dotnet-winui

WinUI 3 / Windows App SDK development: project setup with UseWinUI and Windows 10 TFM, XAML patterns with compiled bindings (x:Bind) and deferred loading (x:Load), MVVM with CommunityToolkit.Mvvm, MSIX and unpackaged deployment modes, Windows integration (lifecycle, notifications, widgets), UWP migration guidance, and common agent pitfalls.

Version assumptions: .NET 8.0+ baseline. Windows App SDK 1.6+ (current stable). TFM net8.0-windows10.0.19041.0. .NET 9 features explicitly marked.

Scope boundary: This skill owns WinUI 3 project setup, XAML patterns, MVVM integration, packaging modes, Windows platform integration, and UWP migration guidance. Desktop testing is owned by [skill:dotnet-ui-testing-core]. Migration decision matrix is owned by [skill:dotnet-wpf-migration].

Out of scope: Desktop UI testing (Appium, WinAppDriver) -- see [skill:dotnet-ui-testing-core]. General Native AOT patterns -- see [skill:dotnet-native-aot]. UI framework selection decision tree -- see [skill:dotnet-ui-chooser]. WPF patterns -- see [skill:dotnet-wpf-modern].

Cross-references: [skill:dotnet-ui-testing-core] for desktop testing, [skill:dotnet-wpf-modern] for WPF patterns, [skill:dotnet-wpf-migration] for migration guidance, [skill:dotnet-native-aot] for general AOT, [skill:dotnet-ui-chooser] for framework selection, [skill:dotnet-native-interop] for general P/Invoke patterns (CsWin32 generates P/Invoke declarations), [skill:dotnet-accessibility] for accessibility patterns (AutomationProperties, AutomationPeer, UI Automation).

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Project Setup

WinUI 3 uses the Windows App SDK (formerly Project Reunion) as its runtime and API layer. Projects target a Windows 10 version-specific TFM.

<!-- MyWinUIApp.csproj -->
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net8.0-windows10.0.19041.0</TargetFramework>
    <UseWinUI>true</UseWinUI>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>

    <!-- Windows App SDK version (auto-referenced via UseWinUI) -->
    <WindowsAppSDKSelfContained>true</WindowsAppSDKSelfContained>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.*" />
    <PackageReference Include="CommunityToolkit.WinUI.Controls.SettingsControls" Version="8.*" />
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="8.*" />
  </ItemGroup>
</Project>

Project Layout

MyWinUIApp/
  App.xaml / App.xaml.cs        # Application entry, resource dictionaries
  MainWindow.xaml / .xaml.cs    # Main window
  ViewModels/                   # MVVM ViewModels
  Views/                        # XAML pages (for Frame navigation)
  Models/                       # Data models
  Services/                     # Service interfaces and implementations
  Assets/                       # Images, icons
  Package.appxmanifest          # MSIX manifest (packaged mode)
  Properties/
    launchSettings.json

Host Builder Pattern

Modern WinUI apps use the generic host for dependency injection and service configuration:

// App.xaml.cs
public partial class App : Application
{
    private readonly IHost _host;

    public App()
    {
        this.InitializeComponent();

        _host = Host.CreateDefaultBuilder()
            .ConfigureServices((context, services) =>
            {
                // Services
                services.AddSingleton<INavigationService, NavigationService>();
                services.AddSingleton<IProductService, ProductService>();

                // ViewModels
                services.AddTransient<MainViewModel>();
                services.AddTransient<ProductDetailViewModel>();

                // Views
                services.AddTransient<MainPage>();
                services.AddTransient<ProductDetailPage>();

                // Windows
                services.AddSingleton<MainWindow>();
            })
            .Build();
    }

    protected override async void OnLaunched(LaunchActivatedEventArgs args)
    {
        await _host.StartAsync();

        var mainWindow = _host.Services.GetRequiredService<MainWindow>();
        mainWindow.Closed += async (_, _) =>
        {
            await _host.StopAsync();
            _host.Dispose();
        };
        mainWindow.Activate();
    }

    public static T GetService<T>() where T : class
    {
        var app = (App)Application.Current;
        return app._host.Services.GetRequiredService<T>();
    }
}

TFM Requirements

The net8.0-windows10.0.19041.0 TFM specifies:

• .NET 8.0 -- the runtime version
• Windows 10 build 19041 (version 2004) -- the minimum Windows SDK version

Windows App SDK features may require higher SDK versions:

• Widgets (Windows 11): net8.0-windows10.0.22000.0 (Windows 11 build 22000)
• Mica backdrop: net8.0-windows10.0.22000.0
• Snap layouts integration: net8.0-windows10.0.22000.0

---

XAML Patterns

WinUI 3 XAML is distinct from UWP XAML. The root namespace is Microsoft.UI.Xaml, not Windows.UI.Xaml.

Compiled Bindings (x:Bind)

x:Bind provides compile-time type checking and better performance than {Binding}. It resolves properties relative to the code-behind class (not the DataContext).

<Page x:Class="MyApp.Views.ProductListPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:vm="using:MyApp.ViewModels">

    <Page.Resources>
        <!-- x:Bind resolves against code-behind, so expose ViewModel as property -->
    </Page.Resources>

    <StackPanel Padding="16" Spacing="12">
        <TextBox Text="{x:Bind ViewModel.SearchTerm, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
        <Button Content="Search" Command="{x:Bind ViewModel.SearchCommand}" />

        <ListView ItemsSource="{x:Bind ViewModel.Products, Mode=OneWay}"
                  SelectionMode="Single">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="vm:ProductViewModel">
                    <StackPanel Orientation="Horizontal" Spacing="12" Padding="8">
                        <Image Source="{x:Bind ImageUrl}" Height="60" Width="60" />
                        <StackPanel>
                            <TextBlock Text="{x:Bind Name}" Style="{StaticResource BodyStrongTextBlockStyle}" />
                            <TextBlock Text="{x:Bind Price}" Style="{StaticResource CaptionTextBlockStyle}" />
                        </StackPanel>
                    </StackPanel>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </StackPanel>
</Page>

// Code-behind: expose ViewModel property for x:Bind
public sealed partial class ProductListPage : Page
{
    public ProductListViewModel ViewModel { get; }

    public ProductListPage()
    {
        ViewModel = App.GetService<ProductListViewModel>();
        this.InitializeComponent();
    }
}

Key differences from {Binding}:

• x:Bind is resolved at compile time (type-safe, faster)
• Default mode is OneTime (not OneWay like {Binding})
• Resolves against the code-behind class, not DataContext
• Requires x:DataType in DataTemplate items

Deferred Loading (x:Load)

Use x:Load to defer element creation until needed, reducing initial page load time:

<StackPanel>
    <TextBlock Text="Always visible" />

    <!-- This panel is not created until ShowDetails is true -->
    <StackPanel x:Load="{x:Bind ViewModel.ShowDetails, Mode=OneWay}" x:Name="DetailsPanel">
        <TextBlock Text="Detail content loaded on demand" />
        <ListView ItemsSource="{x:Bind ViewModel.DetailItems, Mode=OneWay}" />
    </StackPanel>
</StackPanel>

When to use x:Load: Heavy UI sections (complex lists, settings panels, detail views) that are not immediately visible. The element is created when the bound property becomes true and destroyed when it becomes false.

NavigationView Pattern

WinUI apps typically use NavigationView with a Frame for page navigation:

<!-- MainWindow.xaml -->
<Window x:Class="MyApp.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <NavigationView x:Name="NavView"
                    IsBackButtonVisible="Collapsed"
                    SelectionChanged="NavView_SelectionChanged">
        <NavigationView.MenuItems>
            <NavigationViewItem Content="Home" Tag="home" Icon="Home" />
            <NavigationViewItem Content="Products" Tag="products" Icon="Shop" />
            <NavigationViewItem Content="Settings" Tag="settings" Icon="Setting" />
        </NavigationView.MenuItems>

        <Frame x:Name="ContentFrame" />
    </NavigationView>
</Window>

---

MVVM

WinUI 3 integrates with CommunityToolkit.Mvvm (the same MVVM Toolkit used by MAUI). Source generators eliminate boilerplate for properties and commands.

// ViewModels/ProductListViewModel.cs
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

public partial class ProductListViewModel : ObservableObject
{
    private readonly IProductService _productService;

    public ProductListViewModel(IProductService productService)
    {
        _productService = productService;
    }

    [ObservableProperty]
    [NotifyCanExecuteChangedFor(nameof(SearchCommand))]
    private string _searchTerm = "";

    [ObservableProperty]
    private ObservableCollection<ProductViewModel> _products = [];

    [ObservableProperty]
    private bool _isLoading;

    [ObservableProperty]
    private bool _showDetails;

    [RelayCommand]
    private async Task LoadProductsAsync(CancellationToken ct)
    {
        IsLoading = true;
        try
        {
            var items = await _productService.GetProductsAsync(ct);
            Products = new ObservableCollection<ProductViewModel>(
                items.Select(p => new ProductViewModel(p)));
        }
        finally
        {
            IsLoading = false;
        }
    }

    [RelayCommand(CanExecute = nameof(CanSearch))]
    private async Task SearchAsync(CancellationToken ct)
    {
        var results = await _productService.SearchAsync(SearchTerm, ct);
        Products = new ObservableCollection<ProductViewModel>(
            results.Select(p => new ProductViewModel(p)));
    }

    private bool CanSearch() => !string.IsNullOrWhiteSpace(SearchTerm);
}

Key source generator attributes:

• [ObservableProperty] -- generates property with INotifyPropertyChanged from a backing field
• [RelayCommand] -- generates ICommand from a method (supports async, cancellation, CanExecute)
• [NotifyPropertyChangedFor] -- raises PropertyChanged for dependent properties
• [NotifyCanExecuteChangedFor] -- re-evaluates command CanExecute when property changes

---

Packaging

WinUI 3 supports two deployment models: MSIX packaged and unpackaged. The choice affects app identity, capabilities, and distribution.

MSIX Packaged Deployment

MSIX is the default packaging model. It provides app identity, clean install/uninstall, automatic updates, and access to full Windows integration APIs.

<!-- Package.appxmanifest declares app identity and capabilities -->
<Package xmlns="http://schemas.microsoft.com/appx/manifest/foundation/windows10"
         xmlns:mp="http://schemas.microsoft.com/appx/2014/phone/manifest"
         xmlns:uap="http://schemas.microsoft.com/appx/manifest/uap/windows10">

  <Identity Name="MyApp" Publisher="CN=Contoso" Version="1.0.0.0" />

  <Applications>
    <Application Id="App"
                 Executable="$targetnametoken$.exe"
                 EntryPoint="$targetentrypoint$">
      <uap:VisualElements DisplayName="My App"
                          Description="WinUI 3 application"
                          BackgroundColor="transparent"
                          Square150x150Logo="Assets\Square150x150Logo.png"
                          Square44x44Logo="Assets\Square44x44Logo.png" />
    </Application>
  </Applications>

  <Capabilities>
    <Capability Name="internetClient" />
  </Capabilities>
</Package>

# Build MSIX package
dotnet publish -c Release -r win-x64

Unpackaged Deployment

Unpackaged mode removes MSIX requirements. The app runs as a standard Win32 executable without app identity.

<!-- .csproj: enable unpackaged mode -->
<PropertyGroup>
  <WindowsPackageType>None</WindowsPackageType>
</PropertyGroup>

Trade-offs:

Feature	MSIX Packaged	Unpackaged
App identity	Yes	No
Clean install/uninstall	Yes (Add/Remove Programs)	Manual
Auto-update	Yes (Store, App Installer)	Manual
Background tasks	Full support	Limited
Toast notifications	Full support	Requires COM registration
Widgets (Windows 11)	Yes	No
File type associations	Via manifest	Via registry
Distribution	Store, sideload, App Installer	xcopy, installer (MSI/EXE)
Startup time	Slightly slower (package verification)	Faster

When to choose unpackaged:

• Internal enterprise tools with existing deployment infrastructure
• Apps that need xcopy deployment or integration with existing MSI/EXE installers
• Quick prototypes where packaging overhead is unnecessary
• Apps that do not need Windows identity features

---

Windows Integration

App Lifecycle

WinUI 3 apps use the Windows App SDK activation and lifecycle model, distinct from UWP's CoreApplication.

// Handle activation kinds (protocol, file, toast, etc.)
public partial class App : Application
{
    protected override void OnLaunched(LaunchActivatedEventArgs args)
    {
        // Check for specific activation
        var activationArgs = AppInstance.GetCurrent().GetActivatedEventArgs();

        switch (activationArgs.Kind)
        {
            case ExtendedActivationKind.Protocol:
                var protocolArgs = (ProtocolActivatedEventArgs)activationArgs.Data;
                HandleProtocolActivation(protocolArgs.Uri);
                break;

            case ExtendedActivationKind.File:
                var fileArgs = (FileActivatedEventArgs)activationArgs.Data;
                HandleFileActivation(fileArgs.Files);
                break;

            default:
                // Normal launch
                break;
        }
    }
}

Notifications

Toast notifications require the Windows App SDK notification APIs:

using Microsoft.Windows.AppNotifications;
using Microsoft.Windows.AppNotifications.Builder;

// Register for notification activation
var notificationManager = AppNotificationManager.Default;
notificationManager.NotificationInvoked += OnNotificationInvoked;
notificationManager.Register();

// Send a toast notification
var builder = new AppNotificationBuilder()
    .AddText("Order Shipped")
    .AddText("Your order #12345 has shipped.")
    .AddButton(new AppNotificationButton("Track")
        .AddArgument("action", "track")
        .AddArgument("orderId", "12345"));

AppNotificationManager.Default.Show(builder.BuildNotification());

Widgets (Windows 11)

Widgets require Windows 11 (build 22000+) and MSIX packaged deployment. The implementation involves creating a widget provider that implements IWidgetProvider and registering it in the MSIX manifest.

Key steps:

1. Implement IWidgetProvider interface (methods: CreateWidget, DeleteWidget, OnActionInvoked, OnWidgetContextChanged, OnCustomizationRequested, Activate, Deactivate)
2. Register the provider as a COM class in the MSIX manifest
3. Define widget templates using Adaptive Cards JSON format
4. Return updated widget content from provider methods

See the Windows App SDK Widget documentation for the complete interface contract and manifest registration.

Taskbar Integration

Taskbar progress in WinUI 3 requires Win32 COM interop via the ITaskbarList3 interface. Unlike UWP which had a managed TaskbarManager, WinUI 3 does not expose a managed wrapper.

// Taskbar progress requires COM interop in WinUI 3
// Use CsWin32 source generator or manual P/Invoke for ITaskbarList3
// 1. Add CsWin32: <PackageReference Include="Microsoft.Windows.CsWin32" Version="0.3.*" />
// 2. Add to NativeMethods.txt: ITaskbarList3
// See: https://learn.microsoft.com/en-us/windows/win32/api/shobjidl_core/nn-shobjidl_core-itaskbarlist3

---

UWP Migration

Migrating from UWP to WinUI 3 involves namespace changes, API replacements, and project restructuring.

Namespace Changes

UWP Namespace	WinUI 3 Namespace
Windows.UI.Xaml	Microsoft.UI.Xaml
Windows.UI.Xaml.Controls	Microsoft.UI.Xaml.Controls
Windows.UI.Xaml.Media	Microsoft.UI.Xaml.Media
Windows.UI.Xaml.Input	Microsoft.UI.Xaml.Input
Windows.UI.Composition	Microsoft.UI.Composition
Windows.UI.Text	Microsoft.UI.Text
Windows.UI.Colors	Microsoft.UI.Colors

Keep as-is: Windows.Storage, Windows.Networking, Windows.Security, Windows.ApplicationModel, Windows.Devices -- these WinRT APIs remain in the Windows.* namespace.

API Replacements

UWP API	WinUI 3 Replacement
CoreApplication.MainView	App.MainWindow (track your own window reference)
CoreDispatcher.RunAsync	DispatcherQueue.TryEnqueue
Window.Current	Track window reference manually in App class
ApplicationView.Title	window.Title = "..."
CoreWindow.GetForCurrentThread	Not available; use InputKeyboardSource for keyboard APIs
SystemNavigationManager.BackRequested	NavigationView.BackRequested

Migration Steps

1. Create a new WinUI 3 project using the Windows App SDK template
2. Copy source files and update namespaces (Windows.UI.Xaml to Microsoft.UI.Xaml)
3. Update XAML namespaces in all .xaml files
4. Replace deprecated APIs (see table above)
5. Migrate packaging from .appxmanifest UWP format to Windows App SDK format
6. Update NuGet packages to Windows App SDK-compatible versions
7. Test Windows integration features (notifications, background tasks, file associations)

For comprehensive migration path guidance across frameworks, see [skill:dotnet-wpf-migration].

UWP .NET 9 preview path: Microsoft announced UWP support on .NET 9 as a preview. This allows UWP apps to use modern .NET without migrating to WinUI 3. Evaluate this path if full WinUI migration is too costly but you need modern .NET runtime features.

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Agent Gotchas

1. Do not confuse UWP XAML with WinUI 3 XAML. The root namespace changed from Windows.UI.Xaml to Microsoft.UI.Xaml. Code using Windows.UI.Xaml.* types will not compile in WinUI 3 projects.
2. Do not use Window.Current. WinUI 3 does not have a static Window.Current property. Track your window reference manually in the App class and pass it via DI or a static property.
3. Do not use CoreDispatcher. Replace CoreDispatcher.RunAsync() with DispatcherQueue.TryEnqueue(). CoreDispatcher is a UWP API not available in WinUI 3.
4. Do not assume MSIX is required. WinUI 3 supports unpackaged deployment via <WindowsPackageType>None</WindowsPackageType>. Only use MSIX when you need app identity, Store distribution, or Windows integration features that require it.
5. Do not forget x:Bind defaults to OneTime. Unlike {Binding} which defaults to OneWay, x:Bind defaults to OneTime. Always specify Mode=OneWay or Mode=TwoWay for properties that change after initial binding.
6. Do not target Windows 10 builds below 19041. Windows App SDK 1.6+ requires a minimum of build 19041 (version 2004). Targeting lower builds causes runtime failures.
7. Do not use Widgets or Mica in unpackaged apps. These features require MSIX packaged deployment with app identity. Attempting to use them in unpackaged mode fails silently or throws.
8. Do not mix CommunityToolkit.Mvvm with manual INotifyPropertyChanged. Use [ObservableProperty] consistently. Mixing source-generated and hand-written implementations causes subtle binding bugs.
9. Do not forget the Host builder lifecycle. Call _host.StartAsync() in OnLaunched and _host.StopAsync() when the window closes. Forgetting lifecycle management causes DI-registered IHostedService instances to never start or stop.

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Prerequisites

• .NET 8.0+ with Windows desktop workload
• Windows App SDK 1.6+ (auto-referenced via UseWinUI)
• Windows 10 version 2004 (build 19041) or later
• Visual Studio 2022+ with Windows App SDK workload, or VS Code with C# Dev Kit
• For widgets: Windows 11 (build 22000+)

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References

• WinUI 3 Documentation
• Windows App SDK
• CommunityToolkit.Mvvm
• UWP to WinUI Migration
• MSIX Packaging
• Windows App SDK Deployment Guide