WinUI 3 Migration Guide

Use this skill when migrating UWP apps to WinUI 3 / Windows App SDK, or when verifying that generated code uses correct WinUI 3 APIs instead of legacy UWP patterns.

Namespace Changes

All Windows.UI.Xaml.* namespaces move to Microsoft.UI.Xaml.*:

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.Xaml.Data`	`Microsoft.UI.Xaml.Data`
`Windows.UI.Xaml.Navigation`	`Microsoft.UI.Xaml.Navigation`
`Windows.UI.Xaml.Shapes`	`Microsoft.UI.Xaml.Shapes`
`Windows.UI.Composition`	`Microsoft.UI.Composition`
`Windows.UI.Input`	`Microsoft.UI.Input`
`Windows.UI.Colors`	`Microsoft.UI.Colors`
`Windows.UI.Text`	`Microsoft.UI.Text`
`Windows.UI.Core`	`Microsoft.UI.Dispatching` (for dispatcher)

Top 3 Most Common Copilot Mistakes

1. ContentDialog Without XamlRoot

// ❌ WRONG — Throws InvalidOperationException in WinUI 3
var dialog = new ContentDialog
{
    Title = "Error",
    Content = "Something went wrong.",
    CloseButtonText = "OK"
};
await dialog.ShowAsync();

// ✅ CORRECT — Set XamlRoot before showing
var dialog = new ContentDialog
{
    Title = "Error",
    Content = "Something went wrong.",
    CloseButtonText = "OK",
    XamlRoot = this.Content.XamlRoot  // Required in WinUI 3
};
await dialog.ShowAsync();

2. MessageDialog Instead of ContentDialog

// ❌ WRONG — UWP API, not available in WinUI 3 desktop
var dialog = new Windows.UI.Popups.MessageDialog("Are you sure?", "Confirm");
await dialog.ShowAsync();

// ✅ CORRECT — Use ContentDialog
var dialog = new ContentDialog
{
    Title = "Confirm",
    Content = "Are you sure?",
    PrimaryButtonText = "Yes",
    CloseButtonText = "No",
    XamlRoot = this.Content.XamlRoot
};
var result = await dialog.ShowAsync();
if (result == ContentDialogResult.Primary)
{
    // User confirmed
}

3. CoreDispatcher Instead of DispatcherQueue

// ❌ WRONG — CoreDispatcher does not exist in WinUI 3
await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () =>
{
    StatusText.Text = "Done";
});

// ✅ CORRECT — Use DispatcherQueue
DispatcherQueue.TryEnqueue(() =>
{
    StatusText.Text = "Done";
});

// With priority:
DispatcherQueue.TryEnqueue(DispatcherQueuePriority.High, () =>
{
    ProgressBar.Value = 100;
});

Windowing Migration

Window Reference

// ❌ WRONG — Window.Current does not exist in WinUI 3
var currentWindow = Window.Current;

// ✅ CORRECT — Use a static property in App
public partial class App : Application
{
    public static Window MainWindow { get; private set; }

    protected override void OnLaunched(LaunchActivatedEventArgs args)
    {
        MainWindow = new MainWindow();
        MainWindow.Activate();
    }
}
// Access anywhere: App.MainWindow

Window Management

UWP API	WinUI 3 API
`ApplicationView.TryResizeView()`	`AppWindow.Resize()`
`AppWindow.TryCreateAsync()`	`AppWindow.Create()`
`AppWindow.TryShowAsync()`	`AppWindow.Show()`
`AppWindow.TryConsolidateAsync()`	`AppWindow.Destroy()`
`AppWindow.RequestMoveXxx()`	`AppWindow.Move()`
`AppWindow.GetPlacement()`	`AppWindow.Position` property
`AppWindow.RequestPresentation()`	`AppWindow.SetPresenter()`

Title Bar

UWP API	WinUI 3 API
`CoreApplicationViewTitleBar`	`AppWindowTitleBar`
`CoreApplicationView.TitleBar.ExtendViewIntoTitleBar`	`AppWindow.TitleBar.ExtendsContentIntoTitleBar`

Dialogs and Pickers Migration

File/Folder Pickers

// ❌ WRONG — UWP style, no window handle
var picker = new FileOpenPicker();
picker.FileTypeFilter.Add(".txt");
var file = await picker.PickSingleFileAsync();

// ✅ CORRECT — Initialize with window handle
var picker = new FileOpenPicker();
var hwnd = WinRT.Interop.WindowNative.GetWindowHandle(App.MainWindow);
WinRT.Interop.InitializeWithWindow.Initialize(picker, hwnd);
picker.FileTypeFilter.Add(".txt");
var file = await picker.PickSingleFileAsync();

Threading Migration

UWP Pattern	WinUI 3 Equivalent
`CoreDispatcher.RunAsync(priority, callback)`	`DispatcherQueue.TryEnqueue(priority, callback)`
`Dispatcher.HasThreadAccess`	`DispatcherQueue.HasThreadAccess`
`CoreDispatcher.ProcessEvents()`	No equivalent — restructure async code
`CoreWindow.GetForCurrentThread()`	Not available — use `DispatcherQueue.GetForCurrentThread()`

Key difference: UWP uses ASTA (Application STA) with built-in reentrancy blocking. WinUI 3 uses standard STA without this protection. Watch for reentrancy issues when async code pumps messages.

Background Tasks Migration

// ❌ WRONG — UWP IBackgroundTask
public sealed class MyTask : IBackgroundTask
{
    public void Run(IBackgroundTaskInstance taskInstance) { }
}

// ✅ CORRECT — Windows App SDK AppLifecycle
using Microsoft.Windows.AppLifecycle;

// Register for activation
var args = AppInstance.GetCurrent().GetActivatedEventArgs();
if (args.Kind == ExtendedActivationKind.AppNotification)
{
    // Handle background activation
}

App Settings Migration

Scenario	Packaged App	Unpackaged App
Simple settings	`ApplicationData.Current.LocalSettings`	JSON file in `LocalApplicationData`
Local file storage	`ApplicationData.Current.LocalFolder`	`Environment.GetFolderPath(SpecialFolder.LocalApplicationData)`

GetForCurrentView() Replacements

All GetForCurrentView() patterns are unavailable in WinUI 3 desktop apps:

UWP API	WinUI 3 Replacement
`UIViewSettings.GetForCurrentView()`	Use `AppWindow` properties
`ApplicationView.GetForCurrentView()`	`AppWindow.GetFromWindowId(windowId)`
`DisplayInformation.GetForCurrentView()`	Win32 `GetDpiForWindow()` or `XamlRoot.RasterizationScale`
`CoreApplication.GetCurrentView()`	Not available — track windows manually
`SystemNavigationManager.GetForCurrentView()`	Handle back navigation in `NavigationView` directly

Testing Migration

UWP unit test projects do not work with WinUI 3. You must migrate to the WinUI 3 test project templates.

UWP	WinUI 3
Unit Test App (Universal Windows)	Unit Test App (WinUI in Desktop)
Standard MSTest project with UWP types	Must use WinUI test app for Xaml runtime
`[TestMethod]` for all tests	`[TestMethod]` for logic, `[UITestMethod]` for XAML/UI tests
Class Library (Universal Windows)	Class Library (WinUI in Desktop)

// ✅ WinUI 3 unit test — use [UITestMethod] for any XAML interaction
[UITestMethod]
public void TestMyControl()
{
    var control = new MyLibrary.MyUserControl();
    Assert.AreEqual(expected, control.MyProperty);
}

Key: The [UITestMethod] attribute tells the test runner to execute the test on the XAML UI thread, which is required for instantiating any Microsoft.UI.Xaml type.

Migration Checklist