mvvm toolkit

Author's original introduction

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.

SkillPi interpretation

Tasks

mvvm toolkit

Audience

Use the scenarios stated in the author's introduction.

Inputs and outputs

See the complete author document; outputs have not been runtime verified by SkillPi.

Software, accounts and paid services

Check required software, accounts, costs and permissions in the full document; confirm and authorize actual file access and external services in your host. Domains referenced in the documents (not a runtime allowlist): aka.ms, github.com, learn.microsoft.com

Limits and differences

Functionality, version status and outcome statements in the introduction are author claims. Only pinned source and static attachment evidence were checked; runtime behavior, external services and compatibility are not guaranteed.

Testing

Not tested

Security rating and access scope

caution · Static evidence review; not a runtime safety guarantee.

Check required software, accounts, costs and permissions in the full document; confirm and authorize actual file access and external services in your host. Domains referenced in the documents (not a runtime allowlist): aka.ms, github.com, learn.microsoft.com

Before use, confirm and authorize actual file access, external services and network destinations in your host. This review grants no runtime permissions.

Original SKILL.md

---
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

```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`](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`](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`](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]`:

```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/end-to-end-walkthrough.md).

---

## References & companion skills

| Topic | Where |
|-------|-------|
| Source generator attribute reference | [`references/source-generators.md`](references/source-generators.md) |
| RelayCommand recipes | [`references/relaycommand-cookbook.md`](references/relaycommand-cookbook.md) |
| Validation deep dive | [`references/validation.md`](references/validation.md) |
| Full Notes-app walkthrough | [`references/end-to-end-walkthrough.md`](references/end-to-end-walkthrough.md) |
| `MVVMTK0xxx` diagnostics & pitfalls | [`references/troubleshooting.md`](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>

Copyright and license notice

MIT License

Copyright GitHub, Inc.

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
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copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.