Skill v1.0.0

version: "1.0.0" name: winui-design description: "Use when designing, reviewing, or fixing WinUI 3: layout planning, control choice, Fluent Design alignment, Light/Dark/High Contrast theming, typography, spacing, brushes, accessibility, and XAML data-binding design. Load before authoring new XAML, reviewing UI PRs, migrating desktop UI to WinUI, or choosing between WinUI controls/patterns."

Search samples before writing XAML

This skill ships winui-search.exe alongside this SKILL.md (≈100 WinUI Gallery controls, every Windows Community Toolkit scenario, curated platform-integration patterns; each result returns full XAML + C# + pitfall notes). Front-load lookups, then code — don't interleave.

.\winui-search.exe search "<feature 1>" "<feature 2>" ...   # batch one focused query per feature (BM25 likes focused phrasing)
.\winui-search.exe get <id 1> <id 2> ...                     # batch up to 3 IDs — full XAML + C# + pitfall notes
.\winui-search.exe list                                       # browse all patterns (heavy — prefer search)
.\winui-search.exe update                                     # force cache refresh

Search covers controls and platform integration (file pickers, Share, JumpList, drag-drop, app lifecycle, dialogs) — front-load all lookups before writing XAML; don't interleave search with coding.

App-shape anchors

Pick the closest shipping app silhouette before laying out a page:

Reach-for-this control map

Before writing XAML, map the requirement to a platform control. These mappings exist to short-circuit cross-framework instincts (WPF DataGrid, web <select>, HTML <input type=date>):

• Navigation: 2–7 sections → NavigationView; document/session tabs → TabView; breadcrumb trail → BreadcrumbBar; 2–3 modes → SelectorBar.
• Data display: Vertical list → ListView; tiles/grid → GridView or ItemsRepeater + UniformGridLayout; hierarchy → TreeView; tabular → `ListView` with a `Grid`-based `ItemTemplate` and a header `Grid` above (WinUI has no DataGrid; don't default to CommunityToolkit.WinUI.Controls.DataGrid — its columns can't use x:Bind); master-detail → ListView + detail Grid.
• Input: Text → TextBox; number → NumberBox; search → AutoSuggestBox; date → CalendarDatePicker; boolean → ToggleSwitch; pick one from 2–3 → RadioButtons; pick one from 4+ → ComboBox.
• Feedback: Blocking decision → ContentDialog; contextual action → Flyout / MenuFlyout; onboarding / hint → TeachingTip; inline status / async progress → InfoBar; system notification → AppNotification.

If the mapping above doesn't fit, search winui-search.exe before improvising.

Window sizing (WinUI 3 specifics)

WinUI 3 has no `SizeToContent`. Without an explicit size, Windows defaults the main window to ~1024×768 — oversized for most utilities. Size it in MainWindow's constructor.

Rubric. Width = widest row + 48 padding, rounded up to nearest 20. Height = 32 (titlebar) + Σ(row heights) + Σ(spacing) + 48 padding, rounded up to 20. Round up — clipped content is a worse failure than a slightly-wide window. Sanity ranges (derive yours from the rubric):

• Single-purpose utility → ~440–560 wide
• Form / single-page tool → ~600–800 wide, ~640–800 tall
• Multi-pane (nav + content) → ~1100–1300 wide, ~720–840 tall
• Document / canvas / media editor → 1280+ wide

AppWindow.Resize takes physical pixels, not DIPs — multiply by the monitor's DPI scale. XamlRoot.RasterizationScale is null in the constructor and stale after AppWindow.Move, so [DllImport] GetDpiForWindow is the cleanest path:

using Microsoft.UI;
using Microsoft.UI.Windowing;
using System.Runtime.InteropServices;
using Windows.Graphics;
 
public sealed partial class MainWindow : Window
{
    [DllImport("user32.dll")]
    private static extern uint GetDpiForWindow(IntPtr hWnd);
 
    public MainWindow()
    {
        InitializeComponent();
        var hwnd  = Win32Interop.GetWindowFromWindowId(AppWindow.Id);
        var scale = GetDpiForWindow(hwnd) / 96.0;
        // widthDip / heightDip come from the rubric above — derive, don't copy.
        AppWindow.Resize(new SizeInt32((int)(widthDip * scale), (int)(heightDip * scale)));
    }
}

Don't size the window by setting Width/Height on the root Grid — that clips content, not the window.

XAML landmines (the things you'll otherwise ship broken)

x:Bind defaults to OneTime

<!-- ❌ silently never updates -->
<TextBlock Text="{x:Bind Vm.Status}" />
<!-- ✅ -->
<TextBlock Text="{x:Bind Vm.Status, Mode=OneWay}" />

TextBox two-way needs UpdateSourceTrigger=PropertyChanged

<TextBox Text="{x:Bind Vm.Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

Default trigger resolves to LostFocus specifically for TextBox.Text (most other properties default to PropertyChanged). The VM is not updated per keystroke, and UIA keyboard-simulation tests (WinAppDriver SendKeys, etc.) that assert immediately after typing will see stale VM state until focus moves.

Attached properties from C# use static setters, not initializers

using Microsoft.UI.Xaml.Automation;
 
// ❌ WRONG — does not compile. CS0117: 'Button' does not contain a definition for 'AutomationProperties'.
// AutomationProperties is a static class of attached-property accessors, not an instance member.
var btn = new Button { AutomationProperties = { AutomationId = "BtnSave" } };
 
// ✅ CORRECT
var btn = new Button { Content = "Save" };
AutomationProperties.SetAutomationId(btn, "BtnSave");
AutomationProperties.SetName(btn, "Save button");
Grid.SetRow(btn, 1);
ToolTipService.SetToolTip(btn, "Save the current document");

Converter={x:Null} crashes x:Bind at runtime

{x:Bind} requires Converter to be a {StaticResource} lookup. Converter={x:Null} compiles but the generated code calls LookupConverter(""), which returns null, then dereferences it — you get Resource Dictionary Key can only be String-typed / NullReferenceException on first activation of the binding. If you don't want a converter, omit the property entirely.

Prefer x:Bind static functions over IValueConverter

// MainPage.xaml.cs
public static Visibility BoolToVisibility(bool v) => v ? Visibility.Visible : Visibility.Collapsed;
public static Visibility InvertBoolToVisibility(bool v) => v ? Visibility.Collapsed : Visibility.Visible;
public static bool Not(bool v) => !v;

<TextBlock Visibility="{x:Bind local:MainPage.BoolToVisibility(Vm.IsLoading), Mode=OneWay}" />
<Button   IsEnabled="{x:Bind local:MainPage.Not(Vm.IsLoading), Mode=OneWay}" />

Acrylic and ThemeShadow rendering rules

• BackgroundSizing defaults to InnerBorderEdge on both Border and Control, which correctly clips acrylic to the inner stroke. The hazard is the opposite of intuition: don't change it to OuterBorderEdge on a bordered acrylic surface — that's what makes the material bleed past the stroke.
• ThemeShadow casts a shadow from the caster's Translation Z. Microsoft's recommended elevations are 16 for tooltips, 32 for popup/flyout UI, 128 for dialogs — pick by surface type. For non-popup casters, add the surfaces it should land on to ThemeShadow.Receivers; otherwise the shadow has nothing to fall on and looks clipped.

Theming rules (short version)

• {ThemeResource ...} at usage sites (updates on theme switch). {StaticResource} inside ThemeDictionaries for theme-local definitions; SystemAccentColor / SystemColor* are the exceptions and stay {ThemeResource}.
• Custom theme dictionaries cover Light, Dark, and HighContrast explicitly — never Default.
• Name resources by purpose (CardBackgroundBrush, DangerTextBrush), not hue.
• Light/Dark working ≠ High Contrast working. Test in a Contrast theme separately.
• Never set HighContrastAdjustment="None" unless your app already supplies system-aware brushes throughout.

Anti-patterns

Build custom UI only when all are true: no platform/Gallery/Toolkit control fits; you'll implement keyboard, focus, UI Automation, theme resources, High Contrast, and responsive behaviour; you have specs for default/hover/pressed/disabled/selected/focused/error states; you've tested with keyboard and a contrast theme.

References (load on demand)