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

| 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

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;

[Observab