Wpf Mvvm

Overview

Apply MVVM conventions with CommunityToolkit.Mvvm, ensuring view models are UI-agnostic and ready for the widget shell architecture.

Constraints

• Target .NET 8

• CommunityToolkit.Mvvm

• ViewModel-first navigation (custom service)

• No Prism, no heavy region manager

Definition of Done (DoD)

• ViewModel has no WPF dependencies (Window, UserControl, etc.)

• Commands use [RelayCommand] attribute from CommunityToolkit.Mvvm

• Observable properties use [ObservableProperty] attribute

• Async commands have proper cancellation support where applicable

• ViewModel behavior is covered by unit tests

• No Application.Current access in ViewModels

• Services injected via constructor (not resolved in methods)

Core Patterns

ViewModel Base Classes

Base Class Use Case

ObservableObject

Standard ViewModels

ObservableValidator

ViewModels with validation

ObservableRecipient

ViewModels needing messaging

Observable Properties

[ObservableProperty] private string _title = "Widget";

// With property change notification to other properties [ObservableProperty] [NotifyPropertyChangedFor(nameof(CanSave))] private string _name = "";

// With can-execute change notification [ObservableProperty] [NotifyCanExecuteChangedFor(nameof(SaveCommand))] private bool _isValid;

Commands

// Simple command [RelayCommand] private void Refresh() { }

// Async command with cancellation [RelayCommand] private async Task LoadDataAsync(CancellationToken token) { Data = await _service.GetDataAsync(token); }

// Command with can-execute [RelayCommand(CanExecute = nameof(CanSave))] private void Save() { }

private bool CanSave => !string.IsNullOrEmpty(Name);

Async Command Patterns

[RelayCommand] private async Task LoadAsync(CancellationToken token) { IsLoading = true; try { Items = await _service.GetItemsAsync(token); } catch (Exception ex) when (ex is not OperationCanceledException) { ErrorMessage = "Failed to load items"; Log.Error(ex, "Load failed"); } finally { IsLoading = false; } }

Validation

public partial class SettingsViewModel : ObservableValidator { [ObservableProperty] [NotifyDataErrorInfo] [Required(ErrorMessage = "Name is required")] [MinLength(3)] private string _name = "";

[RelayCommand(CanExecute = nameof(CanSave))]
private void Save()
{
    ValidateAllProperties();
    if (!HasErrors)
    {
        // Proceed
    }
}

private bool CanSave => !HasErrors;

}

Navigation (ViewModel-First)

// In Shell or navigation service public void Navigate<TViewModel>() where TViewModel : ObservableObject { var vm = _serviceProvider.GetRequiredService<TViewModel>(); CurrentContent = vm; // DataTemplate maps VM to View }

<!-- In App.xaml or Resources --> <DataTemplate DataType="{x:Type vm:SettingsViewModel}"> <views:SettingsView/> </DataTemplate>

Anti-Patterns

❌ Don't ✅ Do

Reference Window/UserControl in VM Inject INavigationService

Call Application.Current Inject IThemeService, IShellService

Create services in methods Inject via constructor

Use code-behind for business logic Keep logic in ViewModel

Expose ICommand directly Use [RelayCommand]

Testing ViewModels

[Fact] public void Save_WhenValid_CallsService() { var mockService = new Mock<IDataService>(); var vm = new MyViewModel(mockService.Object);

vm.Name = "Valid Name";
vm.SaveCommand.Execute(null);

mockService.Verify(s => s.Save(It.IsAny<Data>()), Times.Once);

}

References

• CommunityToolkit.Mvvm documentation

• 3SC/ViewModels/ for examples

• async-patterns skill for async command details

• input-validation skill for validation patterns

References

• references/viewmodel-patterns.md for structure patterns.

• references/commands-and-async.md for command and async guidance.

• references/validation.md for validation recipes.