boshi-xixixi / mvvm-toolkit

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

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`](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`](references/relaycommand-cookbook.md)
for sync / async / cancellable / concurrency / error-surfacing recipes.

Base class selection

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`](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.

2. PascalCase field name. [ObservableProperty] private string Name;
   collides with the generated property. Use name, _name, or m_name.

3. 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.

4. Forgetting [NotifyCanExecuteChangedFor]. The Save button stays
   disabled even though CanSave() would now return true.

5. **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`](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/end-to-end-walkthrough.md).

References & companion skills

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>