Reactor Getting Started

Coming from React? Read this first.

Reactor concepts are React's, with one C# spelling. If you know React, you already know how Reactor works — components render an element tree, hooks manage state and effects, lists need keys, lifting state up is the same. Trust your React intuition for shape; verify the names against the table below or references/reactor.api.txt.

One important difference: in Reactor, UseState with a List<T> won't re-render on .Add() — same reference. Use UseReducer for lists (just like useReducer is preferred in React for complex state).

Controlled prop note: the factories shown here still take plain values (TextBox(text, setText), Slider(value, ...)) and wrap them for you. Element-record properties behind those factories use Optional<T> so Optional<T>.Unset can mean "the control owns this value". See migration/050-optional-t.md.

Starting a new app

dotnet new reactorapp -n <Name> scaffolds the canonical shape: App.cs (entry point + initial component, with the using block at the top) plus <Name>.csproj. See the anti-probe + mur check notes under "Use a .csproj …" below for what comes out of the scaffold.

For a single-file dotnet run App.cs demo (no .csproj), prepend the file-level #:package / #:property headers — see reactor-build-and-check's single-file-scripts section.

Use a .csproj when you need …

… multiple files, analyzers (single-file .cs builds don't load them), or shared project references. dotnet new reactorapp scaffolds the canonical csproj — you don't need to author one from scratch.

UseWinUI MUST be true. No XAML files of any kind.

Don't change the packaging mode of a project you didn't scaffold — keep whatever the scaffold produced. Reactor works in both shapes:

• Unpackaged — <WindowsPackageType>None</WindowsPackageType>, no Package.appxmanifest. None wins over EnableMsixTooling, so a project carrying both still builds unpackaged. Runs from any folder, no MSIX registration. This is what dotnet new reactorapp produces today.
• Packaged — has a Package.appxmanifest. Either omit WindowsPackageType entirely (the default produces a packaged app) or set it to MSIX; pair it with EnableMsixTooling=true for the single-project MSIX tooling. Launches with package identity, which is what identity-gated APIs require.

Neither is "the" Reactor shape, and neither is signalled by one property alone. If a project has a Package.appxmanifest, it is packaged on purpose — adding <WindowsPackageType>None</WindowsPackageType> to it silently strips its identity, and the build still succeeds, so nothing surfaces the mistake. Switch modes only when the user asks, and switch the whole property set, not a single flag — the packaging guide has both shapes in full, including the explicit-MSIX form.

After dotnet new reactorapp -n <Name>, the workspace contains App.cs (entry point + initial component) and <Name>.csproj, plus a Properties/launchSettings.json for F5 — and nothing else you need to touch. Other Reactor templates scaffold more files (a packaged one adds MSIX packaging inputs); those are not source — leave them alone. There is no Program.cs and no GlobalUsings.cs — modify App.cs in place. App.cs has its own using directives at the top — see the Required imports section below — and that is where you add new namespaces (e.g. using System.Linq; when you reach for .Select(...)). Some templates enable implicit usings, so a namespace may already be in scope. Don't probe the .csproj after scaffolding unless you're adding a PackageReference or changing a property — Restore succeeded. in the scaffold stdout is the only confirmation you need.

The scaffolded csproj ships with WindowsAppSDKSelfContained=true and a Debug-only ItemGroup that adds Microsoft.UI.Reactor.Devtools + Reactor.DevtoolsSupport=true. Together they make dotnet watch run (and the very rough, experimental Visual Studio embedded-preview extension) hot-reload safe and F5 (which passes --devtools from Properties/launchSettings.json) bring up the devtools menu. The VS extension is currently the roughest Reactor surface; do not present it as stable. Release builds drop the devtools package and host-config switch so trim / AOT analyzers stay quiet — see the packaging guide for the full rationale before flipping either knob.

Verify your edits with mur check before declaring done. From the project directory: mur check (no arguments) runs dotnet build and emits one compressed line per diagnostic with a → try: suggestion when the engine recognizes the mistake; mur check --final is the explicit "I am done iterating" sweep that emits the full diagnostic set including suppressed iteration-mode warnings. For anything more involved than the build/fix loop — strict-mode failures, custom diagnostic gating, MSBuild passthrough flags — load the reactor-build-and-check skill.

Required imports

using System;
using Microsoft.UI.Reactor;
using Microsoft.UI.Reactor.Core;
using Microsoft.UI.Reactor.Layout;   // FlexDirection, FlexJustify, FlexAlign
using Microsoft.UI.Xaml;             // Thickness, HorizontalAlignment, VerticalAlignment
using Microsoft.UI.Xaml.Controls;    // Orientation, InfoBarSeverity, etc.
using static Microsoft.UI.Reactor.Factories;

Components

// Class component (primary)
class Counter : Component
{
    public override Element Render()
    {
        var (count, setCount) = UseState(0);
        return VStack(TextBlock($"Count: {count}"),
                      Button("+1", () => setCount(count + 1)));
    }
}

// Function component (inline, small reusable pieces)
var toggle = Memo(ctx =>
{
    var (on, setOn) = ctx.UseState(false);
    return ToggleSwitch(on, setOn);
});

// Typed props — use records for free structural equality
record UserCardProps(string Name, string Role);
class UserCard : Component<UserCardProps>
{
    public override Element Render() =>
        VStack(TextBlock(Props.Name), Caption(Props.Role));
}
Component<UserCard, UserCardProps>(new UserCardProps("Alice", "Admin"))

// Memoized function component
Memo(ctx => TextBlock("Stable"))               // render once + own state
Memo(ctx => TextBlock($"Hi, {name}"), name)    // re-render when deps change

Component skips parent-triggered re-renders by default. Component<TProps> skips when Equals(oldProps, newProps).

App entry point. ReactorApp.Run<MyRoot>("Title", width: W, height: H) against a component class (the scaffolded form). width/height are optional DIPs — omit them to let the OS choose the initial window size. For a tiny demo without a class, the inline form: ReactorApp.Run("Title", ctx => { var (m, setM) = ctx.UseState("hi"); return TextBlock(m); }).

Hooks

Rules: same order every render (no hooks in if/for), only from Render() or function-component body.

// UseState
var (count, setCount) = UseState(0);

// UseReducer for lists (UseState won't re-render on .Add — same reference!)
var (items, updateItems) = UseReducer(new List<Todo>());
updateItems(list => [.. list, new Todo("New", false)]);

// Action-style reducer
var (state, dispatch) = UseReducer<BoardState, BoardAction>(Board.Reduce, BoardState.Initial);

// UseEffect
UseEffect(() => { /* mount */ });                      // empty deps → once
UseEffect(() => { /* on count change */ }, count);
UseEffect(() =>
{
    var timer = new Timer(...);
    return () => timer.Dispose();                      // cleanup
}, deps);

// UseContext
public static readonly Context<string> ThemeCtx = new("light");
VStack(...).Provide(ThemeCtx, "dark")                  // provide
var theme = UseContext(ThemeCtx);                      // consume

// UseRef — mutable value that persists across renders without triggering re-render
var timerRef = UseRef<DispatcherTimer?>(null);
timerRef.Current = new DispatcherTimer();              // .Current is the property (NOT .Value)
timerRef.Current.Start();

Common factories — the 90% cases

The full catalog (every factory, modifier, enum) is in references/reactor.api.txt. The signatures below cover most apps; consult the index when you need a control not listed here.

// Layout
VStack(spacing, children...)                  HStack(spacing, children...)
FlexColumn(children...)                       FlexRow(children...)
// Prefer FlexRow/FlexColumn for linear layout — CSS Flexbox semantics
// (grow/shrink/gap/wrap, justify-content, align-items). VStack/HStack
// remain for StackPanel's shrink-wrap behavior.
Border(child).CornerRadius(8).Background(Theme.CardBackground).Padding(16)
ScrollView(VStack(...))      // modern (default); ScrollViewer(...) is the classic Control
Grid(columns: [GridSize.Star(), GridSize.Px(200)],
     rows:    [GridSize.Auto,   GridSize.Star()],
    TextBlock("Header").Grid(row: 0, column: 0, columnSpan: 2),
    ListView(items, ...).Grid(row: 1, column: 0),
    Sidebar().Grid(row: 1, column: 1))
TitleBar("App") with { Subtitle = "Home", Content = ..., RightHeader = ... }
ContentDialog(string title, Element content, string primaryButtonText = "OK")
Flyout(Element target, Element content)
NavigationView(items, content) with { SelectedTag = "home", IsPaneOpen = true }

// Text
TextBlock("hi")        Heading("Title")        SubHeading("Section")        Caption("note")
// Strings auto-convert to TextBlockElement: VStack("A", "B") works.

// Controls
Button("Click", () => ...)                                    // positional: (label, onClick)
Button("Save", onClick: handler).Background(Theme.Accent)     // named-arg form before modifiers
// `onClick` is a Button ctor parameter — NOT a chained `.OnClick(...)` /
// `.OnTapped(...)`. `.OnTapped` is a gesture event with different input
// semantics (long-press, touch, pen) and is the wrong fix for click intent.
TextBox(value, setValue, placeholderText: "...")
CheckBox(isChecked, onChanged: setChecked, label: "label")
ToggleSwitch(on, setOn)
Slider(v, 0, 100, setV)
ComboBox(items, selectedIndex, setIndex)
ProgressIndeterminate()                  ProgressRing()
InfoBar("Title", "message").Severity(InfoBarSeverity.Error)

// Lists / templated
ListView<T>(items, keySelector, viewBuilder)
GridView<T>(items, keySelector, viewBuilder)
DataGrid<T>(source, columns, ...)        // see reactor.api.txt for full signature

// Conditional / iteration
isLoggedIn ? TextBlock($"Hi, {name}") : Button("Log in", onLogin)
VStack(TextBlock("always"), showExtra ? TextBlock("maybe") : null)   // null filtered
When(items.Any(), () => TextBlock($"{items.Count} items"))
If(isError, () => InfoBar("Error", msg).Severity(InfoBarSeverity.Error),
            () => TextBlock("OK"))
status switch {
    Status.Loading => ProgressIndeterminate(),
    Status.Error   => TextBlock("Oops"),
    Status.Success => Component<SuccessView>(),
    _ => Empty()
}
ForEach(items, item => TextBlock(item.Name))
items.Select(i => Component<Card, CardProps>(new CardProps(i)).WithKey(i.Id)).ToArray()

// Common modifiers
.Margin(10)            .Margin(left: 8, top: 4, right: 8, bottom: 4)
.Padding(16)           .Background(Theme.CardBackground)
.Foreground(Theme.PrimaryText)
.CornerRadius(8)       .WithBorder(Theme.CardStroke, 1)
.Flex(grow: 1, basis: 0)        // CSS `flex: 1` equivalent
.WithKey("id")                  // dynamic list items — see gotcha #6
.OnTapped((s, e) => ...)        // tap on non-Button surfaces — Border, Image, ScrollView, …
                                // (Button click → ctor arg, see Controls section)
                                // ⚠️ For UIA/automation: always pair with .AutomationName("...")
.AutomationName("Submit")       // a11y — sets AutomationProperties.Name
.Ref(targetRef)                 // bind an ElementRef<T>; Compose FocusRequester / React callback-ref shape
.Target(targetRef)              // reference prop, e.g. TeachingTip.Target; also .LabeledBy/.XYFocusRight
.Set(native => native.MaxWidth = 400)   // native escape hatch (lambda receives the WinUI control)

Theme tokens (always)

Use Theme.* for all themed colors — never hardcoded hex on themed surfaces. The full token list with WinUI keys is in the reactor-design skill.

⚠️ Theme.Error, Theme.Success, Theme.Warning, Theme.ErrorText do NOT exist. Use Theme.SystemCritical (red/error), Theme.SystemSuccess (green), Theme.SystemCaution (yellow).

TextBlock("Hi").Foreground(Theme.PrimaryText)
Border(child).Background(Theme.CardBackground).WithBorder(Theme.CardStroke, 1)
Button("Action").Background(Theme.Accent)
TextBlock("Error!").Foreground(Theme.SystemCritical)       // NOT Theme.Error
TextBlock("Saved").Foreground(Theme.SystemSuccess)         // NOT Theme.Success

Critical gotchas

1. Hook order is constant. No hooks inside if/for. Call them all unconditionally; conditionally use the result.
2. Type-specific sugar before generic modifiers. TextBlock("Hi").Bold().Margin(10) ✓ — .Bold() needs TextBlockElement. TextBlock("Hi").Margin(10).Bold() ✗ — .Margin() returns Element.
3. List mutations need UseReducer. UseState(new List<T>()) + list.Add() won't re-render — same reference. Use UseReducer(list => [.. list, item]).
4. Null children are filtered. VStack(a, condition ? b : null, c) is safe.
5. Records with with for init-only properties. NavigationView(items, content) with { SelectedTag = "home", IsPaneOpen = true }.
6. .WithKey("id") on dynamic list items. Without keys, the reconciler matches by position and re-mounts everything on insert/reorder — losing focus, animation state, ElementRef identity. The REACTOR_DSL_001 analyzer catches this in .csproj builds.
7. Memoize expensive computations. UseMemo(() => items.OrderBy(...).ToList(), items).
8. .Flex(grow: 1) is flex-grow, not the CSS flex: 1 shorthand. Default basis is auto (content size), so a growing child with large intrinsic content overflows the container. Pass .Flex(grow: 1, basis: 0) (matches CSS flex: 1) or add .Flex(shrink: 0) to each fixed-size sibling.
9. Don't pass freshly-allocated objects/arrays/lambdas as hook deps. They compare unequal every render → hook never hits its stable path. The REACTOR_HOOKS_004 analyzer catches this. Tuples also trigger this — (x, y) allocates a new ValueTuple each render. Instead, use a string key: $"{x}|{y}", or pass individual values as separate deps: UseEffect(fn, x, y).
10. UseResource is reads-only. Never call Post*/Create*/Delete*/Save* from a UseResource fetcher — it can re-run on deps change, retry, and focus revalidation. Use UseMutation for writes.

Element refs and reference props

Use UseElementRef<T>() when one element needs to point at another real WinUI element. Attach the cell with .Ref(...), then pass it to a reference prop such as TeachingTip.Target, .LabeledBy(...), or .XYFocusRight(...). This is Reactor's callback-ref / Compose FocusRequester shape: the graph updates when the target mounts late, unmounts, or is recreated.

var target = this.UseElementRef<FrameworkElement>();

return HStack(
    Button("Show tip", () => setOpen(true)).Ref(target),
    TeachingTip("Hint", target: target) with { IsOpen = open });

Do not set relationship properties by reading target.Current in a handler; use the reference prop so Reactor can clear and re-resolve it.

Common patterns (paste-ready)

These cover the bulk of "stateful app with lists, dialogs, and per-row actions" — copy and adapt rather than re-deriving from the api index.

Drag and drop (typed payload between two lists)

using Microsoft.UI.Reactor.Input;   // DragOperations, DragData

sealed record Item(string Id, string Title);

Element RenderList(string title,
                   IReadOnlyList<Item> items,
                   Action<IReadOnlyList<Item>> setThis)
{
    var children = new List<Element> { TextBlock(title).SemiBold() };

    foreach (var item in items)
    {
        var captured = item;                                         // capture for the lambda
        children.Add(
            Border(TextBlock(captured.Title))
                .Background(Theme.CardBackground).CornerRadius(6).Padding(10)
                .OnDragStart<BorderElement, Item>(
                    getPayload: () => captured,
                    allowedOperations: DragOperations.Move,
                    onEnd: ctx =>
                    {
                        // Move-on-confirmation: only remove after a confirmed Move
                        // (not on cancel or Copy). Avoids the source losing data
                        // if the drop target rejects.
                        if (!ctx.WasCancelled && ctx.CompletedOperation == DragOperations.Move)
                            setThis(items.Where(i => i.Id != captured.Id).ToList());
                    })
        );
    }

    return VStack(6, children.ToArray())
        .OnDrop<StackElement, Item>(
            onDrop: dropped =>
            {
                if (!items.Any(i => i.Id == dropped.Id))
                    setThis(items.Append(dropped).ToList());
            },
            acceptedOps: DragOperations.Move);
}

For cross-process text drag (drop into Notepad/Word): use .OnDragStart<BorderElement>(() => new DragData().WithText("...")) and .OnDrop<BorderElement>(args => args.Data.TryGetText(out var t)).

ContentDialog (modal — confirm, edit, alert)

ContentDialog is a render-tree element with IsOpen/OnClosed driven by component state — same pattern as React. Don't try to imperatively .ShowAsync(); let the reconciler manage it.

var (showConfirm, setShowConfirm) = UseState(false);
var (lastResult, setLastResult) = UseState("(none)");

VStack(8,
    Button("Delete item", () => setShowConfirm(true)),
    TextBlock($"Last result: {lastResult}").Foreground(Theme.SecondaryText),

    ContentDialog("Confirm delete",
                  TextBlock("Are you sure? This cannot be undone."),
                  primaryButtonText: "Delete") with
    {
        IsOpen = showConfirm,
        SecondaryButtonText = "Cancel",
        =>
        {
            setLastResult(result.ToString());
            setShowConfirm(false);
        },
    }
)

For an "edit existing item" dialog: lift the item being edited into state (UseState<Item?>(null)); when non-null, render the dialog with IsOpen = editing != null; in OnClosed either commit the edit and clear, or just clear.

Flyout / context menu (right-click, dropdown menu)

// Right-click context menu on any element
Border(TextBlock("Right-click me"))
    .Padding(12).Background(Theme.SubtleFill).CornerRadius(6)
    .WithContextFlyout(MenuItems(
        MenuItem("Edit",      () => beginEdit(item)),
        MenuItem("Duplicate", () => duplicate(item)),
        MenuSeparator(),
        MenuItem("Delete",    () => delete(item))
    ))

// Dropdown button with a menu
DropDownButton("Sort", flyout: MenuItems(
    MenuItem("By name", () => setSort(Sort.Name)),
    MenuItem("By date", () => setSort(Sort.Date)),
    MenuSeparator(),
    MenuItem("Reverse", () => setSort(s => s.Reversed()))
))

// Click flyout with custom content (vs. menu items)
Button("Open flyout", null).WithFlyout(ContentFlyout(
    VStack(8,
        TextBlock("Custom content here").SemiBold(),
        Slider(value, 0, 100, setValue)),
    placement: Microsoft.UI.Xaml.Controls.Primitives.FlyoutPlacementMode.Bottom
))

Context (dispatch / theme passed without prop drilling)

When many descendants need dispatch(action) or a theme value, define Context<T> once and .Provide(...) from a parent. Same shape as React's createContext + useContext.

// At module/class scope
sealed record AppAction(string Name);
static readonly Context<Action<AppAction>> DispatchCtx = new(_ => { });

// At the root
class App : Component
{
    public override Element Render()
    {
        var (state, dispatch) = UseReducer<AppState, AppAction>(App.Reduce, AppState.Initial);

        return VStack(
            Heading("My app"),
            Component<ChildView>()
        ).Provide(DispatchCtx, dispatch);     // descendants can read this
    }
}

// Anywhere in the subtree
class ChildView : Component
{
    public override Element Render()
    {
        var dispatch = UseContext(DispatchCtx);
        return Button("Do thing", () => dispatch(new AppAction("ThingClicked")));
    }
}

Nested .Provide() overrides the outer for its subtree only. If no provider is present, UseContext returns the Context<T> default.

Bootstrap

mur pack-local (selfhost) and nuget.config (consumer outside the source clone) — see the top-level SKILL.md's "Which mode are you in?" section. If selfhost restore fails with "package Microsoft.UI.Reactor 0.0.0-local was not found", run mur pack-local.

⚠️ Platform flag required when working inside this repo (selfhost): always build samples / in-repo projects with an explicit platform: dotnet build -p:Platform=x64 (or ARM64). Omitting -p:Platform=... causes WindowsAppSDKSelfContained errors. This applies to dotnet build, dotnet run, and mur check invocations alike.

The dotnet new reactorapp template auto-resolves RuntimeIdentifier from the host SDK when Platform/RuntimeIdentifier aren't explicit, so consumer projects scaffolded outside the repo build with bare dotnet build / F5 — the rule above only applies in the selfhost tree.

Where the skill content comes from (and the api index)

You're reading this through the reactor plugin — the most efficient channel. The plugin SDK preloads reactor-getting-started; topical skills (reactor-async, reactor-design, reactor-forms, reactor-navigation, reactor-input, reactor-recipes, reactor-wrapping for turning any WinUI, vendor, or custom control into a declarative element via the [GenerateReactorWrapper] source generator, reactor-advanced for Win2D canvases via the optional Microsoft.UI.Reactor.Advanced package, etc.) load only when the task explicitly needs them.

Full API index (every factory, modifier, hook, theme token, with parameter lists): references/reactor.api.txt inside the reactor-dsl skill (also bundled into the Microsoft.UI.Reactor nupkg's agentkit/ tree if you need to find it from a consumer).

Read the api index once when you need to confirm an unusual signature — it's ~12K tokens, but cheaper than the equivalent number of grep+read cycles. Don't re-read pages of it; cache what you need in your working memory.

Build output

dotnet run exits with code 1 on build failure. Always read the output — don't assume success. After non-trivial edits, run mur check <path> for one-line diagnostics with skill-file pointers (see reactor-build-and-check).

Remember: always pass -p:Platform=x64 (or ARM64). Without it, the first build always fails.