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

xml
<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:

csharp
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

csharp
public partial class ContactViewModel : ObservableObject
{
    [ObservableProperty]
    private string? name;
}

Hooks: OnXxxChanging / OnXxxChanged

csharp
[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

csharp
[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

csharp
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

csharp
[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

csharp
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

1. Forgetting partial. Class (and every enclosing type) must be

partial. Compile error MVVMTK0008 / MVVMTK0042.

1. PascalCase field name. [ObservableProperty] private string Name;

collides with the generated property. Use name, _name, or m_name.

1. 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.

1. Forgetting [NotifyCanExecuteChangedFor]. The Save button stays

disabled even though CanSave() would now return true.

1. 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]:

csharp
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>