# Wpf Rule Mvvm Constraints

> WPF MVVM layer-separation rules: no System.Windows in ViewModels, BCL-only types, CommunityToolkit.Mvvm base classes. Preloaded into MVVM-related wpf-dev-pack agents.

- Skill: `christian289/wpf-rule-mvvm-constraints` (Agent Skill)
- Install (CLI): `npx skillmds@latest add christian289/wpf-rule-mvvm-constraints`
- Raw SKILL.md: https://api.skillmd.com/api/skills/christian289/wpf-rule-mvvm-constraints/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: christian289 (https://skillmd.com/u/christian289)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/christian289/wpf-rule-mvvm-constraints

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# MVVM Constraints

Rules for enforcing strict MVVM layer separation in wpf-dev-pack projects.

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## Prohibited DLL References in ViewModel Projects

ViewModel projects must NOT reference these assemblies:

- `WindowsBase.dll`
- `PresentationFramework.dll`
- `PresentationCore.dll`

If any of these appear in a `.csproj` targeting the ViewModel layer, remove them immediately.

---

## Prohibited Using Statements

The following namespaces must never appear in ViewModel code:

```csharp
// Prohibited in ViewModel projects
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Media;
using System.Windows.Input;   // except ICommand — use CommunityToolkit.Mvvm instead
```

---

## Allowed Using Statements

These namespaces are safe for ViewModels:

```csharp
// Allowed
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.ComponentModel;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
```

---

## ViewModel Type Restrictions

ViewModel properties and constructor parameters must use BCL types only:

- `string`, `int`, `double`, `bool`, `decimal`, `DateTime`, `Guid`
- `ObservableCollection<T>`, `List<T>`, `IEnumerable<T>`
- Other ViewModels or domain model types

Do NOT expose WPF types (`Brush`, `Visibility`, `ImageSource`, `Thickness`, etc.) as ViewModel properties. Convert in the View layer using converters or triggers.

---

## CommunityToolkit.Mvvm

Prefer `CommunityToolkit.Mvvm` for all ViewModel base classes and commands:

- Inherit from `ObservableObject` or `ObservableRecipient`
- Use `[ObservableProperty]` for bindable properties (source-generated)
- Use `[RelayCommand]` for commands (source-generated)
- Never hand-roll `INotifyPropertyChanged` or `ICommand` implementations