Navigation with MVVM

December 18, 2011

When I first started out with MVVM, I was lost about how you should navigate between pages. I’m a firm believer in using ViewModels to do everything (unless it’s View-specific code), and that the UI is simply a user-friendly interface for your ViewModels. I did not want to create a button on a page that has any kind of code-behind to switch pages, and I didn’t like the idea of my navigation being spread out throughout all the ViewModels.

I finally came to realize the solution was simple: I needed a ViewModel for the Application itself, which contained the application state, such as the CurrentPage.

Here is an example that builds on the Simple MVVM Example.

The ViewModel

Usually I name the ViewModel ApplicationViewModel or ShellViewModel, but you can call it whatever you want. It is the startup page of the application, and it is usually the only page or window object in my project.

It usually contains

    List<ViewModelBase> PageViewModels
    ViewModelBase CurrentPage
    ICommand ChangePageCommand

Here is an example ApplicationViewModel that I would use to go with the Simple MVVM Example.


    public class ApplicationViewModel : ObservableObject
    {
        #region Fields

        private ICommand _changePageCommand;

        private IPageViewModel _currentPageViewModel;
        private List<IPageViewModel> _pageViewModels;

        #endregion

        public ApplicationViewModel()
        {
            // Add available pages
            PageViewModels.Add(new HomeViewModel());
            PageViewModels.Add(new ProductsViewModel());

            // Set starting page
            CurrentPageViewModel = PageViewModels[0];
        }

        #region Properties / Commands

        public ICommand ChangePageCommand
        {
            get
            {
                if (_changePageCommand == null)
                {
                    _changePageCommand = new RelayCommand(
                        p => ChangeViewModel((IPageViewModel)p),
                        p => p is IPageViewModel);
                }

                return _changePageCommand;
            }
        }

        public List<IPageViewModel> PageViewModels
        {
            get
            {
                if (_pageViewModels == null)
                    _pageViewModels = new List<IPageViewModel>();

                return _pageViewModels;
            }
        }

        public IPageViewModel CurrentPageViewModel
        {
            get
            {
                return _currentPageViewModel;
            }
            set
            {
                if (_currentPageViewModel != value)
                {
                    _currentPageViewModel = value;
                    OnPropertyChanged("CurrentPageViewModel");
                }
            }
        }

        #endregion

        #region Methods

        private void ChangeViewModel(IPageViewModel viewModel)
        {
            if (!PageViewModels.Contains(viewModel))
                PageViewModels.Add(viewModel);

            CurrentPageViewModel = PageViewModels
                .FirstOrDefault(vm => vm == viewModel);
        }

        #endregion
    }

This won’t compile right away because I’ve made some changes to it. For one, all my PageViewModels now inherit from an IPageViewModel interface so they can have some common properties, such as a Name.

I also created a new HomeViewModel and HomeView since its hard to demonstrate navigation unless you have at least 2 pages. The HomeViewModel is a blank class that inherits from IPageViewModel, and the HomeView is just a blank UserControl.

In addition, I added an s to ProductsViewModel since it really deals with multiple products, not a single one.

An added advantage to having a ViewModel to control the application state is that it can also be used to handle other application-wide objects, such as Current User, or Error Messages.

The View

I also need an ApplicationView for my ApplicationViewModel. It needs to contain some kind of Navigation that shows the list of PageViewModels, and clicking on a PageViewModel should execute the ChangePage command.

It also needs to contain a control to display the CurrentPage property, and I usually use a ContentControl for that. This allows me to use DataTemplates to tell WPF how to draw each IPageViewModel.

<Window x:Class="SimpleMVVMExample.ApplicationView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:SimpleMVVMExample"
        Title="Simple MVVM Example" Height="350" Width="525">

    <Window.Resources>
        <DataTemplate DataType="{x:Type local:HomeViewModel}">
            <local:HomeView />
        </DataTemplate>
        <DataTemplate DataType="{x:Type local:ProductsViewModel}">
            <local:ProductsView />
        </DataTemplate>
    </Window.Resources>

    <DockPanel>
        <Border DockPanel.Dock="Left" BorderBrush="Black" BorderThickness="0,0,1,0">
            <ItemsControl ItemsSource="{Binding PageViewModels}">
                <ItemsControl.ItemTemplate>
                    <DataTemplate>
                        <Button Content="{Binding Name}"
                                Command="{Binding DataContext.ChangePageCommand, RelativeSource={RelativeSource AncestorType={x:Type Window}}}"
                                CommandParameter="{Binding }"
                                Margin="2,5"/>
                    </DataTemplate>
                </ItemsControl.ItemTemplate>
            </ItemsControl>
        </Border>

        <ContentControl Content="{Binding CurrentPageViewModel}" />
    </DockPanel>
</Window>

In this example, I’m using an ItemsControl to display my PageViewModels. Each item is drawn using a Button, and the Button’s Command property is bound to the ChangePageCommand.

Since the Button’s DataContext is the PageViewModel, I used a RelativeSource binding to find the ChangePageCommand. I know that my Window is the ApplicationView, and it’s DataContext is the ApplicationViewModel, so this binding looks up the VisualTree for the Window tag, and gets bound to Window.DataContext.ChangePageCommand.

Also note that I am putting DataTemplates in Window.Resources to tell WPF how to draw each IPageViewModel. By default, if WPF encounters an object in it’s visual tree that it doesn’t know how to handle, it will draw it using a TextBlock containing the .ToString() method of the object. By defining a DataTemplate, I am telling WPF to use a specific template instead of defaulting to a TextBlock.

If you are continuing from the Simple MVVM Example, I moved the ProductView out of a ResourceDictionary and into a UserControl to make this simpler.

Starting the Example

The last thing to do is change App.xaml to make ApplicationView and ApplicationViewModel our startup, instead of ProductView/ProductViewModel.

public partial class App : Application
{
    protected override void OnStartup(StartupEventArgs e)
    {
        base.OnStartup(e);

        ApplicationView app = new ApplicationView();
        ApplicationViewModel context = new ApplicationViewModel();
        app.DataContext = context;
        app.Show();
    }
}

Run the project and you should see something that looks like the images below, which quickly switches the CurrentPage when clicking on the Navigation buttons.

Screenshot of HomeHome

Screenshot of ProductsProducts

Summary

And there you have it. A simple navigation example with MVVM.

You can download the source code for this sample from here.

Once you get more comfortable with WPF, I would recommend looking into using a Messaging System, such as MVVM Light’s Messenger, or Microsoft Prism’s EventAggregator to broadcast ChangePage commands from any ViewModel so you wouldn’t need to find the ApplicationViewModel to execute the ChangePageCommand, however that’s for another day.

<< Back – A Simple MVVM Example
>> Next – Communication between ViewModels


Navigation with MVVM

July 17, 2011

NOTE: I have re-written this article here to provide a better example, and some code samples.


When I first started out with MVVM, I was lost about how you should navigate between pages. I’m a firm believer in using ViewModels to do everything (unless it’s View-specific code), and that the UI is simply a user-friendly interface for your ViewModels. I did not want to create a button on a page that has any kind of code-behind to switch pages, and I didn’t like the idea of my navigation being spread out throughout all the ViewModels.

I finally came to realize the solution was simple: I needed a ViewModel for the Application itself, which contained the application state, such as the CurrentPage.

The ViewModel

Usually I name the ViewModel ShellViewModel or AppViewModel, but you can call it whatever you want. It is the startup page or window in the application, and is usually the only page/window object in my project.

It looks something like this:

    public class ShellViewModel : INotifyPropertyChanged
    {
        private ICommand _changeViewModelCommand;

        private object _currentViewModel;
        private List<object> _viewModels = new List<object>();

        public ShellViewModel()
        {
            ViewModels.Add(new HomeViewModel());
            CurrentViewModel = ViewModels[0];
        }

        private void ChangeViewModel(object viewModel)
        {
            if (!ViewModels.Contains(viewModel))
                ViewModels.Add(viewModel);

            CurrentViewModel = ViewModels.FirstOrDefault(vm => vm == viewModel);
        }
    }

I have omitted the public property implementation and the INotifyPropertyChanged interface methods for sake of readability. The public properties are your standard get/set properties that raise a PropertyChange notification, and the public ChangeViewModelCommand points to the ChangeViewModel method.

The View

I also need a ShellView for my ShellViewModel, so this is what it usually looks like:

<Grid Margin="20">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!-- Header -->
        <TextBlock Text="Application Name" FontWeight="Bold" FontSize="24" />

        <Line Grid.Row="1" Stroke="Black" Margin="0,5" StrokeThickness="1" Stretch="Fill" X2="1" />        <!-- Content -->
        <ContentControl Grid.Row="2" Content="{Binding CurrentViewModel}"/>
</Grid>

It contains a standard frame for the application, in this case the application name with a horizontal line under it, followed by the CurrentPage in a ContentControl. When a page is displayed in the ContentControl, WPF will lookup the appropriate DataTemplate to use to display that ViewModel (page).

Changing Pages

The ChangePage command can either be called directly through the XAML using something like this:

<Button Command="{Binding RelativeSource={RelativeSource
            AncestorType={x:Type Window}},
            Path=DataContext.ChangeViewModelCommand}
        CommandParameter="{Binding EditCustomerViewModel}"
        />

or you can use an Event system such as PRISM’s EventAggregator or MVVM Light’s Messenger, and the ViewModels would raise a ChangePage event. I prefer to use an Event system personally, but use whatever works for you.

For more on switching between Pages/Views, see this article


Communication between ViewModels with MVVM

June 5, 2011

Communication between ViewModels can be tricky at first glance, but there are some easy frameworks and patterns that can help you out.

For example, lets say you had a ShellViewModel that controlled your entire application, and it has an Exit command. Now lets say one of the sub-view-models in the application would like to call this exit command.

The easiest way is through a Relative Binding to the Window’s DataContext.ExitCommand.

<Button Command="{Binding 
    RelativeSource={RelativeSource AncestorType={x:Type Window},
    Path=DataContext.ExitCommand}}

Of course, this only works well if you can find the control containing the command you want through a binding.

Enter the Event System

Another way is through a loosely coupled event system. I would recommend either MVVM Light’s Messenger or Microsoft Prism’s EventAggregator. You could always build your own event system as well if you really wanted to.

Both event systems makes me think of a paging system. Any ViewModel can broadcast a message, and any ViewModel can subscribe to receive broadcasted messages. Messages are often packaged into different types, so a ViewModel can subscribe to only listen for specific message types such as CloseApplicationMessages.

This kind of loosely coupled event system is ideal for MVVM because the ViewModels don’t need to know about each other to be able to do their job.

Using either of these systems, your ViewModel that handles the event would subscribe to receive a specific message type. It would tell the event system that if a message of type X is broadcasted, send it to a specified delegate method. The ViewModel that wishes to raise the event simply has to broadcast the message and include any parameters needed in the body of the message.

Example using Prism’s EventAggregator

// Subscribe
eventAggregator.GetEvent<CloseAppliactionMessage>().Subscribe(ExitMethod);

// Broadcast
eventAggregator.GetEvent<CloseAppliactionMessage>().Publish();

(I also have some code posted here which simplifies PRISM’s EventAggregator for smaller applications)

Example using MVVM Light’s Messenger

//Subscribe
Messenger.Default.Register<CloseAppliactionMessage>(ExitMethod);

// Broadcast
Messenger.Default.Send<CloseAppliactionMessage>()

Switching between Views/UserControls using MVVM

May 28, 2011

I often like to browse questions tagged with WPF or MVVM on StackOverflow, and one question I see come up frequently is how to switch between Views or UserControls in MVVM.

Once you get used to the fact that the ViewModels run your application, it is very simple. Your ViewModel contains a property which defines your CurrentView and to switch views you simply switch your ViewModel’s properties.

There is more than one way to do this, but I’ll show  two common examples.

Using a DataTemplate

The simplest way is to use a DataTemplate. Lets say you have a window with two screens you want to switch between: Home and Work. You have a Menu at the top with two options: View Home Page and View Work Page. Your ViewModel behind this page would have a CurrentView property, along with two commands: ViewHomeCommand and ViewWorkCommand.

public class AppViewModel
{
    // simplified properties
    public ViewModelBase CurrentView {get; set;}
    public ICommand ViewHomeCommand {get;}
    public ICommand ViewWorkCommand {get;}
}

The page’s content would simply be a ContentControl with it’s content bound to CurrentView.

<ContentControl Content="{Binding CurrentView}" />

Now all you need to do is define two DataTemplates, one for the Home and one for the Work

<DataTemplate DataType="{x:Type HomeViewModel}">
    <TextBlock Text="This is the Home Page" />
</DataTemplate>
<DataTemplate DataType="{x:Type WorkViewModel}">
    <localControls:WorkViewUserControl />
</DataTemplate>

When you execute ViewHome/Work commands, all it is doing is setting the CurrentView to an instance of either HomeViewModel, or WorkViewModel. That ViewModel is getting bound to the ContentControl, and the DataTemplate is used to determine how to display the object.

Using a DataTrigger

Sometimes you may want to switch views based on a property instead of a type. Lets say you’re viewing a list of Consumers, and you want to display one View if the consumer is a Business, and a different View if the Consumer is a Person. In this case, you would use a DataTrigger

<DataTemplate x:Key="PersonTemplate" DataType="{x:Type local:ConsumerViewModel}">
     <TextBlock Text="I'm a Person" />
</DataTemplate> 

<DataTemplate x:Key="BusinessTemplate" DataType="{x:Type local:ConsumerViewModel}">
     <TextBlock Text="I'm a Business" />
 </DataTemplate>

<DataTemplate DataType="{x:Type local:ConsumerViewModel}">
     <ContentControl Content="{Binding }">
         <ContentControl.Style>
             <Style TargetType="{x:Type ContentControl}">
                 <Setter Property="ContentTemplate" Value="{StaticResource PersonTemplate}" />
                 <Style.Triggers>
                     <DataTrigger Binding="{Binding ConsumerType}" Value="Business">
                         <Setter Property="ContentTemplate" Value="{StaticResource BusinessTemplate}" />
                     </DataTrigger>
                 </Style.Triggers>
             </Style>
         </ContentControl.Style>
     </ContentControl>
 </DataTemplate>

Here we have a DataTemplate for a Consumer object which defaults to the PersonTemplate. In a DataTrigger, it checks to see if the ConsumerType is a Business and if so switches to a BusinessTemplate. All you have left to do is bind the Consumer object to a ContentControl to display it.

<ContentControl Content="{Binding CurrentConsumer}" />

Summary

Switching between views using MVVM isn’t hard at all. You just need to remember that the ViewModel is driving the application, not the View. The way I see it, your application should be able to run through a command-line interface, so everything your application needs to know about its state should be in the ViewModels somewhere. This includes the current view.


A Simple MVVM Example

May 8, 2011

In my opinion, if you are using WPF or Silverlight you should be using the MVVM design pattern. It is perfectly suited to the technology and allows you to keep your code clean and easy to maintain.

The problem is, there are a lot of online resources for MVVM, each with their own way of implementing the design pattern and it can be overwhelming. I would like to present MVVM in the simplest way possible using just the basics.

So lets start at the beginning.

MVVM

MVVM is short for Model-View-ViewModel.

Models are simple class objects that hold data. They should only contain properties and property validation. They are not responsible for getting data, saving data, click events, complex calculations, business rules, or any of that stuff.

Views are the UI used to display data. In most cases, they can be DataTemplates which is simply a template that tells the application how to display a class. It is OK to put code behind your view IF that code is related to the View only, such as setting focus or running animations.

ViewModels are where the magic happens. This is where the majority of your code-behind goes: data access, click events, complex calculations, business rules validation, etc. They are typically built to reflect a View. For example, if a View contains a ListBox of objects, a Selected object, and a Save button, the ViewModel will have an ObservableCollection ObectList, Model SelectedObject, and ICommand SaveCommand.

MVVM Example

I’ve put together a small sample showing these 3 layers and how they relate to each other. You’ll notice that other than property/method names, none of the objects need to know anything about the others. Once the interfaces have been designed, each layer can be built completely independent of the others.

Sample Model

For this example I’ve used a Product Model. You’ll notice that the only thing this class contains is properties and change notification code.

Usually I would also implement IDataErrorInfo here for property validation, however I have left this out for now.

    public class ProductModel : ObservableObject
    {
        #region Fields

        private int _productId;
        private string _productName;
        private decimal _unitPrice;

        #endregion // Fields

        #region Properties

        public int ProductId
        {
            get { return _productId; }
            set
            {
                if (value != _productId)
                {
                    _productId = value;
                    OnPropertyChanged("ProductId");
                }
            }
        }

        public string ProductName
        {
            get { return _productName; }
            set
            {
                if (value != _productName)
                {
                    _productName = value;
                    OnPropertyChanged("ProductName");
                }
            }
        }

        public decimal UnitPrice
        {
            get { return _unitPrice; }
            set
            {
                if (value != _unitPrice)
                {
                    _unitPrice = value;
                    OnPropertyChanged("UnitPrice");
                }
            }
        }

        #endregion // Properties
    }

The class inherits from ObservableObject, which is a custom class I use to avoid having to rewrite the property change notification code repeatedly. I would actually recommend looking into Microsoft PRISM’s NotificationObject  or MVVM Light’s ViewModelBase which does the same thing once you are comfortable with MVVM, but for now I wanted to keep 3rd party libraries out of this and to show the code.

    public abstract class ObservableObject : INotifyPropertyChanged
    {
        #region INotifyPropertyChanged Members

        /// <summary>
        /// Raised when a property on this object has a new value.
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;

        /// <summary>
        /// Raises this object's PropertyChanged event.
        /// </summary>
        /// <param name="propertyName">The property that has a new value.</param>
        protected virtual void OnPropertyChanged(string propertyName)
        {
            this.VerifyPropertyName(propertyName);

            if (this.PropertyChanged != null)
            {
                var e = new PropertyChangedEventArgs(propertyName);
                this.PropertyChanged(this, e);
            }
        }

        #endregion // INotifyPropertyChanged Members

        #region Debugging Aides

        /// <summary>
        /// Warns the developer if this object does not have
        /// a public property with the specified name. This
        /// method does not exist in a Release build.
        /// </summary>
        [Conditional("DEBUG")]
        [DebuggerStepThrough]
        public virtual void VerifyPropertyName(string propertyName)
        {
            // Verify that the property name matches a real,
            // public, instance property on this object.
            if (TypeDescriptor.GetProperties(this)[propertyName] == null)
            {
                string msg = "Invalid property name: " + propertyName;

                if (this.ThrowOnInvalidPropertyName)
                    throw new Exception(msg);
                else
                    Debug.Fail(msg);
            }
        }

        /// <summary>
        /// Returns whether an exception is thrown, or if a Debug.Fail() is used
        /// when an invalid property name is passed to the VerifyPropertyName method.
        /// The default value is false, but subclasses used by unit tests might
        /// override this property's getter to return true.
        /// </summary>
        protected virtual bool ThrowOnInvalidPropertyName { get; private set; }

        #endregion // Debugging Aides
    }

In addition to the INotifyPropertyChanged methods, there is also a debug method to validate the PropertyName. This is because the PropertyChange notification gets passed in as a String, and I have caught myself forgetting to change this string when I change the name of a Property.

NoteThe PropertyChanged notification exists to alert the View that a value has changed so it knows to update. I have seen suggestions to drop it from the Model and to expose the Model’s properties to the View from the ViewModel instead of the Model, however I find in most cases this complicates things and requires extra coding. Exposing the Model to the View via the ViewModel is much simpler, although either method is valid.

Sample ViewModel

I am doing the ViewModel next because I need it before I can create the View. This should contain everything the User would need to interact with the page. Right now it contains 4 properties: a ProductModel, a GetProduct command, a SaveProduct command, an a ProductId used for looking up a product.

    public class ProductViewModel : ObservableObject
    {
        #region Fields

        private int _productId;
        private ProductModel _currentProduct;
        private ICommand _getProductCommand;
        private ICommand _saveProductCommand;

        #endregion

        #region Public Properties/Commands

        public ProductModel CurrentProduct
        {
            get { return _currentProduct; }
            set
            {
                if (value != _currentProduct)
                {
                    _currentProduct = value;
                    OnPropertyChanged("CurrentProduct");
                }
            }
        }

        public ICommand SaveProductCommand
        {
            get
            {
                if (_saveProductCommand == null)
                {
                    _saveProductCommand = new RelayCommand(
                        param => SaveProduct(),
                        param => (CurrentProduct != null)
                    );
                }
                return _saveProductCommand;
            }
        }

        public ICommand GetProductCommand
        {
            get
            {
                if (_getProductCommand == null)
                {
                    _getProductCommand = new RelayCommand(
                        param => GetProduct(),
                        param => ProductId > 0
                    );
                }
                return _getProductCommand;
            }
        }

        public int ProductId
        {
            get { return _productId; }
            set
            {
                if (value != _productId)
                {
                    _productId = value;
                    OnPropertyChanged("ProductId");
                }
            }
        }

        #endregion

        #region Private Helpers

        private void GetProduct()
        {
            // You should get the product from the database
            // but for now we'll just return a new object
            ProductModel p = new ProductModel();
            p.ProductId = ProductId;
            p.ProductName = "Test Product";
            p.UnitPrice = 10.00;
            CurrentProduct = p;
        }

        private void SaveProduct()
        {
            // You would implement your Product save here
        }

        #endregion
    }

There is another new class here: the RelayCommand. This is essential for MVVM to work. It is a command that is meant to be executed by other classes to run code in this class by invoking delegates. Once again, I’d recommend checking out the MVVM Light Toolkit’s version of this command when you are more comfortable with MVVM, but I wanted to keep this simple so have included this code here.

    /// <summary>
    /// A command whose sole purpose is to relay its functionality to other
    /// objects by invoking delegates. The default return value for the
    /// CanExecute method is 'true'.
    /// </summary>
    public class RelayCommand : ICommand
    {
        #region Fields

        readonly Action<object> _execute;
        readonly Predicate<object> _canExecute;

        #endregion // Fields

        #region Constructors

        /// <summary>
        /// Creates a new command that can always execute.
        /// </summary>
        /// <param name="execute">The execution logic.</param>
        public RelayCommand(Action<object> execute)
            : this(execute, null)
        {
        }

        /// <summary>
        /// Creates a new command.
        /// </summary>
        /// <param name="execute">The execution logic.</param>
        /// <param name="canExecute">The execution status logic.</param>
        public RelayCommand(Action<object> execute, Predicate<object> canExecute)
        {
            if (execute == null)
                throw new ArgumentNullException("execute");

            _execute = execute;
            _canExecute = canExecute;
        }

        #endregion // Constructors

        #region ICommand Members

        [DebuggerStepThrough]
        public bool CanExecute(object parameters)
        {
            return _canExecute == null ? true : _canExecute(parameters);
        }

        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        public void Execute(object parameters)
        {
            _execute(parameters);
        }

        #endregion // ICommand Members
    }

Sample View

And now the Views. These are DataTemplates which define how a class should be displayed to the User. There are many ways to add these templates to your application, but the simplest way is to just add them to the startup window’s Resources.

<Window.Resources>
    <DataTemplate DataType="{x:Type local:ProductModel}">
        <Border BorderBrush="Black" BorderThickness="1" Padding="20">
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition />
                    <ColumnDefinition />
                </Grid.ColumnDefinitions>
                <Grid.RowDefinitions>
                    <RowDefinition />
                    <RowDefinition />
                    <RowDefinition />
                </Grid.RowDefinitions>

                <TextBlock Grid.Column="0" Grid.Row="0"  Text="ID" VerticalAlignment="Center" />
                <TextBox Grid.Row="0" Grid.Column="1"  Text="{Binding ProductId}" />

                <TextBlock Grid.Column="0" Grid.Row="1"  Text="Name" VerticalAlignment="Center" />
                <TextBox Grid.Row="1" Grid.Column="1"  Text="{Binding ProductName}" />

                <TextBlock Grid.Column="0" Grid.Row="2"  Text="Unit Price" VerticalAlignment="Center" />
                <TextBox Grid.Row="2" Grid.Column="1"  Text="{Binding UnitPrice}" />

            </Grid>
        </Border>
    </DataTemplate>

    <DataTemplate DataType="{x:Type local:ProductViewModel}">
        <DockPanel Margin="20">
            <DockPanel DockPanel.Dock="Top">
                <TextBlock Margin="10,2" DockPanel.Dock="Left" Text="Enter Product Id" VerticalAlignment="Center" />

                <TextBox Margin="10,2" Width="50" VerticalAlignment="Center" Text="{Binding Path=ProductId, UpdateSourceTrigger=PropertyChanged}" />

                <Button Content="Save Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
                        Command="{Binding Path=SaveProductCommand}" Width="100" />

                <Button Content="Get Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
                        Command="{Binding Path=GetProductCommand}" IsDefault="True" Width="100" />
            </DockPanel>

            <ContentControl Margin="20,10" Content="{Binding Path=CurrentProduct}" />
        </DockPanel>
    </DataTemplate>
</Window.Resources>

The View defines two DataTemplates: one for the ProductModel, and one for the ProductViewModel. You’ll need to add a namespace reference  to the Window definition pointing to your Views/ViewModels so you can define the DataTypes. Each DataTemplate only binds to properties belonging to the class it is made for.

In the ViewModel template, there is a ContentControl that is bound to ProductViewModel.CurrentProduct. When this control tries to display the CurrentProduct, it will use the ProductModel DataTemplate.

Starting the Sample

And finally, to start the application add the following on startup:

MainWindow app = new MainWindow();
ProductViewModel viewModel = new ProductViewModel();
app.DataContext = viewModel;
app.Show();

This is found in the code behind the startup file – usually App.xaml.cs.

This creates your Window (the one with the DataTemplates defined in Window.Resources), creates a ViewModel, and it sets the Window’s DataContext to the ViewModel.

And there you have it. A basic look at MVVM.

UPDATE
Sample code can be found here.

Notes

There are many other ways to do the things shown here, but I wanted to give you a good starting point before you start diving into the confusing world of MVVM.

The important thing to remember about using MVVM is your Forms, Pages, Buttons, TextBoxes, etc (the Views) are NOT your application. Your ViewModels are. The Views are merely a user-friendly way to interact with your ViewModels.

So if you want to change pages, you should not be changing pages in the View, but instead you should be setting something like the AppViewModel.CurrentPage = YourPageViewModel. If you want to run a Save method, you don’t put that behind a button’s Click event, but rather bind the Button.Command to a ViewModel’s ICommand property.

I started with  Josh Smith’s article on MVVM, which was a good read but for a beginner like me, some of these concepts flew right over my head.

I’ve never done a blog or tutorial before, but I noticed there is a lot of confusion about what MVVM is and how to use it. Since I struggled through the maze of material online to figure out what MVVM is and how its used, I thought I’d try and write a simpler explanation. I hope this clarifies things a bit and doesn’t make it worse 🙂

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