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>
  <!-- Pin to a specific version for deterministic, secure restores. -->
  <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.0" />
</ItemGroup>

Reproducible restores. Add the following to your .csproj or Directory.Build.props and commit the generated packages.lock.json to source control. This ensures every restore fetches an integrity-verified, content-identical artifact:

<RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>

When upgrading the toolkit, update the pinned version above intentionally — do not use floating version wildcards (e.g. 8.*).

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).

For the full attribute reference with generated-code samples, consult the external source documentation linked in the References section below.

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.

For sync / async / cancellable / concurrency / error-surfacing recipes, consult the external source documentation linked in the References section below.

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.

For the full validator surface area, consult the external source documentation linked in the References section below.

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, consult the external source documentation linked in the References section below.

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 a full sample (DI wiring, View code-behind, XAML, unit tests), consult the external source documentation and samples linked in the References section below.

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