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.

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Namespace Changes

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

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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;
});

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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

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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.

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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
}

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App Settings Migration

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GetForCurrentView() Replacements

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

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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.

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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