--- name: maui-dependency-injection description: > Guidance for configuring dependency injection in .NET MAUI apps — service registration in MauiProgram.cs, lifetime selection (Singleton / Transient / Scoped), constructor injection, Shell navigation auto-resolution, platform-specific registrations, and testability patterns. USE FOR: "dependency injection", "DI setup", "AddSingleton", "AddTransient", "AddScoped", "service registration", "constructor injection", "IServiceProvider", "MauiProgram DI", "register services", "BindingContext injection". DO NOT USE FOR: data binding (use maui-data-binding), Shell route configuration (use maui-shell-navigation), unit-test mocking frameworks (use standard xUnit and NSubstitute patterns). license: MIT --- # Dependency Injection in .NET MAUI .NET MAUI uses the same `Microsoft.Extensions.DependencyInjection` container as ASP.NET Core. All service registration happens in `MauiProgram.CreateMauiApp()` on `builder.Services`. The container is built once at startup and is immutable thereafter. ## When to Use - Registering services, ViewModels, and Pages in `MauiProgram.cs` - Choosing between `AddSingleton`, `AddTransient`, and `AddScoped` - Wiring constructor injection for Pages and ViewModels - Leveraging Shell navigation to auto-resolve DI-registered Pages - Registering platform-specific service implementations with `#if` directives - Designing interfaces for testable service layers ## When Not to Use - XAML data-binding syntax or compiled bindings — use the **maui-data-binding** skill - Shell route registration and query parameters — use the **maui-shell-navigation** skill - Mocking frameworks or test runners — use standard .NET testing tools (xUnit, NUnit, MSTest) and mocking libraries (NSubstitute, Moq) ## Inputs - A .NET MAUI project with a `MauiProgram.cs` file - Knowledge of which services, ViewModels, and Pages need registration - Target platforms (Android, iOS, Mac Catalyst, Windows) for conditional registrations ## Workflow 1. Identify all services, ViewModels, and Pages that need to participate in dependency injection. 2. Choose the correct lifetime for each type — `AddSingleton` for shared services, `AddTransient` for Pages and ViewModels. 3. Register all types in `MauiProgram.CreateMauiApp()` on `builder.Services`, grouping by category (services, HTTP, ViewModels, Pages). 4. Register Pages as Shell routes in `AppShell.xaml.cs` so Shell navigation auto-resolves the full dependency graph. 5. Wire each Page to its ViewModel via constructor injection, assigning the ViewModel as `BindingContext`. 6. Add platform-specific registrations with `#if` directives, ensuring every target platform is covered or has a fallback. 7. Verify resolution works by running the app and confirming no `null` dependencies or missing-registration exceptions at runtime. --- ## Lifetime Selection | Lifetime | When to Use | Typical Types | |---|---|---| | `AddSingleton()` | Shared state, expensive to create, app-wide config | `HttpClient` factory, settings service, database connection | | `AddTransient()` | Lightweight, stateless, or needs a fresh instance per use | Pages, ViewModels, per-call API wrappers | | `AddScoped()` | Per-scope lifetime with manually created `IServiceScope` | Scoped unit-of-work (rare in MAUI) | **Key rule:** Register Pages and ViewModels as **Transient**. Register shared services as **Singleton**. > ⚠️ **Avoid `AddScoped` unless you manually manage `IServiceScope`.** MAUI has no built-in request scope like ASP.NET Core. A Scoped registration without an explicit scope silently behaves as a Singleton, leading to subtle bugs. --- ## Registration Pattern in MauiProgram.cs ```csharp public static MauiApp CreateMauiApp() { var builder = MauiApp.CreateBuilder(); builder.UseMauiApp(); // Services — Singleton for shared state builder.Services.AddSingleton(); builder.Services.AddSingleton(); // HTTP — use typed or named clients via IHttpClientFactory // Requires NuGet: Microsoft.Extensions.Http builder.Services.AddHttpClient(); // ViewModels — Transient for fresh state per navigation builder.Services.AddTransient(); builder.Services.AddTransient(); // Pages — Transient so constructor injection fires each time builder.Services.AddTransient(); builder.Services.AddTransient(); return builder.Build(); } ``` --- ## Constructor Injection Inject dependencies through constructor parameters. The container resolves them automatically when the type is itself resolved from DI. ```csharp public class MainViewModel { private readonly IDataService _dataService; public MainViewModel(IDataService dataService) { _dataService = dataService; } public async Task LoadAsync() => Items = await _dataService.GetItemsAsync(); } ``` ### ViewModel → Page Wiring Register both Page and ViewModel. Inject the ViewModel into the Page and assign it as `BindingContext`: ```csharp public partial class MainPage : ContentPage { public MainPage(MainViewModel viewModel) { InitializeComponent(); BindingContext = viewModel; } } ``` --- ## Shell Navigation Auto-Resolution When a Page is registered in DI **and** as a Shell route, Shell resolves it (and its full dependency graph) automatically on navigation: ```csharp // MauiProgram.cs builder.Services.AddTransient(); builder.Services.AddTransient(); // AppShell.xaml.cs Routing.RegisterRoute(nameof(DetailPage), typeof(DetailPage)); // Navigate — DI resolves DetailPage + DetailViewModel await Shell.Current.GoToAsync(nameof(DetailPage)); ``` --- ## Platform-Specific Registration Use preprocessor directives to register platform implementations. Always cover every target platform or provide a no-op fallback to avoid runtime `null`. ```csharp #if ANDROID builder.Services.AddSingleton(); #elif IOS || MACCATALYST builder.Services.AddSingleton(); #elif WINDOWS builder.Services.AddSingleton(); #else builder.Services.AddSingleton(); #endif ``` --- ## Explicit Resolution (Last Resort) Prefer constructor injection. Use explicit resolution only where injection is genuinely unavailable (custom handlers, platform callbacks): ```csharp // From any Element with a Handler var service = this.Handler.MauiContext.Services.GetService(); ``` For dynamic resolution, inject `IServiceProvider`: ```csharp public class NavigationService(IServiceProvider serviceProvider) { public T ResolvePage() where T : Page => serviceProvider.GetRequiredService(); } ``` --- ## Interface-First Pattern for Testability Define interfaces for every service so implementations can be swapped in tests: ```csharp public interface IDataService { Task> GetItemsAsync(); } // Production registration builder.Services.AddSingleton(); // Test registration — swap without touching production code var services = new ServiceCollection(); services.AddSingleton(); ``` --- ## Common Pitfalls ### 1. Singleton ViewModels Cause Stale Data ```csharp // ❌ ViewModel keeps stale state across navigations builder.Services.AddSingleton(); // ✅ Fresh instance each navigation builder.Services.AddTransient(); ``` ### 2. Unregistered Page Silently Skips Injection If a Page appears in Shell XAML via `` but is **not** registered in `builder.Services`, MAUI creates it with the parameterless constructor. Dependencies are silently `null` — no exception is thrown. ```csharp // ❌ Missing — injection silently skipped // builder.Services.AddTransient(); // ✅ Always register pages that need injection builder.Services.AddTransient(); builder.Services.AddTransient(); ``` ### 3. XAML Resource Parsing vs. DI Timing XAML resources in `App.xaml` are parsed during `InitializeComponent()` — before the container is fully available. Defer service-dependent work to `CreateWindow()`: ```csharp public partial class App : Application { private readonly IServiceProvider _services; public App(IServiceProvider services) { _services = services; InitializeComponent(); } protected override Window CreateWindow(IActivationState? activationState) { // Safe — container is fully built // Requires: builder.Services.AddTransient() in MauiProgram.cs var appShell = _services.GetRequiredService(); return new Window(appShell); } } ``` ### 4. Service Locator Anti-Pattern ```csharp // ❌ Hides dependencies, hard to test var svc = this.Handler.MauiContext.Services.GetService(); // ✅ Constructor injection — explicit and testable public class MyViewModel(IDataService dataService) { } ``` ### 5. Missing Platform in Conditional Registration Forgetting a platform in `#if` blocks means `GetService()` returns `null` at runtime on that platform. Always include an `#else` fallback or cover every target. ### 6. AddScoped Without Manual Scope `AddScoped` in MAUI without creating `IServiceScope` manually gives Singleton behavior silently. Use `AddTransient` or `AddSingleton` instead unless you explicitly manage scopes. --- ## Checklist - [ ] Every Page and ViewModel that needs injection is registered in `MauiProgram.cs` - [ ] Pages and ViewModels use `AddTransient`; shared services use `AddSingleton` - [ ] Constructor injection used everywhere possible; service locator only as last resort - [ ] Interfaces defined for services that need test substitution - [ ] Platform-specific `#if` registrations cover all target platforms or include a fallback - [ ] Service-dependent work deferred to `CreateWindow()`, not run during XAML parse - [ ] `AddScoped` only used alongside manually created `IServiceScope` ## References - [Dependency injection in .NET MAUI](https://learn.microsoft.com/dotnet/maui/fundamentals/dependency-injection) - [.NET dependency injection fundamentals](https://learn.microsoft.com/dotnet/core/extensions/dependency-injection)