npx skills add ...
npx skills add github/awesome-copilot --skill mvvm-toolkit
npx skills add github/awesome-copilot --skill mvvm-toolkit
CommunityToolkit.Mvvm (the MVVM Toolkit) core: source generators ([ObservableProperty], [RelayCommand], [NotifyPropertyChangedFor], [NotifyCanExecuteChangedFor], [NotifyDataErrorInfo]), base classes (ObservableObject / ObservableValidator / ObservableRecipient), commands (RelayCommand / AsyncRelayCommand), and validation. Companion skills: mvvm-toolkit-messenger for pub/sub, mvvm-toolkit-di for Microsoft.Extensions.DependencyInjection wiring. Works across WPF, WinUI 3, MAUI, Uno, and Avalonia.
Use this skill when authoring or reviewing ViewModels, properties,
commands, or validation in apps that use CommunityToolkit.Mvvm 8.x.
Companion skills. Load
mvvm-toolkit-messengerforIMessengerpub/sub patterns. Loadmvvm-toolkit-diforMicrosoft.Extensions.DependencyInjectionintegration.
Quick recap.
[ObservableProperty]on private fields inpartialclasses;[RelayCommand]on instance methods; inherit fromObservableObject(orObservableValidatorfor input forms,ObservableRecipientwhen usingIMessenger).
Targets: netstandard2.0, netstandard2.1, net6.0+. Works on .NET, .NET
Framework, Mono. Source generators ship in the same NuGet — no extra
analyzer reference required.
Namespaces:
Universal rule. Every type that uses
[ObservableProperty]or[RelayCommand]— and every enclosing type, if nested — must be declaredpartial. Without it, the generators emitMVVMTK0008/MVVMTK0042.
| Attribute | Applied to | Generates |
|---|---|---|
[ObservableProperty] | private field | Public INotifyPropertyChanged property + OnXxxChanging/OnXxxChanged partial-method hooks |
[NotifyPropertyChangedFor(nameof(Other))] | observable field | Also raises PropertyChanged for the listed property |
[NotifyCanExecuteChangedFor(nameof(MyCommand))] | observable field | Calls MyCommand.NotifyCanExecuteChanged() on change |
[NotifyDataErrorInfo] | observable field on ObservableValidator | Calls ValidateProperty(value) from the setter |
[NotifyPropertyChangedRecipients] | observable field on ObservableRecipient | Broadcast(old, new) after the change |
[RelayCommand] | instance method | Lazy RelayCommand / AsyncRelayCommand exposed as IRelayCommand / IAsyncRelayCommand |
[RelayCommand(CanExecute = nameof(CanX))] | instance method | Wires CanExecute to a method or property |
[RelayCommand(IncludeCancelCommand = true)] | async method with CancellationToken | Also generates XxxCancelCommand |
[RelayCommand(AllowConcurrentExecutions = true)] | async method | Allows queued/parallel invocations (default disables while running) |
[RelayCommand(FlowExceptionsToTaskScheduler = true)] | async method | Surfaces exceptions via ExecutionTask instead of awaiting and rethrowing |
[property: SomeAttr] | observable field or [RelayCommand] method | Forwards SomeAttr onto the generated property (e.g., [JsonIgnore]) |
Naming. Field name / _name / m_name → Name. Method LoadAsync →
LoadCommand (the Async suffix is stripped; a leading On is also
stripped).
See references/source-generators.md for
the full attribute reference with generated-code samples.
OnXxxChanging / OnXxxChangedBoth single-arg (value) and two-arg (oldValue, newValue) overloads
are available. Implement only the ones you need; unimplemented hooks are
elided by the compiler (zero runtime cost).
Pass a static lambda (no captured state) to keep the call allocation-free.
Reach for manual RelayCommand / AsyncRelayCommand constructors only
when you must own the command's lifetime explicitly or compose it from
non-trivial sources. The attribute style covers ~95% of cases.
See references/relaycommand-cookbook.md
for sync / async / cancellable / concurrency / error-surfacing recipes.
| Base class | Use when |
|---|---|
ObservableObject | Default. INotifyPropertyChanged + INotifyPropertyChanging + SetProperty overloads + SetPropertyAndNotifyOnCompletion for Task properties |
ObservableValidator | The VM needs INotifyDataErrorInfo (forms, settings input) |
ObservableRecipient | The VM sends or receives IMessenger messages — see the mvvm-toolkit-messenger skill |
C# is single-inheritance: ObservableValidator and ObservableRecipient
both extend ObservableObject, so combining them requires composition
(e.g., inject IMessenger into an ObservableValidator).
Other entry points: TrySetProperty, ValidateProperty(value, name),
ClearAllErrors(), GetErrors(propertyName). Custom rules support
[CustomValidation] methods and custom ValidationAttribute subclasses.
See references/validation.md for the full
validator surface area.
partial. Class (and every enclosing type) must be
partial. Compile error MVVMTK0008 / MVVMTK0042.[ObservableProperty] private string Name;
collides with the generated property. Use name, _name, or m_name.async void on [RelayCommand]. The generator only wraps
Task-returning methods as IAsyncRelayCommand. async void becomes
a sync RelayCommand and exceptions are unobserved. Always return
Task.[NotifyCanExecuteChangedFor]. The Save button stays
disabled even though CanSave() would now return true.[ObservableProperty]
field. EqualityComparer<T>.Default returns true, no notification
fires. Replace the instance instead of mutating it.For the full diagnostic table (MVVMTK0xxx) and more pitfalls, see
references/troubleshooting.md.
A two-pane Notes app demonstrating generators + commands +
[NotifyCanExecuteChangedFor]:
For the full sample (DI wiring, View code-behind, XAML, unit tests), see
references/end-to-end-walkthrough.md.
| Topic | Where |
|---|---|
| Source generator attribute reference | references/source-generators.md |
| RelayCommand recipes | references/relaycommand-cookbook.md |
| Validation deep dive | references/validation.md |
| Full Notes-app walkthrough | references/end-to-end-walkthrough.md |
MVVMTK0xxx diagnostics & pitfalls | references/troubleshooting.md |
| Messenger pub/sub | Companion skill: mvvm-toolkit-messenger |
Microsoft.Extensions.DependencyInjection wiring | Companion skill: mvvm-toolkit-di |
External sources:
public partial class ContactViewModel : ObservableObject
{
[ObservableProperty]
private string? name;
}[ObservableProperty]
private string? name;
partial void OnNameChanged(string? value) =>
Logger.LogInformation("Name changed to {Name}", value);[ObservableProperty]
[NotifyPropertyChangedFor(nameof(FullName))]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
private string? firstName;
[ObservableProperty]
[NotifyPropertyChangedFor(nameof(FullName))]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
private string? lastName;
public string FullName => $"{FirstName} {LastName}".Trim();public sealed class ObservableUser(User user) : ObservableObject
{
public string Name
{
get => user.Name;
set => SetProperty(user.Name, value, user, (u, n) => u.Name = n);
}
}[RelayCommand]
private void Refresh() => Items.Reset();
[RelayCommand]
private async Task LoadAsync()
{
foreach (var item in await service.GetItemsAsync())
Items.Add(item);
}
[RelayCommand(IncludeCancelCommand = true)]
private async Task DownloadAsync(CancellationToken token)
{
await using var stream = await http.GetStreamAsync(url, token);
// ...
}
[RelayCommand(CanExecute = nameof(CanSave))]
private Task SaveAsync() => repo.SaveAsync(Name!);
private bool CanSave() => !string.IsNullOrWhiteSpace(Name);using System.ComponentModel.DataAnnotations;
public sealed partial class RegistrationViewModel : ObservableValidator
{
[ObservableProperty]
[NotifyDataErrorInfo]
[Required, MinLength(2), MaxLength(100)]
private string? name;
[ObservableProperty]
[NotifyDataErrorInfo]
[Required, EmailAddress]
private string? email;
[RelayCommand]
private void Submit()
{
ValidateAllProperties();
if (HasErrors) return;
// submit...
}
}public sealed partial class NoteViewModel(INotesService notes,
IMessenger messenger) : ObservableRecipient(messenger)
{
[ObservableProperty]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
[NotifyCanExecuteChangedFor(nameof(DeleteCommand))]
private string? filename;
[ObservableProperty]
[NotifyCanExecuteChangedFor(nameof(SaveCommand))]
private string? text;
[RelayCommand(CanExecute = nameof(CanSave))]
private Task SaveAsync()
{
Messenger.Send(new NoteSavedMessage(Filename!));
return notes.SaveAsync(Filename!, Text!);
}
[RelayCommand(CanExecute = nameof(CanDelete))]
private Task DeleteAsync() => notes.DeleteAsync(Filename!);
private bool CanSave() =>
!string.IsNullOrWhiteSpace(Filename) && !string.IsNullOrEmpty(Text);
private bool CanDelete() => !string.IsNullOrWhiteSpace(Filename);
}