winui3-migration-guide

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.*:

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

Title Bar

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

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

GetForCurrentView() Replacements

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

Testing Migration

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

// ✅ 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

1. Replace all Windows.UI.Xaml.* using directives with Microsoft.UI.Xaml.*
2. Replace Windows.UI.Colors with Microsoft.UI.Colors
3. Replace CoreDispatcher.RunAsync with DispatcherQueue.TryEnqueue
4. Replace Window.Current with App.MainWindow static property
5. Add XamlRoot to all ContentDialog instances
6. Initialize all pickers with InitializeWithWindow.Initialize(picker, hwnd)
7. Replace MessageDialog with ContentDialog
8. Replace ApplicationView/CoreWindow with AppWindow
9. Replace CoreApplicationViewTitleBar with AppWindowTitleBar
10. Replace all GetForCurrentView() calls with AppWindow equivalents
11. Update interop for Share and Print managers
12. Replace IBackgroundTask with AppLifecycle activation
13. Update project file: TFM to net10.0-windows10.0.22621.0, add <UseWinUI>true</UseWinUI>
14. Migrate unit tests to Unit Test App (WinUI in Desktop) project; use [UITestMethod] for XAML tests
15. Test both packaged and unpackaged configurations