Recommendations and best practices for implementing MVVM and XAML/.NET applications

January 30, 2015, (updated on June 15, 2018), 37 comments, Software Development

In this article I’ll describe the rules and practices I’m following for XAML and MVVM application development. I’m using the described techniques since multiple years and they have proven themselves for me. Most of the described rules apply for all types of XAML projects – Windows Store, Windows Phone, Silverlight and WPF projects.

Sample code: The source code of the application Visual JSON Editor contains sample implementations for most of the discussed recommendations. Some sample code snippets in this article use the MVVM classes from the MyToolkit library which is developed by myself.

Project file structure

All MVVM/XAML applications should have a similar directory structure. The following tree shows a possible structure of a XAML application project:

  • App.xaml
  • Controls: Reusable UI controls (application independent views) without view models.
  • Localization: Classes and resources for application localization
    • Strings.resx/.resw
  • Models: Model and domain classes
  • ViewModels: View models classes
    • MainWindowModel.cs
    • MyViewModel.cs
    • Dialogs
      • SelectItemDialogModel.cs
  • Views: Contains the views
    • MainWindow.xaml
    • MyView.xaml
    • Dialogs
      • SelectItemDialog.xaml

As shown in the list, the name of a view should end with its type:

  • *Window: A non-modal window
  • *Dialog: A (modal) dialog window
  • *Page: A page view (mostly used in Windows Phone and Windows Store apps)
  • *View: A view which is used as subview in another view, page, window or dialog

The name of a view model should be composed of the corresponding view’s name and the word “Model”. The view models are stored in the same location in the ViewModels directory as their corresponding views in the Views directory.

For bigger projects or projects where the view models are used for multiple platforms, the ViewModels as well as the Localization directory (the UI independent components) can be moved to a separate (PCL) project which can be referenced by multiple application projects.

Rules for views and view models

All views should follow the MVVM pattern to separate the user interface from the application logic. There is always a one-to-one relationship between the view and view model classes and instances. In other words, a view model class or instance belongs to exactly one view and a view has exactly one view model. Only the “parent” view has a reference to the view model, other views are not allowed to access another view’s view model (except unit test classes).

It is not allowed to directly access the view object from the view model (also not through an interface). To send commands from the view model to the view or call methods on the view, use a messenger (described later) or other IoC mechanisms (e.g. services).

If a view becomes too large, it is recommended to split it up into multiple views.

You should use a view model base class as well as an INotifyPropertyChanged base class to simplify observable property implementations like this:

public class MySampleViewModel : ViewModelBase // from MyToolkit
    private Document _selectedDocument; 

    public Document SelectedDocument
        get { return _selectedDocument; }
        set { Set(ref _selectedDocument, value); }

Tipp: The page ViewModelBase from my library shows a sample implementation of a view model as well as the code of a view model base class. Also check out the page ObservableObject for a INotifyPropertyChanged base implementation and its usage.

User controls vs templated controls

When a new user interface element is needed, you have to decide whether to implement a user control (view with view model) or a templated control (UI control).

My general rule for choosing between these two types is:

  • Create a user control when implementing an application specific view with a view model and few dependency properties which have to be synchronized with properties of the view model. These views should be put into the Views directory.
  • Create a templated control when implementing a reusable UI control without view model and/or lots of dependency properties which are directly bound to attributes in XAML. These controls should be put into the Controls directory.

Further reading: Read this article for more information about the differences of the two control types.

View model instantiation and assignment

My recommendation is that the view model should be instantiated in XAML in the Resources element of the root element. Then it is bound to the DataContext property of the child element of the view’s root element. The following sample XAML shows the described implementation:

<UserControl x:Class="My.Namespace.MySampleView" ...> 
        <viewModels:MySampleViewModel x:Key="ViewModel" /> 

    <Grid DataContext="{StaticResource ViewModel}"> 

To access the view model in the code-behind of the view, simply add the following property to your view’s code-behind class (for example in MySampleView.xaml.cs):

protected MySampleViewModel Model 
    get { return (MySampleViewModel)Resources["ViewModel"]; } 

The benefit of instantiating the view models as a resource in XAML are:

  • The Visual Studio’s XAML editor “knows” the type of the view model and provides code completion and IntelliSense; also the XAML code is correctly updated when renaming bound properties.
  • The view model can always be accessed even if the current data context is different – for example when binding to view model properties inside an item template.

Access the view model when current binding context is different:

<Grid DataContext="{StaticResource ViewModel}"> 
    <ItemsControl ItemsSource="{Binding Items}">
                <TextBlock FontSize="{Binding SelectedFontSize, Source={StaticResource ViewModel}}"
                           Text="{Binding}" />

Bind root element properties to view model properties

Because the view model object is only accessible after the Resources element, it cannot directly be used in the root element – for example to set the Title attribute of Window. To solve this problem, define the property using the tag syntax after the resource declaration:

<Window ...> 
        <viewModels:MyWindowModel x:Key="ViewModel" /> 
        <Binding Source="{StaticResource ViewModel}" Path="Document.Title" /> 


Command implementation

All button, context menu or other actions are implemented as commands which are implemented either directly in the view model class itself or encapsulated in an own ICommand implementation.

All command property names have the postfix “Command”. The following sample code shows a command property in a view model:

public ICommand SaveProjectCommand { get; private set; } 

The commands are instantiated in the view model constructor like this:

public MyViewModel()
    SaveDocumentCommand = new RelayCommand<Document>(SaveDocument, document => document != null);
    SaveDocumentAsCommand = new SaveDocumentAsCommand(); 

public void SaveDocument(Document document)

The next code snippet shows the implementation of a command using the “original” command pattern:

public class SaveDocumentAsCommand : CommandBase<Document> // or ICommand

In XAML, a control can be bound to a command in the view model in the following way:

<Button Command="{Binding SaveDocumentCommand}"
        CommandParameter="{Binding SelectedDocument}" /> 

If the view model is not directly accessible in the current context, for example when inside a context menu, you can access the view model using the StaticResource keyword:

<Button Command="{Binding MyButtonCommand, Source={StaticResource ViewModel}}" /> 

This trick can also be used when accessing other properties of the view model in a location where the view model is not directly accessible, i.e. the current data context is not the view model.

Use dependency properties to pass parameters to a view

Important rule: It is not allowed to set the DataContext property in the XAML code where a view is instantiated. For example this is NOT allowed:

<MySubView DataContext="{Binding MySubViewModel}" />

As described in the previous chapter, only the view itself is allowed to set its DataContext property. To pass parameters to a view, always use dependency properties which then are propagated to the view’s view model. The following code shows how to do this:

public class MySubView
    public static readonly DependencyProperty ProjectProperty = DependencyProperty.Register( 
        "Project", typeof(Project), typeof(MySubView),  
        new PropertyMetadata(default(Project), OnProjectChanged)); 

    public Project Project 
        get { return (Project)GetValue(ProjectProperty); } 
        set { SetValue(ProjectProperty, value); } 

    private static void OnProjectChanged(DependencyObject obj,  
        DependencyPropertyChangedEventArgs args) 
        ((MySubView)obj).Model.Project = (Project)args.NewValue; 

    public ProjectPropertiesViewModel Model 
        get { return (ProjectPropertiesViewModel)Resources["ViewModel"]; } 

Now, a Project object can be passed “into” the MySubView instance:

<MySubView Project="{Binding SelectedProject}" />

This rule enforces the development of more encapsulated and decoupled views.

Handling the view model life cycle

Sometimes, a view model needs life cycle methods to initialize and cleanup itself. The following code samples show how to implement them in a save manner. All life cycle methods in the view models should be implemented in the same way.

In the XAML code-behind:

public MyView() 
    Loaded += delegate { Model.OnLoaded(); }; 
    Unloaded += delegate { Model.OnUnloaded(); }; 

And the view model should implement these two methods this way:

public class MyViewModel  
    private bool _isLoaded = false;  

    public MyViewModel()
        // TODO: Add your constructor code here
        // The ctor is always called, initialize view model so that it also works in designer

    public void Initialize()
        // TODO: Add your initialization code here 
        // This method is only called when the application is running

    public void OnLoaded() 
        if (!_isLoaded) 
            // TODO: Add your loaded code here 
            _isLoaded = true;  

    public void OnUnloaded() 
        if (_isLoaded) 
            // TODO: Add your cleanup/unloaded code here 
            _isLoaded = false; 

Now, you may ask why we need a separate Initialize method when we can put all code into the view model’s constructor. The problem here is, that the constructor gets called from the Visual Studio designer but your code may only run when the application is running. To avoid this problem, it is recommended to put all logic which needs a running application (e.g. access to the service locator) into the Initialize method and the rest in the constructor (e.g. instantiation of command objects). This way, the Visual Studio designer only calls the view model constructor but not the view’s constructor with the Initialize call. This ensures – when applied correctly – that the XAML designer is working correctly. Another benefit of the Initialize method is that it allows to inject objects into the view model – this is described in the next section.

Note: The MyToolkit library provides view model base classes which simplify the implementation of these life cycle methods.

View model constructors and dependency injection

This chapter describes how to inject services into the view model using the service locator pattern or another IoC mechanism. I usually use one of the following two ways of injecting services into the view models: Using an initialization method or using constructor overloads.

Using an initialization method

If you want to avoid dependencies between the view model and the IoC framework, you should implement an Initialize method on the view model with the injected objects as parameters and call this method in the view’s constructor:

public class MySampleView
    public void MySampleView()

    protected MySampleViewModel Model 
        get { return (MySampleViewModel)Resources["ViewModel"]; } 

public void MySampleViewModel
    private IMyService _myService;

    public void Initialize(IMyService myService)
        _myService = myService;

Note: The ServiceLocator class can be found in the MyToolkit library or in many other IoC frameworks.

In this implementation the view models have no dependencies to the IoC framework and are well suited for unit tests. However the client has to call the Initialize method which may not be obvious.

Using constructor overloads

The other variant for implementing dependency injection is by providing two constructors: A default constructor with no parameters (for instantiation in XAML) and a constructor with all required service objects as parameters. The second constructor is used in unit tests with constructor dependency injection.

public MySampleViewModel() 
    : this(ServiceLocator.Default.Resolve<IMyService>()) { } 

public MySampleViewModel(IMyService myService) { } 

As you can see in the sample code, the default constructor always calls the other constructor with resolved service objects. The default constructor must not contain any logic. This implementation has the disadvantage that the view model has a dependency to the IoC framework. This is why I would prefer the first solution where a Initialize method is implemented.

View model decoupling and messaging

It is recommended to use the messenger pattern (also known as publish-subscribe pattern) which essentially is a message bus to send messages from any location to unknown receivers. Every component – for example a view or a service – may subscribe to receive messages of a particular type.

In the MVVM context, this facility is used to send messages from the view model to its view or other UI components – for example to show a dialog or navigate to another page. It is not required to use the messenger pattern – you can also use an IoC container and use service objects instead: For example, you can use a dialog service instead of sending messages to show message boxes.

The following libraries provide a messenger implementation:

To use the messenger, register a given message type (in the sample code ShowFileOpenDialog is the method which is called when the message has been sent):

Messenger.Default.Register<ShowFileOpenDialogMessage>(this, ShowFileOpenDialog); 

To send a message use the following code:

Messenger.Default.Send(new ShowFileOpenDialogMessage()); 

If a message provides a result, for example the result of a “YesNo”-dialog, the result should be “returned” using a callback. This is because messages should be immutable and the result may be returned asynchronously.

The messenger implementation of the MyToolkit library provides the class CallbackMessageBase which can be used with the messenger’s SendAsync method. The CallbackMessageBase class provides two callbacks: A success and failure callback. The receiver of the message performs the desired action and must call either the ProcessSuccessCallback or ProcessFailCallback method.

The following code shows how to send a callback message (MyToolkit only):

var msg = new TextMessage(..., MessageBoxButton.YesNo); 
msg.SuccessCallback = result => 
    if (result == MessageBoxResult.Yes)
        // TODO: Add your code

The same using async/await:

var result = await Messenger.Default.SendAsync(msg); 
if (result.Result == MessageBoxResult.Yes)
    // TODO: Add your code

The message receiver, which actually shows the dialog box, would look like this:

Messenger.Default.Register<TextMessage>(this, ShowMessageBox); 


private void ShowMessageBox(TextMessage msg) 
    var result = MessageBox.Show( 
        msg.Content, msg.Title, 
        msg.Button, msg.Icon); 


Note: Message object should be immutable and results should be passed back by using callbacks as shown in the samples before.

Location for message handler registrations

Global message handler registrations like showing dialogs, navigating to pages, etc. should be registered in the App.xaml.cs class on application startup. View specific messages should be registered in the Loaded event of a view and deregistered in the Unloaded event to prevent memory leaks and problems multiple callback registrations.

Always use typed XAML bindings

Important rule: Every binding in the XAML code must be typed.

This rule is needed to safely refactor view model and model classes and to find usages of bound properties. Usually this is no problem, because the view model is instantiated in the view’s resources and automatically provides a type. If the type is not automatically provided, you should manually set the type as shown in the following XAML code:

<Window xmlns:MyNamespace="clr-namespace:MyNamespace"
    <StackPanel d:DataContext="{d:DesignInstance Type=MyNamespace:MyType}"> 


Localized strings – provided by a .resx or .resw file – should be loaded from a Strings property of the view model. The following code shows how to implement the base view model. In the sample, the resource file is called Strings.resx and automatically generates the Strings class which is instantiated in the property:

public class ViewModelBase
    private Strings _strings = new Strings();

    public Strings Strings 
        get { return _strings; }


The XAML binding to load a localized string:

<TextBlock Text="{Binding Strings.LabelPerson" />

Strings in dialog messages and other modal dialogs, can directly be loaded using the Strings class.

Tip: For .resw files (used in Windows Store apps) this resource access class is not automatically generated. This article shows a way on how to solve this problem.

Key prefixes for localized strings

All localized strings should be located in the same Strings.resx or .resw resource file. If you have bigger projects, you can distribute the strings into multiple resource files. To have better structure, the string keys should start with a prefix depending on their usage.

The following prefixes or postfixes are recommended:

  • Label*: Labels which belong to an input control
  • GridLabel*: Data grid headers
  • Message*Title: Titles for message boxes
  • Message*Content: Content messages for message boxes
  • Log*: Log messages
  • Button*: Button content texts
  • Header*: Header texts to use in window titles, tab headers and similar locations
  • Menu*: Strings to use in menu items (e.g. context menu)
  • Task*: Texts which are shown in the task manager (e.g. “Loading Document”)
  • RibbonButton*: Headers for ribbon buttons
  • RibbonMenu*: Labels used in the ribbon menu

Final words

This article showed my way of doing MVVM. Do you have any suggestions or improvements? Are you doing it in a completely different way? Is something unclear or missing?

Tags: , , , , , , ,

37 responses to “Recommendations and best practices for implementing MVVM and XAML/.NET applications”

  1. Shane says:

    I came across this article recently and found it to be very helpful. One thing that you did not talk much about is the role and interactions of model classes. I read that views should not bind to models directly, the view should bind to properties of the view model which forwards on properties from the model. Is this something that you advocate? In your example of MySubView when you instantiate in the parent view with is the Project property a model object (as opposed to a view model) which would be manged by the view model of the sub view?

  2. Claudio says:

    Hi Rico

    In case of the DataContext for a view, why not using the static resource of the view model and bind it to the data context of the Window element with <Window.DataContext><Binding Source="{StaticResource ViewModel}" /></Window.DataContext>.. You are binding it to the first content element. In this case, e.g. the commands of the view model can’t be used in the <Window.InputBindings>.

    <Window x:Class="Demo.Views.MainWindow"
    Height="350" Width="525"
    Title="{Binding Title}">
    <vm:MainWindowModel x:Key="ViewModel" />
    <Binding Source="{StaticResource ViewModel}" />

    • Rico Suter says:

      I think this way you change the data context of the element itself so that when it is used it breaks bindings which should flow from the parent view into the view…

  3. peebo says:

    Thank you for laying out some principles, I’m just new to WPF and there’s so much confusion around that we’d really need more of this. However, I’m a bit skeptical about your way of assigning view-model properties from view properties. Apart from adding a lot of boilerplate, it doesn’t update the view property in case the view-model changes! What I tried is adding property setters in styles everywhere, as in: <Style TargetType="local:MySubView">
    <Setter Property="MySubViewProperty"
    Value="{Binding MySubViewModelProperty,
    but it turns out that doesn’t work as soon as you actually bind the MySubViewProperty to something externally, e.g. in Isn’t there a general, easy way to keep the two properties in sync?

    • Rico Suter says:

      You have to properly implement the INotifyPropertyChanged interface and raise the event in the property setters (as described). This way changes are propagated from the view model to the view.

      • peebo says:

        Sure, changes are propagated everywhere the view-model’s property is used in the view.
        However they’re not propagated to the actual dependency property on the view, e.g. ProjectProperty in your article. This is important if there is a 2-way binding between that property and some other property anywhere else in the application.

        Never had this situation? Imagine your user control exposes some property to be changed from the outside but has also a way to change this property from the inside, e.g. following some user action, which has then to be reported back to where it was set from.

        • Rico Suter says:

          Ah, then you need to register a PropertyChanged callback on the view model and update the DependencyProperty when the vm property changed?

  4. Jumes says:

    About this section “Handling the view model life cycle”, I should point out that one view model may be depended by multiple views, say there are two views, one view is visible and the other is hiden, how to handle this scene?
    How to control view visiblity from view model logic code, let one view be visible and the other be hiden?
    OnLoaded() and OnUnloaded() are UI specific code, View model have no knowledge of displaying, these should be moved to viwe layer.

    Best regards

    • Rico Suter says:

      In general you are right – UI logic belongs to the view and not in the view model. But sometimes the vm has to run specific code depending on its life cycle (e.g. save its state before it is destroyed, clean up resources, etc.). This is why i think its ok to have loded and unloaded methods in the vm.

  5. Radu says:

    Great article! Thanks for the time taken to write it.

    2 other important topics are View/Model IsDirty and Validation.
    Do you plan to write something about that as well? Or can you reply really short to this message?

    I am thinking to implement Validation and IsDirty Logic in the Model. Is that a bad approach?
    Downside is that the Models will get preatty heavy.

  6. Hristo says:

    Only the “parent” view has a reference to the view model, other views are not allowed to access another view’s view model

    Yeah… I disagree. What about when you have a relationship between the view models? You could have a view model which has two sub-viewmodels (e.g. when you have a window composed of two views – each one gets a view model). Surely the parent can access its child view models.

    • Rico Suter says:

      Of course, you could allow the concept of partial views with sub view models. I think its sometimes necessary if you use the DataContext also for sub views for example to keep the files shorter. However keep in mind, that all the views and view models are tightly coupled to each other. They should only be used in one location and not be shared. I think for most views its better to completely encapsulate them, to keep the public surface as small as possible.

  7. Shirstok says:

    Hello! I have one question, if I understand you correctly, you have a heavy usage of singleton here : Messenger.Default, ServiceLocator.Default

    Don’t you lose the benefits of MVVM with this approach? How about testing, esp. running multiple tests in parralel?

    • Rico Suter says:

      The main problem is, that XAML does not natively support DI (adding this feature would be great). This is why I recommend to implement two constructors, one default constructor which uses a global dependency resolver and one which accepts the injected services (as shown in the article). In the unit tests the default constructor is forbidden, only the constructor with parameters must be used with mocks or other service implementations. To improve the architecture, I recommend to completely avoid the global messenger (i.e. Messenger.Default), and also inject the messenger service (or call it message bus service) with the same technique described before. With this approach, you don’t directly rely on singletons and you can run your unit tests in parallel. What do you think?

      • Alok Arya says:

        I think what he means, is the ViewModel First approach wherein you use data templates. While, your blog greatly helped me in building this WPF app I am working on. There are certain times where i find it really flexible to use the ViewModel first approach. It’s powerful to have inheritance in viewModels and what view gets rendered is based on the subclass of viewmodel

  8. Alok Arya says:

    public MyView()
    Loaded += delegate { Model.OnLoaded(); };
    Unloaded += delegate { Model.OnUnloaded(); };

    public void Initialize()
    // TODO: Add your initialization code here
    // This method is only called when the application is running

    I think you mean

    Initialized += delegate { Model.Initialize(); };

    instead of,


    Am I correct ?

    Because otherwise initialize method will also be called by XAML

    • Rico Suter says:

      It is correct as is, because the code-behind class constructor is not called in the designer – only the XAML code is loaded and rendered (i.e. the vm ctor is called).

  9. Petr says:


    when you recommend to create viewmodel in view’s resources, how do you pass parameters to the view?

    I.e. I have a “Customers view”, and there is a command to edit customer record. What information and how you would recommend to pass from “masterViewModel” to “detailViewModel”?

    Thank you very much.


    • Rico Suter says:

      You should treat your detail view as an independent component and add all required properties to the view and pass them from the detail view to the detail view model (i.e. add a Customer property to the detail view and propagate changes to the VM as described in the article). This way the detail view is decoupled from the master view and can also be used in other application parts.

  10. […] Rico Suter: Recommendations and best practices implementing MVVM WPF applications […]

  11. Stefan says:

    Hi Rico,

    thank you very much for this very good and helpful article! Anyway I have some open questions where I miss some explanation.

    Why do you only suggest to put the view models into a separate assembly? I’m a beginner in WPF/MVVM but my thought was that it would be a good idea to put also the models into a separate assembly which the view layer is not allowed to reference. This will ensure decoupling of view and model. Or is it allowed/okay to use the model in the view directly?
    I have a big problem understanding the “flow” of the model objects in your szenario? How do you put the model into the view model when the view model is instantiated in XAML?Imaging the following: there is a MainView which is bound to MainViewModel which has a MainModel with all the configuration data inside, for example a DatabaseConnectionModel. Now I want to edit the database connection. I can open the EditDatabaseView with dialog service or messaging. The EditDatbaseViewModel is created in XAML of EditDatabaseView. How will the DatabaseConnectionModel be put into the corresponding EditDatbaseViewModel? Which mechanism do you use to tell the (EditDatabase)ViewModel which model it has to change?

    Thanks and best regards,

    • Rico Suter says:

      Hi Stefan,

      Question 1: Separation into multiple assemblies

      You also have to put the models in a separate assembly, otherwise you would end up with circular references. In your scenario, you would ideally implement three assemblies: Views => ViewModels => Models.

      Question 2: Directly use models

      In every MVVM project, you have to decide whether to directly use your model classes or wrap them with a view model class (e.g. directly use the Person class or implement a PersonModel class). I think it depends on the project, but I mostly take the pragmatic way and directly use the model classes. Also see:

      Question 3: Pass data to other view

      If your EditDatabaseView is opened via message bus, you would pass the DatabaseConnectionModel as a parameter message. The message listener would create an instance of EditDatabaseView and assign the DatabaseConnectionModel via a dependency property on the EditDatabaseView view to the EditDatabaseViewModel view model.

      Is now everything clear?

      • Stefan says:

        Hi Rico,

        thank you for your detailed answer, which helped me understanding how it’s working. Anyway one more question. How can I share data between different view models? For example when I have a main view model which has the state “IsInProductionMode”, how can I use this information in other view models which have been created in XAML later. Normally I would do composition of view models but when I create the view models in XAML that’s now working. Do I have to pass the main view model as parameter to the new view model like described in the answer of question 3 or is there another more elegant possiblity to do this?


        • Rico Suter says:

          I think you have three options:

          1. Pass view model as parameter (not very elegant, high coupling between the two views)
          2. Use a global event (i.e. messenger or message bus): Seems good for a global IsInProductionMode property
          3. Think of the other views as independent components with properties for all required state (i.e. add a IsInProductionMode property to the sub view and bind the state from main view model)

          As always, there’s no right or wrong here, but I’d recommend option 2 or 3.

  12. Dan Corwin says:

    Hi Rico,

    You have a lot of good principles in this article which is very helpful to me since I am still learning MVVM. I have a best practice question for how to implement opening a dialog from the menu of a main window. For example opening a page setup dialog. I have found 2 ways to do this:

    Define the Click property of the menu item to call a method on the main window (e.g. Click=”OnPageSetup”). Then the View.MainWindow.OnPageSetup(object sender, EventArgs e) can open the View.PageSetupDialog and associate it with the corresponding ViewModel object.
    Have an ICommand property on the MainWindowViewModel like PageSetupCommand that could be invoked to open the PageSetupDialog. Then in the App startup code after it constructs the MainWindowViewModel it could assign the PageSetupCommand with an appropriate implementation.

    Notice that in both cases the ViewModel doesn’t reference the View. Case 1 goes from View to View directly. Case 2 is routed via the ViewModel. I’m wondering if there is any benefit to case 2 or if there are any principle that are violated with case 1.

    Any thoughts are welcome.

    Best regards,

    • Rico Suter says:

      Hi Dan,

      I’d recommend to implement an ICommand property on the view model whose action sends a new ShowPageSetupMessage message with all required parameters. The message handler is registered on app startup and shows the page setup dialog. You can also use a dialog service interface (injected via DI) in your VM which has a method to show the dialog and which can be mocked in your unit tests…

      Is your question answered?

      Best regards Rico

  13. Panos Roditakis says:


    Any suggestions or best practice for triggering a new view (ex a Dialog) to collect user input, from inside the ViewModel?

    Thank you.

    • Rico Suter says:

      I see two possibilities:

      • Use the messenger pattern and a message with a callback providing the user input
      • Inject a dialog service (interface) into your view model so that it can be easily mocked in your unit tests
  14. Mike says:

    Hi Rico!
    Question about “Project file structure”:

    I don’t need to use your entire toolkit (although it’s great), I need only part of ObservableObject.cs with implemented INotifyPropertyChanged interface + one Set method.

    Where should ObservableObject class go in file structure? “Helpers”, “Common”? Or should I name it “ViewModelBase” and put it in ViewModels folder?

    Thanks for advice in advance
    Best regards

    • Rico Suter says:

      Hi Mike,

      There’s no correct or wrong answer. Personally, I’d put it into a “Common” directory and use the name ObservableObject as it has no relation to view models but only provides the functionality to track object changes. If you need view model specific logic, you can create a ViewModelBase class and inherit from ObservableObject. But it is really up to you and your taste how to name the classes and where to store them…

      Best regards

  15. Veux says:

    thank you for the great blog entry.
    I have some question whenI try to follow your rules.
    I often have the Situation that I put model classes or data transfer objects into the view models constructur. How do I do that if I follow the rule to instantiate the view model in the XAML code ?
    Or if I have to jump to another page e.g master-detail pages ?
    What you suggest to “insert” Parameters into the view model ?

    Cheers Veux

    • Rico Suter says:

      Hi Veux,

      As described in the article, I’d recommend to define the parameters as dependency properties on the view and push them from the view to the view model. As shown in the chapter “Use dependency properties to pass parameters to a view” you can use simple property assignments and evaluate all loaded properties in the OnLoaded method. Does this answer your questions? Or can you describe your problem in more detail?

      Best regards

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