maui-dependency-injection（毛伊岛依赖注入）

Dependency Injection in.NET MAUI

Lifetime Decision Framework

⚠️ Avoid AddScoped in MAUI — there is no built-in scope per page. Using it without manually creating IServiceScope gives you singleton behaviour silently, which is confusing and error-prone.

Singleton traps

// ❌ ViewModel registered as Singleton — stale data across navigations
builder.Services.AddSingleton<DetailViewModel>();

// ✅ ViewModels are Transient — fresh instance each navigation
builder.Services.AddTransient<DetailViewModel>();

Register Pages and ViewModels as Transient. Register services that hold shared state as Singleton (e.g., IDataService, HttpClient factory).

Gotcha: XAML Resource Parsing vs. DI Timing

XAML resources (App.xaml styles, converters) are parsed during InitializeComponent() — before the DI container is fully available. If a resource or converter needs a service, resolve it in CreateWindow(), not in the constructor.

// ❌ Resolving services during XAML parse — container may not be ready
public App(IDataService data)
{
    InitializeComponent(); // XAML parses here
    _data = data;          // may fail for types not yet resolved
}

// ✅ Defer service resolution 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
        var mainPage = _services.GetRequiredService<MainPage>();
        return new Window(new AppShell());
    }
}

Gotcha: Unregistered Page Silently Skips DI

If a Page is used in Shell XAML (<ShellContent ContentTemplate="...">) but not registered in builder.Services, MAUI instantiates it with the parameterless constructor. Dependencies are silently null — no exception.

// ❌ Page not registered — constructor injection silently skipped
// builder.Services.AddTransient<DetailPage>(); // missing!

// ✅ Always register pages that need injection
builder.Services.AddTransient<DetailPage>();
builder.Services.AddTransient<DetailViewModel>();

Anti-Pattern: Service Locator Overuse

// ❌ Service locator scattered through code — hard to test, hides dependencies
public void DoWork()
{
    var service = this.Handler.MauiContext.Services.GetService<IDataService>();
    service.Load();
}

// ✅ Constructor injection — explicit, testable
public class MyViewModel(IDataService dataService)
{
    public void DoWork() => dataService.Load();
}

Use explicit resolution (Handler.MauiContext.Services) only when constructor injection is genuinely unavailable (e.g., inside a custom handler or platform callback).

Platform-Specific Registration Pitfall

When using #if directives for platform services, ensure the interface is always registered — otherwise consumers on untargeted platforms get a runtime null.

// ❌ No registration on Windows — GetService returns null
#if ANDROID
builder.Services.AddSingleton<INotificationService, AndroidNotificationService>();
#elif IOS || MACCATALYST
builder.Services.AddSingleton<INotificationService, AppleNotificationService>();
#endif

// ✅ Cover all platforms or provide a no-op fallback
#if ANDROID
builder.Services.AddSingleton<INotificationService, AndroidNotificationService>();
#elif IOS || MACCATALYST
builder.Services.AddSingleton<INotificationService, AppleNotificationService>();
#elif WINDOWS
builder.Services.AddSingleton<INotificationService, WindowsNotificationService>();
#endif

Checklist

• Every Page and ViewModel that needs injection is registered in MauiProgram.cs
• Pages/ViewModels are AddTransient; shared services are AddSingleton
• Constructor injection used everywhere possible; service locator only as last resort
• Interfaces defined for any service you need to mock in tests
• Platform-specific #if registrations cover all target platforms (or provide fallback)
• Late-bound services resolved in CreateWindow(), not during XAML parse
• AddScoped only used when you manually manage IServiceScope