--- description: 'CommunityToolkit.Mvvm (MVVM Toolkit) coding conventions for ViewModels, commands, messaging, validation, and DI across WPF, WinUI 3, .NET MAUI, Uno Platform, and Avalonia.' applyTo: '**/*.cs, **/*.xaml, **/*.csproj' --- # CommunityToolkit.Mvvm (MVVM Toolkit) These rules apply whenever a project references `CommunityToolkit.Mvvm`. For deep reference and end-to-end examples, load the `mvvm-toolkit` skill. ## Package & language - Reference `CommunityToolkit.Mvvm` 8.x (or newer) in `.csproj`. Do not install the legacy `Microsoft.Toolkit.Mvvm` (7.x) for new projects. - C# `LangVersion` must support source generators (default in modern SDKs). ## ViewModel base class - Inherit ViewModels from `ObservableObject` by default. - Use `ObservableValidator` only when the ViewModel needs `INotifyDataErrorInfo` (forms, settings, input validation). - Use `ObservableRecipient` only when the ViewModel sends or receives `IMessenger` messages. - Never hand-implement `INotifyPropertyChanged` when one of the toolkit base classes can be used. If the type cannot inherit from a toolkit base (e.g., a custom control), apply the class-level `[ObservableObject]` or `[INotifyPropertyChanged]` attribute instead. ## Properties - Declare every type that uses `[ObservableProperty]` as `partial` (and every enclosing type, if nested). - Apply `[ObservableProperty]` to private fields named `name`, `_name`, or `m_name` — never PascalCase. Let the generator emit the public property. - Do not write manual `SetProperty(ref field, value)` boilerplate when the field qualifies for `[ObservableProperty]`. - Use `[NotifyPropertyChangedFor(nameof(Derived))]` to raise change notifications for derived/computed properties. - Use `[NotifyCanExecuteChangedFor(nameof(XxxCommand))]` so commands re-evaluate `CanExecute` when their inputs change. - Implement `OnXxxChanging` / `OnXxxChanged` partial-method hooks for side-effects on property changes — do not subscribe to your own `PropertyChanged` event. - Use `[property: SomeAttribute]` to forward an attribute (e.g., `[JsonIgnore]`, `[JsonPropertyName(...)]`) onto the generated property. ## Commands - Use `[RelayCommand]` on instance methods over manually constructed `RelayCommand` / `AsyncRelayCommand` instances. - `[RelayCommand]` methods must return `void` or `Task` (or `Task`). Never use `async void` — exceptions become unobserved. - For cancellable async work, declare a `CancellationToken` parameter and optionally set `IncludeCancelCommand = true` to expose a paired `XxxCancelCommand`. - Use `CanExecute = nameof(...)` plus `[NotifyCanExecuteChangedFor]` on the inputs to keep button enable/disable state in sync. - Default `AllowConcurrentExecutions` to `false` (the default). Only set `true` when overlapping invocations are explicitly safe and intended. - Default error policy is await-and-rethrow. Only set `FlowExceptionsToTaskScheduler = true` when the UI binds to `ExecutionTask` to render error states. ## Messaging - Default to `WeakReferenceMessenger.Default`. Only switch to `StrongReferenceMessenger.Default` when profiling shows the messenger is hot, and document the lifetime guarantees. - Register handlers with the `(recipient, message)` lambda form using the `static` modifier — never capture `this` in the lambda. - Prefer `IRecipient` interfaces on `ObservableRecipient` ViewModels so `RegisterAll(this)` wires everything automatically when `IsActive = true`. - Set `IsActive = true` on activation (e.g., `OnNavigatedTo`) and `IsActive = false` on deactivation (e.g., `OnNavigatedFrom`). - Inheritance is not considered when delivering messages — register each concrete message type explicitly. - Use channel tokens (the `int` / `string` / `Guid` overloads) to scope messages to a sub-system or window when more than one consumer would otherwise collide. ## Dependency injection - Use `Microsoft.Extensions.DependencyInjection` for service and ViewModel registration. Prefer the .NET Generic Host (`Host.CreateDefaultBuilder()`) so configuration, logging, and scope validation are wired automatically. - Register services and ViewModels in the composition root (typically `App.xaml.cs`). Resolve the page's root ViewModel from DI in the page constructor or via the navigation framework. - Inject services and child ViewModels through constructors. Do not call `Ioc.Default.GetService()` from inside ViewModels, services, or any type the DI container can construct. - Lifetimes: - `AddSingleton()` — shell/main-window VMs, settings, file/HTTP services, the shared `IMessenger`. - `AddTransient()` — per-page or per-document VMs. - `AddScoped()` — only with explicit `IServiceScope` usage; rarely needed in client apps. - Register `IMessenger` once (`services.AddSingleton(WeakReferenceMessenger.Default)`) and inject it via `ObservableRecipient(messenger)` constructors. ## Validation - Use `ObservableValidator` plus `[NotifyDataErrorInfo]` and DataAnnotation attributes (`[Required]`, `[Range]`, `[EmailAddress]`, `[MinLength]`, `[MaxLength]`, `[CustomValidation]`). - Call `ValidateAllProperties()` before submitting a form; check `HasErrors` and bail out if `true`. - Reset error state with `ClearAllErrors()` after a successful submit or when resetting a form. - For cross-property rules, call `ValidateProperty(value, nameof(Other))` from the changed property's `OnXxxChanged` hook. ## XAML - For WinUI 3 / UWP, prefer `{x:Bind}` (compiled bindings) over `{Binding}`. Set `Mode=OneWay` or `Mode=TwoWay` explicitly — `{x:Bind}` defaults to `OneTime`. - Bind `Command="{x:Bind ViewModel.SaveCommand}"` directly to the generated command property. - Bind async-command status (`IsRunning`, `ExecutionTask.Status`, `ExecutionTask.Exception`) to surface progress/errors instead of blocking the UI thread. ## Things to avoid - `[ObservableProperty] private string Name;` — PascalCase field collides with the generated property; use lowerCamel. - Manual `RaisePropertyChanged(nameof(X))` calls alongside `[ObservableProperty]` — produces duplicate notifications. - `Ioc.Default.GetService()` from inside a ViewModel constructor — hides dependencies, breaks unit tests. - `StrongReferenceMessenger` without `OnDeactivated` / `UnregisterAll` — pins recipients and leaks them. - Capturing `this` in messenger lambdas — closure allocation and lifetime confusion. Always use `(r, m) => r.OnX(m)` with `static`. - `async void` on `[RelayCommand]` methods — return `Task` instead. - Mutating the same reference held by an `[ObservableProperty]` field — the equality comparer returns `true` and no change notification fires. Replace the instance instead. - Inheriting from both `ObservableValidator` and `ObservableRecipient` — not possible; use composition (inject `IMessenger` or implement validation manually). --- Source: awesome-copilot (github/awesome-copilot) https://github.com/github/awesome-copilot/blob/cb64ad12e32737ae764becd6885462e9b4c7cb00/instructions/mvvm-toolkit.instructions.md MIT License Copyright GitHub, Inc. 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