Getting Started with the WPF Model-View-ViewModel Toolkit: A Beginner’s Guide

Migrating Existing WPF Apps to the Model-View-ViewModel Toolkit: Step-by-StepMigrating a legacy WPF application to a Model-View-ViewModel (MVVM) toolkit can dramatically improve maintainability, testability, and developer productivity. This guide provides a step-by-step migration path, practical examples, and tips for avoiding common pitfalls. It assumes basic familiarity with WPF, data binding, and the MVVM pattern.

Why migrate to an MVVM toolkit?

Separation of concerns: MVVM clearly separates UI (View), presentation logic (ViewModel), and business/data logic (Model).
Testability: ViewModels can be unit-tested without UI dependencies.
Scalability: Toolkits offer utilities (commands, messaging, DI integration) that scale well for large apps.
Consistency: Standardized patterns reduce onboarding time and bugs.

Pre-migration checklist

Before starting, ensure you have:

A working solution under version control (git recommended).
A test plan and automated tests if available.
A list of third-party controls and libraries in use.
Identification of views with heavy code-behind logic.
A chosen MVVM toolkit (examples: MVVM Toolkit from Microsoft, Prism, Caliburn.Micro, ReactiveUI). This guide uses the Microsoft MVVM Toolkit (Community Toolkit MVVM) for examples, but concepts apply broadly.

Step 1 — Pick a migration strategy

Choose one of these approaches based on app size and risk tolerance:

Incremental: Migrate view-by-view. Low risk, allows gradual refactor.
Module-based: Migrate whole features or modules at once. Best for modular apps.
Big bang: Replace UI layer largely at once. Higher risk, faster if well-resourced.

Recommended: Incremental for most apps.

Step 2 — Add the MVVM toolkit to your project

For Community Toolkit MVVM:

Install NuGet package: Microsoft.Toolkit.Mvvm (or CommunityToolkit.Mvvm for newer versions).

Add using statements where needed:


using CommunityToolkit.Mvvm.ComponentModel; using CommunityToolkit.Mvvm.Input;

Step 3 — Establish conventions and folder structure

Create folders and namespaces:

Models/
ViewModels/
Views/
Services/ (for data access, navigation, dialogs)
Converters/
Behaviors/

Define conventions: one ViewModel per View, ViewModels named XyzViewModel, Views named XyzView.

Step 4 — Extract ViewModel from code-behind

Identify a simple View with code-behind handling logic (event handlers, state). Example: MainWindow.xaml.cs handling a button click that updates UI.

Create ViewModel class:

public partial class MainViewModel : ObservableObject { [ObservableProperty] private string _status; public MainViewModel() {     Status = "Ready"; } [RelayCommand] private void DoWork() {     Status = "Working...";     // long running tasks should be async — see below } }

Move logic from code-behind to ViewModel methods/commands. Replace event handlers with ICommand using [RelayCommand] or RelayCommand.
Bind View to ViewModel:

In XAML:


<Window ...     xmlns:vm="clr-namespace:YourApp.ViewModels"> <Window.DataContext>     <vm:MainViewModel /> </Window.DataContext> ... </Window>

Or register via DI and resolve in App.xaml.

Step 5 — Replace direct control manipulation with data binding

Move UI state into properties on ViewModel. Replace calls like myTextBox.Text = “…” with a bound property:

XAML:


<TextBox Text="{Binding Status, UpdateSourceTrigger=PropertyChanged}" />

ViewModel: ObservableProperty as shown above.

For visibility, enablement, selection — expose relevant properties and use converters where needed.

Centralize navigation into a service:

public interface INavigationService {     void NavigateTo(string viewName, object parameter = null); }

Implement with Frame navigation, content control swapping, or DI-based view resolution. Inject service into ViewModels and call it instead of manipulating windows from code-behind.

For dialogs, create IDialogService to show modal dialogs and return results; implement using Window.ShowDialog wrapped behind the interface.

Step 7 — Handle async work safely

Use async commands to keep UI responsive:

[RelayCommand] private async Task LoadDataAsync() {     IsBusy = true;     try     {         Items = await _dataService.GetItemsAsync();     }     finally     {         IsBusy = false;     } }

Use CancellationToken for long-running operations.

Step 8 — Migrate event aggregators and messaging

Replace static events or tight coupling with a messenger or event aggregator provided by the toolkit:

WeakReferenceMessenger.Default.Register<RefreshMessage>(this, (r, m) => Refresh());

This decouples ViewModels and avoids memory leaks.

Step 9 — Replace behaviors, attached properties, and control tricks

If code-behind used attached properties or behaviors, prefer XAML behaviors or implement reusable attached properties. Use Blend behaviors for drag/drop, focus management, etc., keeping code declarative.

Step 10 — Introduce dependency injection

Add a DI container (Microsoft.Extensions.DependencyInjection works well). Register services and ViewModels:

services.AddSingleton<INavigationService, NavigationService>(); services.AddTransient<MainViewModel>();

Resolve ViewModels via constructor injection in Views (set DataContext in code-behind) or use a ViewModelLocator.

Step 11 — Unit test ViewModels

Write unit tests for ViewModel logic without UI:

Test commands change state.
Test properties raise PropertyChanged.
Test interactions with services using mocks.

Example using xUnit + Moq:

[Fact] public async Task LoadDataCommand_PopulatesItems() {     var mockService = new Mock<IDataService>();     mockService.Setup(s => s.GetItemsAsync()).ReturnsAsync(new[] { "a", "b" });     var vm = new MainViewModel(mockService.Object);     await vm.LoadDataCommand.ExecuteAsync(null);     Assert.Equal(2, vm.Items.Count); }

Step 12 — Gradually remove code-behind, keep pragmatic exceptions

Not all code-behind is bad: pure UI concerns like animations or control templates can remain. Move logic that affects app state into ViewModels. Keep code-behind minimal: set DataContext, handle view-only animations.

Step 13 — Performance and memory considerations

Avoid large object graphs in DataContext. Use lightweight ViewModels.
Unregister messenger listeners when appropriate to avoid leaks.
Virtualize long lists (VirtualizingStackPanel, ItemsControl virtualization).
Use INotifyPropertyChanged efficiently (ObservableProperty helps).

Step 14 — QA and rollout

Run automated and manual UI tests.
Feature-flag major UI changes for safer rollout.
Monitor performance and crash reports post-deployment.

Common pitfalls & tips

Moving too much at once increases risk — prefer incremental.
Don’t over-architect: keep simple ViewModels for simple Views.
Use design-time data for XAML designer productivity.
Name commands clearly (LoadDataCommand, SaveCommand).
Keep synchronous blocking off the UI thread.

Example: Full small refactor

MainWindow.xaml.cs before:

public partial class MainWindow : Window {     public MainWindow()     {         InitializeComponent();     }     private async void OnLoadClicked(object sender, RoutedEventArgs e)     {         progressBar.Visibility = Visibility.Visible;         var items = await DataService.GetItemsAsync();         listBox.ItemsSource = items;         progressBar.Visibility = Visibility.Collapsed;     } }

After: MainViewModel.cs

public partial class MainViewModel : ObservableObject {     private readonly IDataService _dataService;     public MainViewModel(IDataService dataService)     {         _dataService = dataService;     }     [ObservableProperty]     private ObservableCollection<string> _items;     [ObservableProperty]     private bool _isBusy;     [RelayCommand]     private async Task LoadAsync()     {         IsBusy = true;         try         {             Items = new ObservableCollection<string>(await _dataService.GetItemsAsync());         }         finally         {             IsBusy = false;         }     } }

MainWindow.xaml (binding):

<Button Content="Load" Command="{Binding LoadCommand}" /> <ProgressBar IsIndeterminate="True" Visibility="{Binding IsBusy, Converter={StaticResource BoolToVisibilityConverter}}"/> <ListBox ItemsSource="{Binding Items}" />

Conclusion

Migrating to an MVVM toolkit is an investment that pays off in maintainability, testability, and developer productivity. Use an incremental approach, centralize services (navigation, dialogs, data), keep ViewModels lean, and write unit tests. Prioritize pragmatic decisions: not every line of code-behind must move — keep UI-only behavior where it belongs.