Adding variables Now, in the workflow designer open the variables window and add a new variable called player of type String. This will hold the player name received through the “Start A New Game” Receive activity. You will also see a variable of type CorrelationHandle. This is used to connect several receive and send activities through correlation. Rename this to gameHandle.We’ll use this handle later to setup content-based correlation. Now add two new variables theNumber and guess, both of type Int32. The first is the number the user needs to guess, to you need to initialize it to a random number between 0 and 100. Use the Random type to do this: Go back to the first receive activity. Click on the content property. This opens the Content Definition window. Select the player as the message data (and set the Message type to String): Do the same for the send activity, but now use following expression (of type String): 1: player + " guess the number between 0 and 100" You might also get validation errors because you renamed the correlation handle to gameHandle. Change the CorrelationInitilizers property to use gameHandle (click on the … button). Testing the service Press F5 in Visual Studio. The WCF Test Client will start and automatically fetch meta-data from the service. Double click the Begin method. This opens a new tab allowing you to invoke the Begin operation on the service. Enter your name and click Invoke. In the bottom area you should see the result of invoking the begin method. This concludes the first step. Adding the data contracts Let’s add a couple of data contracts to the project, one for a game and another for a guess. Note that both contain the player name, this way our service will be able to distinguish between multiple games, as long as each player uses a unique player name: 1: [DataContract] 2:  3: public class Game 4:  5: { 6:  7: [DataMember] 8:  9: public string PlayerName { get; set; } 10:  11: [DataMember] 12:  13: public string Message { get; set; } 14:  15: } 16:  17: [DataContract] 18:  19: public class Guess 20:  21: { 22:  23: [DataMember(IsRequired = true)] 24:  25: public string PlayerName { get; set; } 26:  27: [DataMember(IsRequired = true)] 28:  29: public int Try { get; set; } 30:  31: } Make the “Send Game Started” send activity use a Game instance as the result. To do this you will need to add another variable of type Game, and initialize it to a new instance: Now add to assign activities after the Begin receive activity, and assign appropriate values to the game: Adding the guessing logic Add another variable, calling it guess of type Guess. Then add another receive activity after the send activity, but now with an operation name called Guess, taking guess variable as its content. Check if the guess was right using a decision shape. If so, congratulate the user. If not, decide whether the guess was too small or large. Confirm this back to the user using three SendReply activities (create each one by right-clicking the Guess receive activity and select Create SendReply). Here is an example of the result: Adding Content-based Correlation Of course we want to make a single player play the same game, but what if multiple players are playing on the same server? We need correlation. In this part we’re going to correlate the guess activity to the Begin activity using the player’s name. That is why our data contract both contain the player’s name (by coincidence the properties have the same name, but this is not required). To do correlation we need a correlation handle. This handle will act like a key to a workflow. If a request comes in, we’re going to find this key through part of the message, in our case the player’s name. So, if you haven’t done so, add a new variable to the workflow called gameHandle of type CorrelationHandle. The make sure that all the first sendReply activity is set to initialize this handle by ensuring the CorrelationInitializer is set like this: When the workflow sends this message, we’re saying that the key of the gameHandle is the player’s name. So when another message arrives, the workflow runtime can then see is the message contains a valid key (again the player’s name), use this key to find the right workflow instance, and then send the message to it. So next step is to set the Guess receive activity’s Correlates On like this: Now you should be able to play a game, making sure that the first and following messages all have the same player name. You can also start two or more games, each with their own player name!

ObservableCollection<T> now part of .NET 4 (No need to reference WPF)

ObservableCollection<T> is a generic collection added as part of WPF and Silverlight. WinForms has BindingList<T>. So writing code that targets both WinForms and WPF would mean using BindingList<T> (the common thing) and writing code targetting WPF and Silverlight would mean ObservableCollection<T>. So there would be no way to write code that targets all three platforms. Luckily now (in .NET 4) we can use ObservableCollection<T> anywhere because Microsoft made it part of System.dll. Nice! Two others were also moved here: ReadOnlyObservableCollection<T> and INotifyCollectionChanged. To double check if I could use these collections outside WPF projects I created a simple console application using them: 1: class Program 2: { 3: static void Main(string[] args) 4: { 5: ObservableCollection<string> noWpf = new ObservableCollection<string> { "Hello", "World" }; 6: INotifyCollectionChanged watchCollection = noWpf as INotifyCollectionChanged; 7:  8: 9:  10: if (watchCollection != null) 11: { 12: watchCollection.CollectionChanged += (sender, e) => { Console.WriteLine("Collection action = {0}", e.Action); }; 13: } 14:  15: noWpf.Add("Love it!"); 16: } 17: } Compiles. Runs. Could I use it in WinForms? So I created a simple WinForms application like this: 1: public partial class Form1 : Form 2: { 3: ObservableCollection<string> noWpf; 4:  5: public Form1() 6: { 7: InitializeComponent(); 8:  9: noWpf = new ObservableCollection<string> { "Hello", "World" }; 10: var bs = new BindingSource() { DataSource = noWpf }; 11: bs.ListChanged += (sender, e) => { MessageBox.Show(e.ListChangedType.ToString()); }; 12: listBox1.DataSource = bs; 13: } 14:  15: private void button1_Click(object sender, EventArgs e) 16: { 17: noWpf.Add("Test"); 18: } 19: } But when I click the button, which adds a new element to the observable collection, the listbox doesn’t update. It looks like winforms databinding doesn’t support ObservableCollection…

Fixing Application Pool not starting problem by editing ApplicationHost.config

While playing around with Windows Server AppFabric I created a new Application pool set for .NET 4. However this application pool would immediately throw an error when starting; “The worker process failed to pre-load .Net Runtime version v4.0.21006.” A little experimentation showed that changing to the built in application pool worked. So it looks like IIS, when creating a new application pool, configures for the wrong .NET version. I then found that this kind of information is stored in ApplicationHost.config, but I couldn’t find it anywhere on my system (except backups in history). Hmmm. And I wasn’t the only one. So using notepad (!) I could open this file in “C:\Windows\System32\inetsrv\config”. Best of course is to first stop IIS and make a backup of the file. Then I edited the <applicationPools> section, looking for v4.0 and replacing it with v4.0.30128. And yes! That fixed it.

Using Model-View-ViewModel with WPF

In this blog post I want to show my way of implementing the Model-View-ViewModel pattern for WPF. I hope it serves as a simple example for those of you who want to start using it. The advantage of MVVM is that the view, which is a WPF thing, doesn’t contain any code. Instead the view uses a lot of data binding to bind itself to the data, but also to the functionality through commands. Because the model itself should not be dependent on any specific technology, we use a viewmodel, which is an adapter, to add the model things like extra data and commands. So again, the view doesn’t contain any code, the viewmodel does. This means that it is possible to download the view from a database, so you can vary the view per company/user, or make it very easy to update the view. Second advantage is that is makes it very easy to test your functionality using unit tests, which I think is the most important advantage… Use INotifyPropertyChanged So let’s get started. First of all you should understand INotifyPropertyChanged. To better understand the importance of INotifyPropertyChanged, you should understand how databinding works, for example in WPF. In WPF any control can data bind any of its dependency properties to any data through its data binding mechanism. The control doesn’t need to know about the data object (actually it can’t know the type of the data object because the control is usually written several years before the data object :) Dependency properties have this special feature, namely that you can register for the changed event of the dependency property. So when someone changes the control’s property, the databinding object will be notified and it can then update the data object’s property. INotifyPropertyChanged does the same thing. A databinding object from WPF knows about this interface and will query the data object for it. When it has this interface, the databinding object registers itself for any changed events, and updates the control’s property. This way you get two-way data binding, when one side changes, the other side gets updated as well. Likewise should you use ObservableCollection<T> for any collections, so these can notify any controls that the list has changed. Implementing INotifyPropertyChanged is simple. Start by defining a base class that implements the interface: 1: public class Entity : INotifyPropertyChanged 2: { 3: public event PropertyChangedEventHandler PropertyChanged; 4:  5: public virtual void RaisePropertyChanged(string propName) 6: { 7: if (PropertyChanged != null) 8: PropertyChanged(this, new PropertyChangedEventArgs(propName)); 9: } 10: } INotifyPropertyChanged requires you to implement the PropertyChanged event, and trigger it each time a property changes. This base class does it all… Then any object requiring it can derive from this base class; both model and viewmodel should derive and raise the PropertyChanged event when they change. In this example I’m going to use a Person class: 1: public class Person : Entity 2: { 3: public class Properties 4: { 5: public const string Name = "Name"; 6: public const string Age = "Age"; 7: } 8:  9: private string name; 10:  11: public string Name 12: { 13: get { return name; } 14: set 15: { 16: if (name != value) 17: { 18: name = value; 19: RaisePropertyChanged(Person.Properties.Name); 20: } 21: } 22: } 23:  24: private int age; 25:  26: public int Age { 27: get { return age; } 28: set { 29: if (age != value) { 30: age = value; 31: RaisePropertyChanged(Person.Properties.Age); 32: } 33: } 34: } 35: } There has been much discussion going on on how to implement the setter of your property, mainly in how to pass the property name to the event. I like to use a nested “Properties” class that contains a const string for each property name. This way I find it easy to refactor my property since there are no more strings. Other people like to use reflection but I hate the runtime overhead this brings. This technique also makes is easier to register for PropertyChanged events from other objects. If each class has a nested Properties class, you can easily code find the name of each property using intelli-sense. The Model. Your model should be exactly that. The representation of your business data, such as lists of things. In my example the model is very simple; it contains a list of Person objects. In real life these would the several lists of interrelated objects, such as products, customers, their orders, etc… The model can also have methods implementing business functionality. Any changes to the model should trigger events notifying any controls of changes… 1: public class PeopleModel /* : Entity */ 2: { 3: private ObservableCollection<Person> people 4: = new ObservableCollection<Person>() 5: { // Let's add some data to make it easier to verify data binding in view 6: new Person { Name = "Wesley", Age = 31 }, 7: new Person { Name = "Dimitri", Age = 28 }, 8: new Person { Name = "Gert", Age = 24 }, 9: new Person { Name = "Peter", Age = 24 } 10: }; 11:  12: public ObservableCollection<Person> People 13: { 14: get { return people; } 15: } 16:  17: /* and many more other collections and objects */ 18: } Please note that normally the model would also implement INotifyPropertyChanged, but because this model doesn’t need it, I haven’t gone to the trouble… Uncommenting the Entity base class will not change any functionality in this example so if you want, go right ahead! The ViewModel For me the ViewModel is an adapter, that only exposes stuff so the view can only access what is necessary. For example the view will contain a list of people, will have a current selected person and two fields so you can create a new person. The ViewModel will use data binding to couple itself to the view, but doesn’t know any specific things about the controls used. It does however implement INotifyPropertyChanged so changes to the ViewModel will update the view: 1: public class PersonVM : Entity 2: { 3: private PeopleModel model; 4:  5: public PersonVM(PeopleModel m) { model = m; } 6:  7: public ObservableCollection<Person> People 8: { 9: get { return model.People; } 10: } 11:  12: private string name; 13: public string Name 14: { 15: get { return name; } 16: set 17: { 18: if (name != value) 19: { name = value; RaisePropertyChanged("Name"); } 20: } 21: } 22:  23: private int age; 24: public int Age 25: { 26: get { return age; } 27: set 28: { 29: if (age != value) 30: { age = value; RaisePropertyChanged("Age"); } 31: } 32: } 33: 34: private Person current; 35:  36: public Person Current 37: { 38: get { return current; } 39: set 40: { 41: if (current != value) 42: { 43: current = value; 44: RaisePropertyChanged("Current"); 45: } 46: } 47: } 48:  49: public void DeleteCurrentPerson() 50: { 51: model.People.Remove(Current); 52: Current = model.People[0]; 53: } 54:  55: public ICommand DeleteCommand 56: { 57: get { return new RelayCommand(DeleteCurrentPerson); } 58: } 59:  60: public ICommand AddCommand 61: { 62: get { return new RelayCommand(AddPerson); } 63: } 64:  65: public void AddPerson() 66: { 67: Person newPerson = new Person { Name = this.Name, Age = this.Age }; 68: model.People.Add(newPerson); 69: Current = newPerson; 70: } 71: } So this ViewModel has a People property, which is the list we’re going to work with, a Current property to allow us to access the currently selected person (we’ll databind the selected item property of the list control to it), and a Name and Age property for entering new people. These properties will be data bound to two text boxes, so the user can enter stuff in the text boxes, but the command (which is on the viewmodel level) can access these properties, again not knowing which controls are being used. NOTE: the RaisePropertyChanged is not implemented here without the PersonVM.Properties.Property technique. That is because for this demo I also created a code snippet to make it easier to type in properties that implement INotifyPropertyChanged. It’s very easy to create your own, I’ve actually uploaded a little video on how to do this. The View If you’d ask me what is the best thing in WPF, then I would say DataTemplate! Our UI is going to databind to the ViewModel and we’re going to associate the View using a DataTemplate. WPF will automatically select our view when it databinds to our ViewModel. You can do this as follows: 1: <Window x:Class="ModelViewViewModelLab.Window1" 2: xmlns="" 3: xmlns:x="" 4: xmlns:viewModels="clr-namespace:ModelViewViewModelLab.ViewModels" 5: xmlns:views="clr-namespace:ModelViewViewModelLab.Views" 6: Title="Window1" Height="300" Width="300"> 7: <Window.Resources> 8: <!-- voor elk soort van viewmodel mappen we dit naar een view --> 9: <DataTemplate DataType="{x:Type viewModels:PersonVM }"> 10: <views:PersonView /> 11: </DataTemplate> 12: </Window.Resources> 13: <Grid> 14: <ContentControl Content="{Binding}" /> 15: </Grid> 16: </Window> This way the view will be created with its DataContext set to the view model, so we can use DataContext relative data binding: 1: <UserControl x:Class="ModelViewViewModelLab.Views.PersonView" 2: xmlns="" 3: xmlns:x="" 4: xmlns:model="clr-namespace:ModelViewViewModelLab.Models" 5: Height="300" Width="300"> 6: <UserControl.Resources> 7: <DataTemplate DataType="{x:Type model:Person}"> 8: <StackPanel Orientation="Vertical"> 9: <StackPanel Orientation="Horizontal"> 10: <TextBox Text="{Binding Name}" /> 11: <TextBlock Text=" - " /> 12: <TextBox Text="{Binding Age}" /> 13: </StackPanel> 14: <ProgressBar Width="100" Height="16" Value="{Binding Age}" /> 15: </StackPanel> 16: </DataTemplate> 17: </UserControl.Resources> 18: <Grid> 19: <Grid.ColumnDefinitions> 20: <ColumnDefinition /> 21: <ColumnDefinition /> 22: </Grid.ColumnDefinitions> 23: <!-- People listbox --> 24: <ListBox ItemsSource="{Binding People}" 25: Name="theList" 26: SelectedItem="{Binding Current}" /> 27: <!-- Command part --> 28: <StackPanel Orientation="Vertical" Grid.Column="1"> 29: <ContentControl Content="{Binding Current}" /> 30: <!-- New person --> 31: <TextBox Text="{Binding Name}" /> 32: <TextBox Text="{Binding Age}" /> 33: <Button Content="Add" Command="{Binding AddCommand, Mode=OneWay}" /> 34: <Button Content="Delete" Command="{Binding DeleteCommand, Mode=OneWay}"/> 35: </StackPanel> 36: </Grid> 37: </UserControl> Note the buttons in the view; these are data bound to the RelayCommand instances on the view model. Clicking these will execute the associated command. 1: public class RelayCommand : ICommand 2: { 3: Action action; 4:  5: public RelayCommand(Action execute) 6: { 7: action = execute; 8: } 9:  10: public bool CanExecute(object parameter) 11: { 12: return true; 13: } 14:  15: public event EventHandler CanExecuteChanged; 16:  17: public void Execute(object parameter) 18: { 19: action(); 20: } 21: } The Commands RelayCommand is a (too) simple implementation of the command pattern in WPF, but it allows us to easily execute a method on the model or viewModel. Our viewModel uses it to expose two commands: add and delete: 1: public ICommand DeleteCommand 2: { 3: get { return new RelayCommand(DeleteCurrentPerson); } 4: } 5:  6: public ICommand AddCommand 7: { 8: get { return new RelayCommand(AddPerson); } 9: } Thanks to delegates this is easy! The commands each execute a method on the viewModel: 1: public void AddPerson() 2: { 3: Person newPerson = new Person { Name = this.Name, Age = this.Age }; 4: model.People.Add(newPerson); 5: Current = newPerson; 6: } AddPerson requires a couple of inputs, the name and age of the new person to add; its gets these from the Name and Age properties on the ViewModel. How did these get filled in? The view has a Textbox data bound to each! 1: public void DeleteCurrentPerson() 2: { 3: model.People.Remove(Current); 4: Current = model.People[0]; 5: } DeleteCurrentPerson will remove the currently selected person from the list of people. Again the Current property is data bound to the selectedItem property of the listbox, so when you select something from the listbox, the current property will be automatically updated. After deleting the current person, we select the first person from the list. Carefull, this will give a nice bug when the last person is removed from the list, but fixing this should be easy… So, MVVM. That’s it. Of course you could add a lot using other patterns, but that is for another time…