mvvm-patterns-dotnet

# .NET MVVM Patterns Agent structures .NET UI features so models hold domain data, views hold presentation markup, and view models expose observable state and commands without business logic in code-behind. ## When to Use | Condition | Use | |---|---| | Agent creates a new WPF, MAUI, WinUI 3, UWP, or Avalonia screen. | Use this skill. | | Agent moves business logic from code-behind into a testable presentation layer. | Use this skill. | | Agent needs manual `INotifyPropertyChanged` or `ObservableCollection<T>` patterns. | Use this skill. | | Compare MVVM against MVC or MVP for an existing UI architecture choice. | Use this skill. | ## When Not to Use | Condition | Route | |---|---| | Agent edits backend services, APIs, or data access only. | Use the matching service skill. | | Agent changes styling, layout, or control templates only. | Use the platform UI skill. | | Agent works in a project with an established non-MVVM presentation pattern. | Follow the existing pattern. | | Agent focuses on command wiring details only. | Use `commands-behaviors-patterns`. | ## Required Inputs | Input | Required | Notes | |---|---|---| | Screen workflow | Yes | Workflow identifies state, user actions, and transitions. | | Domain model boundaries | Yes | Boundaries identify model ownership and transformation points. | | Target platforms | Yes | Scope identifies WPF, MAUI, WinUI 3, UWP, or Avalonia. | | Validation scope | No | Scope identifies editable fields and error rules. | | Navigation pattern | No | Pattern identifies page, window, or shell transitions. | ## Pattern Comparison | Pattern | State Owner | View Responsibility | Interaction Style | Best Fit | |---|---|---|---|---| | MVVM | View model | Bindings, templates, visual states | Data binding and commands | XAML-first UI stacks | | MVC | Controller | Passive rendering | Controller actions update model and view | Request-response UI or web apps | | MVP | Presenter | Thin interface surface | Presenter pushes values into view contract | UI stacks with weak binding support | | Code-behind driven UI | View | Layout and behavior mix | Event handlers mutate control state directly | Small throwaway screens only | ## Observable State Matrix | Concern | Pattern | Pass | |---|---|---| | Scalar state | `INotifyPropertyChanged` or shared observable base type | One logical property change raises one notification. | | Collection state | `ObservableCollection<T>` for mutable bound lists | Add, remove, and reset operations refresh the UI. | | Derived state | Explicit dependent-property notifications | Derived values stay synchronized with source state. | | Bulk refresh | Replace collection contents or the view-model instance, not control state | Data and selection stay coherent. | | Thread affinity | Marshal UI-bound updates onto the UI thread | Async refresh paths avoid cross-thread UI exceptions. | ## Platform-Specific Guidance | Platform | View and binding | View-model and hooks | |---|---|---| | WPF | `DataContext`, `Binding`, validation rules, `CollectionViewSource` | Manual `INotifyPropertyChanged`, `ObservableObject`, Prism regions, or MVVM Toolkit generators | | .NET MAUI | `BindingContext`, `ObservableCollection<T>`, Shell or page navigation | Constructor-injected services plus MVVM Toolkit or MAUI Community Toolkit | | WinUI 3 | `Page`, `NavigationView`, `x:Bind`, or `Binding` | Async initialization with CommunityToolkit.Mvvm or Prism for WinUI | | UWP | Adaptive XAML with `x:Bind` or `Binding` | Activation-aware view models with Prism or MVVM Toolkit | | Avalonia | `DataContext` and compiled bindings where enabled | Dispatcher-aware observable models with CommunityToolkit.Mvvm or ReactiveUI interop | ## Workflow | Step | Action | Pass | |---|---|---| | 1 | Map model, view, and view-model responsibilities before edits. | Each touched file has one clear owner. | | 2 | Move business logic from code-behind into view models or injected services. | Code-behind keeps composition only. | | 3 | Implement observable scalar, collection, derived, and validation state. | Bound UI updates match state changes. | | 4 | Align binding and lifecycle choices with the target platform. | XAML and navigation follow platform conventions. | | 5 | Compare the feature to MVVM, MVC, and MVP fit criteria. | The chosen pattern matches the UI stack and interaction style. | ## Integration Hooks | Integration | Trigger | Pass | |---|---|---| | CommunityToolkit.Mvvm | Boilerplate properties or commands appear across files. | Generated members replace repeated notification plumbing. | | Prism | Existing app conventions already use Prism navigation or regions. | New work matches the container and navigation style. | | Source generators | .NET 10+ UI projects already allow MVVM Toolkit generators. | Generated members compile and reduce manual code. | | Manual implementation | The project avoids extra helper packages. | Notification behavior stays explicit and testable. | ## MCP Hooks | Task | MCP Tool | Assistance | |---|---|---| | Find view, view-model, and code-behind ownership drift | `github-mcp-server-search_code` | Locates bindings, commands, notification paths, and code-behind logic. | | Inspect versioned screen files in branches or pull requests | `github-mcp-server-get_file_contents` | Reads XAML, view-model, and project files without local branch switches. | | Review prior MVVM architecture decisions | `github-mcp-server-search_pull_requests` | Surfaces earlier refactors, naming patterns, and reviewer guidance. | ## Verification Checklist | Check | Test | Pass | |---|---|---| | Layer boundaries | Review touched files and search code-behind handlers. | Responsibilities stay separated and handlers delegate to commands or coordinators. | | Notifications | Exercise direct, derived, and collection updates. | UI refreshes correctly with no manual control mutation. | | Platform fit | Compare implementation to the platform guidance table. | Binding and lifecycle choices match the target stack. | ## Common Pitfalls | Pitfall | Agent Fix | |---|---| | View model exposes control types or visual primitives that belong to the view. | Expose plain state and lets XAML handle presentation. | | Property setter raises notifications for unchanged values. | Agent guards setters with equality checks. | | Derived property notifications stay missing. | Raise explicit notifications for dependent properties. | | Collections use `List<T>` in bound screens. | Switch bound mutable collections to `ObservableCollection<T>`. | | Service location appears inside the view model. | Constructor-inject dependencies. | ## Outputs | Output | Contents | |---|---| | Responsibility map | Model, view, and view-model ownership for the feature | | State pattern | Observable property, collection, and validation choices | | Platform notes | Binding, navigation, and lifecycle decisions | | Verification plan | Focused UI update and architecture checks |