# mvvm-toolkit

Guides GitHub Copilot on CommunityToolkit.Mvvm 8.x conventions across WPF, WinUI 3, .NET MAUI, and Avalonia. Eliminates boilerplate for properties, commands, and validation.

- rule · `type=rule` · GitHub Copilot · ★ 40418
- Author: github · official GitHub repository
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/github/awesome-copilot/tree/HEAD/instructions/mvvm-toolkit.instructions.md
- Updated: 2026-10-23
- HTML: https://affpapa.org/en/nevibecode/rules/mvvm-toolkit

## What it does

This instruction set activates for `.cs`, `.xaml`, and `.csproj` files. It enforces `CommunityToolkit.Mvvm` 8.x or newer and explicitly blocks the legacy `Microsoft.Toolkit.Mvvm` package. Classes utilizing `[ObservableProperty]` must remain `partial`. The attribute attaches exclusively to lowerCamelCase private fields, letting source generators emit the public properties.

Methods use `[RelayCommand]` and must return `void` or `Task`. Returning `async void` is prohibited because exceptions turn unobserved. Asynchronous handlers should accept a `CancellationToken` to support cancellation. Without C# source generator support in your compiler SDK, the toolkit attributes will not generate code.

Dependency injection stays clean by requiring constructor injection over `Ioc.Default.GetService<T>()`. For messaging, the rule defaults to `WeakReferenceMessenger` with static lambdas to prevent closure leaks. When forms require validation, ViewModels inherit from `ObservableValidator` and run `ValidateAllProperties()` prior to submission.

## Install

### GitHub Copilot (native platform)

**Manually: put the files in place**
- File: `.github/instructions/mvvm-toolkit.instructions.md` — The applyTo field sets which files it applies to

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/mvvm-toolkit.md` — The paths frontmatter field limits the files: move applyTo from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### Cursor

**Manually: put the files in place**
- File: `.cursor/rules/mvvm-toolkit.mdc` — Frontmatter: description, globs, alwaysApply. Move applyTo from the source into globs, or the rule will not attach on its own

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/mvvm-toolkit.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/mvvm-toolkit.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/github/awesome-copilot/tree/HEAD/instructions/mvvm-toolkit.instructions.md

## When to use it
- generate reactive properties via `[ObservableProperty]` without boilerplate
- configure async commands with cancellation tokens using `[RelayCommand]`
- decouple ViewModel communication with `WeakReferenceMessenger`
- validate form inputs across XAML platforms with `ObservableValidator`

## Example prompts
- Create an edit profile ViewModel with input validation via ObservableValidator
- Add an async save command with a CancellationToken and cancellation support
- Set up message handling with WeakReferenceMessenger to broadcast item selection

## Source text · mvvm-toolkit.instructions.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 5d7863ec369b51b1.

BEGIN THIRD-PARTY FILE 5d7863ec369b51b1
```text
---
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<T>`).
  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<TMessage>` interfaces on `ObservableRecipient`
```
END THIRD-PARTY FILE 5d7863ec369b51b1

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