Winui Design

UI Planning

Before picking controls, search the catalogue. This skill ships winui-search.exe alongside this SKILL.md. It indexes 100+ WinUI Gallery controls, every Windows Community Toolkit scenario, and a curated set of platform integration patterns (JumpList, Share, file pickers, drag-drop). Use it to ground every control choice in a real shipping sample before writing any XAML — this is the difference between guessing property names and copying canonical code.

.\winui-search.exe search "<feature 1>" "<feature 2>" ...   # batch one focused query per feature
.\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 refresh now

Workflow: in one search call, list every feature you need for the current page (one focused query per feature, not a bag of keywords) → from each shortlist pick the best ID → grab full code with get (batch up to 3 IDs per call) → then write XAML using those samples as reference. Do NOT interleave searching with coding — front-load all lookups, then code. BM25 rewards focused per-query phrasing, so keep each query about one control or pattern.

Step 1: Identify App Type and Anchor Control

Step 2: Map Requirements to Controls

Navigation: 2-7 sections → NavigationView; document 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 Grid column headers; 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 → TeachingTip; inline status → InfoBar; system notification → AppNotification.

Step 3: Plan Layout

• Content fills the window — no floating cards on empty backgrounds
• Grid for structure, StackPanel only for simple stacking of few items
• Sidebar: fixed 300-360px width; main content: Width="*" with 24px padding
• Status bar: Grid row at bottom; toolbar: CommandBar or title bar buttons

Step 4: Size the Window to the App

WinUI 3 has no SizeToContent. Without an explicit size, Windows defaults the main window to ~1024×768 — oversized for most utilities. Size the window in MainWindow's constructor; derive from the layout, not a generic.

Rubric. Width = widest row + 48 padding (24 each side), 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 check (ranges, not targets — 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

If your derived number is well below its range, you missed a row — re-check.

AppWindow.Resize takes physical pixels, not DIPs — multiply by the monitor's DPI scale:

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;
        AppWindow.Resize(new SizeInt32((int)(460 * scale), (int)(860 * scale)));
    }
}

XamlRoot.RasterizationScale is null in the ctor and stale after AppWindow.Move, so [DllImport] is the cleanest path. Don't try to size the window by setting Width/Height on the root Grid — that clips content, not the window.

If the user asks for UI validation, see winui-ui-testing Step 3.5 to verify the rubric against the visual checklist.

Step 5: Design Anti-Patterns

XAML Correctness

Theming Rules

• {ThemeResource BrushName} at usage sites — updates on theme change
• {StaticResource} with ResourceKey redirects inside theme dictionaries — zero allocation
• ResourceKey must end in Brush (target the SolidColorBrush, not the Color)
• Always define all three variants: x:Key="Light", x:Key="Dark", x:Key="HighContrast" — never use x:Key="Default"
• Verify runtime theme switching: {ThemeResource} updates; {StaticResource} does not

<!-- Correct: StaticResource redirect in theme dictionary -->
<StaticResource x:Key="MyBrush" ResourceKey="ControlFillColorDefaultBrush" />

<!-- Wrong: inline SolidColorBrush allocates new object -->
<SolidColorBrush x:Key="MyBrush" Color="{StaticResource ControlFillColorDefault}" />

High Contrast

Only 8 system color brushes allowed in HC dictionaries:

HC prohibitions: No hardcoded colors, no opacity, no accent colors, no regular WinUI brushes, no SystemColor* in Light/Dark dicts. Use empty HC dict when WinUI defaults suffice. Set HighContrastAdjustment = None at app level.

Typography — Use Styles, Not Raw FontSize

Use SemiBold, never Bold. Minimum 12px. BasedOn styles must not re-declare inherited properties.

Spacing and Layout

• 4px grid: margins, padding, sizes must be multiples of 4 (4, 8, 12, 16, 24, 32, 48)
• ControlCornerRadius (4px) for controls, OverlayCornerRadius (8px) for overlays — never hardcode
• RowSpacing/ColumnSpacing instead of spacer elements
• MinHeight/MinWidth instead of fixed sizing
• No negative margins

Remove Defaults

Don't set WinUI default values — blocks future updates:

• BodyTextBlockStyle on TextBlock, TextFillColorPrimaryBrush foreground, TextWrapping="NoWrap", Padding="0", Margin="0"

Acrylic Pairings

Use BackgroundSizing="InnerBorderEdge" on bordered acrylic. ThemeShadow requires Translation="0,0,32" and 12px parent padding.

Data Binding

• {x:Bind} over {Binding}, explicit Mode=OneWay/TwoWay, x:DataType on DataTemplate
• TextBox x:Bind TwoWay — always add UpdateSourceTrigger=PropertyChanged so the ViewModel updates on each keystroke instead of waiting for LostFocus. Without it, UIA automation (set-value) and programmatic changes won't commit to the ViewModel.

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

• Commands over Click/Tapped handlers (MVVM)
• VisualStateManager for visual property changes, not code-behind
• No IValueConverter — prefer x:Bind with functions

Bool negation and Visibility functions — define static methods in code-behind:

// In code-behind (e.g., MainPage.xaml.cs)
public static Visibility BoolToVisibility(bool value) =>
    value ? Visibility.Visible : Visibility.Collapsed;
public static Visibility InvertBoolToVisibility(bool value) =>
    value ? Visibility.Collapsed : Visibility.Visible;
public static bool IsNotBusy(bool isLoading) => !isLoading;

<!-- Usage in XAML -->
Visibility="{x:Bind local:MainPage.BoolToVisibility(ViewModel.IsLoading), Mode=OneWay}"
IsEnabled="{x:Bind local:MainPage.IsNotBusy(ViewModel.IsLoading), Mode=OneWay}"

❌ NEVER use Converter={x:Null} — it crashes at runtime.

Accessibility

• AutomationProperties.Name on icon-only controls
• AutomationProperties.AutomationId on all interactive controls
• Semantic controls (Button, HyperlinkButton) — not clickable Border/TextBlock
• DividerStrokeColorDefaultBrush for dividers

Setting attached properties in code-behind — WinUI attached properties use static methods, NOT object initializer syntax:

using Microsoft.UI.Xaml.Automation; // required for AutomationProperties

// ❌ WRONG — object initializer doesn't work for attached properties
var btn = new Button { AutomationProperties = { AutomationId = "BtnSave" } };

// ✅ CORRECT — static setter method
var btn = new Button { Content = "Save" };
AutomationProperties.SetAutomationId(btn, "BtnSave");
AutomationProperties.SetName(btn, "Save button");
Grid.SetRow(btn, 1);
Grid.SetColumn(btn, 0);
ToolTipService.SetToolTip(btn, "Save the current document");

Formatting

• Self-closing tags for childless elements
• Styles referenced with {StaticResource} not {ThemeResource}
• No px suffix on numeric values, no commented-out XAML
• Consistent attribute order: x:Name, AutomationProperties, layout, content, style

References