mvvm-toolkit

name mvvm-toolkit description 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. CommunityToolkit.Mvvm (core) 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-messenger for IMessenger pub/sub patterns. Load mvvm-toolkit-di for Microsoft.Extensions.DependencyInjection integration. Quick recap. [ObservableProperty] on private fields in partial classes; [RelayCommand] on instance methods; inherit from ObservableObject (or ObservableValidator for input forms, ObservableRecipient when using IMessenger ). Package & setup < ItemGroup > < PackageReference Include = "CommunityToolkit.Mvvm" Version = "8.*" /> </ ItemGroup > 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: using CommunityToolkit.Mvvm.ComponentModel; // ObservableObject, [ObservableProperty] using CommunityToolkit.Mvvm.Input; // [RelayCommand], RelayCommand, AsyncRelayCommand Universal rule. Every type that uses [ObservableProperty] or [RelayCommand] — and every enclosing type, if nested — must be declared partial . Without it, the generators emit MVVMTK0008 / MVVMTK0042 . Source generators cheat sheet 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. ViewModel patterns Simple observable property public partial class ContactViewModel : ObservableObject { [ ObservableProperty ] private string ? name; } Hooks: OnXxxChanging / OnXxxChanged [ ObservableProperty ] private string ? name; partial void OnNameChanged ( string ? value ) => Logger.LogInformation( "Name changed to {Name}" , value ); Both 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). Dependent properties + dependent commands [ 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(); Wrapping a non-observable model public sealed class ObservableUser ( User user ) : ObservableObject { public string Name { get => user.Name; set => SetProperty(user.Name, value , user, (u, n) => u.Name = n); } } Pass a static lambda (no captured state) to keep the call allocation-free. Commands [ 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); 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 selection 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 ). Validation 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... } } 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. Top pitfalls Forgetting partial . Class (and every enclosing type) must be partial . Compile error MVVMTK0008 / MVVMTK0042 . PascalCase field name. [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 . Forgetting [NotifyCanExecuteChangedFor] . The Save button stays disabled even though CanSave() would now return true . Mutating the same reference held by an [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 . End-to-end mini walkthrough A two-pane Notes app demonstrating generators + commands + [NotifyCanExecuteChangedFor] : 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); } For the full sample (DI wiring, View code-behind, XAML, unit tests), see references/end-to-end-walkthrough.md . References & companion skills 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: Toolkit overview: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/ WinUI MVVM Toolkit tutorial: https://learn.microsoft.com/en-us/windows/apps/tutorials/winui-mvvm-toolkit/intro Source: https://github.com/CommunityToolkit/dotnet Samples: https://github.com/CommunityToolkit/MVVM-Samples