---

name: wpf-mvvm description: 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.

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)

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Core Patterns

ViewModel Base Classes

Observable Properties

Copy[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

Copy// 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

Copy[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;
    }
}

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Validation

Copypublic 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;
}

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Navigation (ViewModel-First)

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

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

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Anti-Patterns

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Testing ViewModels

Copy[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);
}

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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.