Agent skill

wpf-mvvm

Build and maintain WPF MVVM patterns using CommunityToolkit.Mvvm for a .NET 8 widget-host app. Use when creating ViewModels, commands, observable state, validation, view bindings, and viewmodel-first navigation behaviors. Avoid Prism and heavy region managers; keep ViewModels testable and UI-agnostic.

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npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/wpf-mvvm

SKILL.md

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

csharp
[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

csharp
// 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

csharp
[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

csharp
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)

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

csharp
[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.

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