maui-dependency-injection

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

Rules That Change the Answer

Situation	Do this	Why
Registering a Page or ViewModel	Prefer AddTransient	A fresh instance per navigation avoids stale state, and a Singleton page cannot be re-added to the visual tree after it is removed. Singleton is defensible for a genuinely single-instance page (e.g. a root tab you want to keep warm)
Registering shared/expensive state	AddSingleton	One instance app-wide (settings, DB connection, HttpClient handler)
Tempted to use AddScoped	Use AddTransient (or AddSingleton if sharing is intended)	MAUI has no built-in request scope like ASP.NET Core's HTTP pipeline. MAUI does create one IServiceScope per window, so a Scoped service lives as long as that window — and resolved from the root provider it behaves like a Singleton. Neither gives you per-navigation freshness
Navigating to a DI-registered page	Register the page and its ViewModel, then Routing.RegisterRoute	Shell.Current.GoToAsync resolves the page through DI and injects its constructor dependencies
Platform-specific implementation	#if per platform with every platform covered	A missing platform branch leaves the service unregistered and throws at resolution time

Do not introduce DI into a project that isn't using it, swap a working service lifetime, or add an interface purely for symmetry — only when the user asked or it fixes a real defect.

Answer narrowly, but completely. When you recommend a lifetime change, show the registration code, and give the realistic alternatives rather than a single verdict — for a unit-of-work or DbContext question that means AddTransient, an explicit IServiceScopeFactory.CreateScope(), and the factory pattern (AddDbContextFactory), with a note on when each fits. A one-line prescription is usually a worse answer than a short menu with trade-offs.

// Explicit scope when you genuinely need unit-of-work semantics
using var scope = scopeFactory.CreateScope();
var db = scope.ServiceProvider.GetRequiredService<MyDbContext>();

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.

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Lifetime Selection

Lifetime	When to Use	Typical Types
AddSingleton<T>()	Shared state, expensive to create, app-wide config	HttpClient factory, settings service, database connection
AddTransient<T>()	Lightweight, stateless, or needs a fresh instance per use	Pages, ViewModels, per-call API wrappers
AddScoped<T>()	Per-window lifetime, or a manually created IServiceScope	Scoped unit-of-work (rare in MAUI)

Key rule: Register Pages and ViewModels as Transient by default. Register shared services as Singleton.

⚠️ Avoid AddScoped unless you manually manage IServiceScope. MAUI has no built-in request scope like ASP.NET Core. MAUI creates one IServiceScope per window, so a Scoped service lives as long as that window; resolved from the root provider it silently behaves as a Singleton. Neither gives per-navigation freshness.

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Registration Pattern in MauiProgram.cs

public static MauiApp CreateMauiApp()
{
    var builder = MauiApp.CreateBuilder();
    builder.UseMauiApp<App>();

    // Services — Singleton for shared state
    builder.Services.AddSingleton<IDataService, DataService>();
    builder.Services.AddSingleton<ISettingsService, SettingsService>();

    // HTTP — use typed or named clients via IHttpClientFactory
    // Requires NuGet: Microsoft.Extensions.Http
    builder.Services.AddHttpClient<IApiClient, ApiClient>();

    // ViewModels — Transient for fresh state per navigation
    builder.Services.AddTransient<MainViewModel>();
    builder.Services.AddTransient<DetailViewModel>();

    // Pages — Transient so constructor injection fires each time
    builder.Services.AddTransient<MainPage>();
    builder.Services.AddTransient<DetailPage>();

    return builder.Build();
}

---

Constructor Injection

Inject dependencies through constructor parameters. The container resolves them automatically when the type is itself resolved from DI.

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:

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:

// MauiProgram.cs
builder.Services.AddTransient<DetailPage>();
builder.Services.AddTransient<DetailViewModel>();

// AppShell.xaml.cs
Routing.RegisterRoute(nameof(DetailPage), typeof(DetailPage));

// Navigate — DI resolves DetailPage + DetailViewModel
await Shell.Current.GoToAsync(nameof(DetailPage));

Passing parameters to a DI-resolved ViewModel

DI supplies the ViewModel's dependencies; navigation parameters arrive separately. Don't try to inject them through the constructor — implement IQueryAttributable on the ViewModel so it receives both:

public class DetailViewModel : ObservableObject, IQueryAttributable
{
    readonly IDataService _data;   // ← injected by DI

    public DetailViewModel(IDataService data) => _data = data;

    public void ApplyQueryAttributes(IDictionary<string, object> query)
    {
        // ← supplied by navigation
        if (query.TryGetValue("id", out var id))
            LoadAsync(id.ToString()!);
    }
}

// Navigate with a parameter — the page and its ViewModel still come from DI
await Shell.Current.GoToAsync($"{nameof(DetailPage)}?id={product.Id}");

Shell applies query attributes to the page and its BindingContext, so the ViewModel receives them without any wiring in the page.

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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.

#if ANDROID
builder.Services.AddSingleton<INotificationService, AndroidNotificationService>();
#elif IOS || MACCATALYST
builder.Services.AddSingleton<INotificationService, AppleNotificationService>();
#elif WINDOWS
builder.Services.AddSingleton<INotificationService, WindowsNotificationService>();
#else
builder.Services.AddSingleton<INotificationService, NoOpNotificationService>();
#endif

---

Explicit Resolution (Last Resort)

Prefer constructor injection. Use explicit resolution only where injection is genuinely unavailable (custom handlers, platform callbacks):

// From any Element with a Handler
var service = this.Handler.MauiContext.Services.GetService<IDataService>();

For dynamic resolution, inject IServiceProvider:

public class NavigationService(IServiceProvider serviceProvider)
{
    public T ResolvePage<T>() where T : Page
        => serviceProvider.GetRequiredService<T>();
}

---

Interface-First Pattern for Testability

Define interfaces for every service so implementations can be swapped in tests:

public interface IDataService
{
    Task<List<Item>> GetItemsAsync();
}

// Production registration
builder.Services.AddSingleton<IDataService, DataService>();

// Test registration — swap without touching production code
var services = new ServiceCollection();
services.AddSingleton<IDataService, FakeDataService>();

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Common Pitfalls

1. Singleton ViewModels Cause Stale Data

// ❌ ViewModel keeps stale state across navigations
builder.Services.AddSingleton<DetailViewModel>();

// ✅ Fresh instance each navigation
builder.Services.AddTransient<DetailViewModel>();

2. ContentTemplate Pages Are Not Created Through DI

Pages declared in Shell XAML via <ShellContent ContentTemplate="{DataTemplate views:DetailPage}"> are instantiated with Activator.CreateInstance (ElementTemplate.cs), not through the service provider. Constructor injection does not run on that path: if the page's only constructor takes dependencies, you get a MissingMethodException — not a silently null dependency.

Pages reached through Routing.RegisterRoute + GoToAsync are different: they go through ActivatorUtilities.GetServiceOrCreateInstance (Routing.cs), which injects registered dependencies even if the page type itself was never registered, and throws if a required dependency cannot be resolved.

// Registering the page and its dependencies keeps both paths working
builder.Services.AddTransient<DetailPage>();
builder.Services.AddTransient<DetailViewModel>();

If you need DI for a tab/flyout page, give it a parameterless constructor that resolves what it needs, or navigate to it by route instead of embedding it in ContentTemplate.

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():

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<AppShell>() in MauiProgram.cs
        var appShell = _services.GetRequiredService<AppShell>();
        return new Window(appShell);
    }
}

4. Service Locator Anti-Pattern

// ❌ Hides dependencies, hard to test
var svc = this.Handler.MauiContext.Services.GetService<IDataService>();

// ✅ Constructor injection — explicit and testable
public class MyViewModel(IDataService dataService) { }

5. Missing Platform in Conditional Registration

Forgetting a platform in #if blocks means GetService<T>() returns null at runtime on that platform. Always include an #else fallback or cover every target.

6. AddScoped Without Manual Scope

See the rule table above: AddScoped gives you either window lifetime or Singleton behaviour, never per-navigation freshness. Use AddTransient or AddSingleton unless you explicitly create and manage an IServiceScope.

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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 used only when window lifetime is intended, or alongside a manually created IServiceScope

References

• Dependency injection in .NET MAUI
• .NET dependency injection fundamentals