Reactor App

Reactor — Getting Started

Prefer the plugin. This file is preserved for environments that don't support the Copilot CLI / Claude plugin loading model. If you have a plugin SDK available, install / load the reactor plugin (under plugins/reactor/ in source, or agentkit/plugins/reactor/ in the NuGet). The plugin splits this content into focused per-skill files and is materially cheaper to load than this monolith.

Reactor is a React-inspired functional projection for WinUI 3. You write functions that return lightweight element descriptions; a reconciler diffs old vs new trees and patches real WinUI controls. State changes trigger re-renders automatically. No XAML. No data binding. No ViewModels.

Which mode are you in? (read this first)

Reactor ships as a NuGet package — apps reference it as <PackageReference Include="Microsoft.UI.Reactor" Version="…" /> (or #:package Microsoft.UI.Reactor@… for single-file). The package carries the framework, the analyzers, and an agent kit (signatures index + this SKILL.md). Two paths:

If you're in selfhost and local-nupkgs/ is empty, restore will fail with "package Microsoft.UI.Reactor 0.0.0-local was not found." Run mur pack-local to fix it.

Bootstrap (selfhost, fresh clone)

# Build the CLI; on first build the SignaturesGen project also writes
# skills/reactor.api.txt as part of its AfterBuild target.
dotnet build src/Reactor.Cli -p:Platform=ARM64

# `mur` mirrors itself to <repo>/bin/<arch>/. Add that to PATH or invoke directly.
.\bin\arm64\mur.exe pack-local

After this, any project under the clone resolves Microsoft.UI.Reactor 0.0.0-local from local-nupkgs/ automatically (the repo-level nuget.config configures it). A consumer outside the clone needs a project-local nuget.config pointing at the absolute path of <repo>/local-nupkgs/.

Where to find docs (mur --skill, mur --api)

The mur CLI ships these embedded — works from any directory:

A consumer who doesn't have mur can read the same files directly from the NuGet cache:

%USERPROFILE%\.nuget\packages\microsoft.ui.reactor\<version>\agentkit\
├─ SKILL.md                  ← this file
├─ reactor.api.txt           ← signatures index
└─ skills\
   ├─ async.md, design.md, commanding.md, navigation.md, forms.md,
   │  input.md, charts.md, dsl-reference.md, devtools.md, perf-tips.md
   └─ recipes\
      ├─ index.md            ← intent → recipe map
      └─ <name>.cs           ← paste-ready single-file programs

When SKILL.md or a recipe references skills/foo.md, a consumer agent reads it from agentkit/skills/foo.md in the package cache. Selfhost agents read it from <repo>/skills/foo.md.

API signatures index — load this before grepping source

skills/reactor.api.txt is a generated, alphabetized flat list of every public Factory, Modifier, Hook, Theme token (with WinUI resource key), and enum in Reactor. Load this when you need to confirm a signature. It replaces grepping src/Reactor/Elements/*.cs and walking the sub-skills' tables.

• Local / selfhost: the file is committed at skills/reactor.api.txt. Run mur --api to print it. Run mur --regen-api after framework changes.
• NuGet consumer: the same file ships in the package at <package-cache>/microsoft.ui.reactor/<version>/agentkit/reactor.api.txt (typically %USERPROFILE%\.nuget\packages\microsoft.ui.reactor\<version>\agentkit\reactor.api.txt). If mur is on PATH, mur --api prints the embedded copy.

Recipes — paste-ready snippets indexed by intent

skills/recipes/ holds compilable single-file recipes for the most common Reactor patterns. Load a recipe instead of synthesizing from skill prose. See skills/recipes/index.md for the intent → recipe map. Available today: list-add-delete, sidebar-nav, form-with-validation, async-fetch-list, themed-card, canvas-positioning, named-styles, calendar-multiselect.

Wrapping native / third-party controls (source generator)

Need a control Reactor has no built-in factory for (a vendor control or your own custom Control)? Annotate an empty partial record and the [GenerateReactorWrapper] source generator emits the element, descriptor, registration, and a factory named after the control — no hand-written wrapper:

using Microsoft.UI.Reactor.Wrappers;
[GenerateReactorWrapper(typeof(Acme.Controls.RatingDial))]
public partial record RatingDialElement;   // → using static …RatingDialElement; RatingDial(value: …, onValueChanged: …)

Full guidance — auto-inference, the [Wrap*] annotation cheat-sheet, imperative lifecycle, diagnostics, and a gallery of worked examples — is in the reactor-wrapping skill (plugins/reactor/skills/reactor-wrapping/, or agentkit/plugins/reactor/skills/reactor-wrapping/ in the nupkg). Spec: docs/specs/058-control-wrapper-generator.md.

mur check — fast feedback with skill pointers

mur check is the build, not a separate check step. It runs dotnet build under the hood and returns the same exit code. When mur check exits 0, the build is green — do not re-run dotnet build to confirm. They're the same compilation; a redundant dotnet build afterwards is wasted work.

Two enrichments over raw dotnet build:

1. Skill pointers for known REACTOR_* IDs.
2. Did-you-mean → try: suggestions for unknown identifiers, computed against the live Reactor surface for the exact diagnostic.

C:\path\Program.cs:15:23  W  REACTOR_DSL_001  Element produced by Select(...)…   → SKILL.md gotcha #6 (.WithKey on dynamic list items)
C:\path\Program.cs:34:16  E  CS1061  'ButtonElement' does not contain a definition for 'OnClick'   → try: Button(label, onClick: ...)  // [factory has Action onClick parameter]

<path> defaults to . and accepts a .csproj or directory. Single-file .cs builds work but don't load analyzers — for analyzer coverage, use a .csproj.

Trust → try: suggestions directly. Use the suggested name verbatim in your next edit; don't grep adjacent or sibling names to second-guess it. The suggestion has been computed against the actual Reactor surface for this exact diagnostic. If wrong, the next mur check will tell you — that self-correcting loop is cheaper than manual verification.

Don't introspect via [System.Reflection] to enumerate Reactor types or members — the skill files plus mur check's did-you-mean suggestions plus skills/reactor.api.txt cover the surface.

Workflow modes (Phase-2 ranker):

• mur check — iteration mode (default). A ranker suppresses cosmetic noise (CS1591 XML doc, CS0168 unused-var, IDE0xxx style, NuGet restore chatter) mid-iteration so the real blocker doesn't scroll off attention. When this exits 0, you are done — the build is green.
• mur check --final — optional pre-merge sweep. Emits the cosmetic diagnostics suppressed during iteration (XML doc, unused locals, style hints, nullable warnings, transient restore noise). For human code review or a CI ship-readiness gate; not a task-completion requirement and skipping it is fine.
• mur check -- <msbuild args> — anything after a bare -- is forwarded verbatim to dotnet build (override platform, config, restore, verbosity).

Sub-skills — load when the task calls for them

Project Setup

.csproj (copy exactly)

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net10.0-windows10.0.22621.0</TargetFramework>
    <Platforms>x64;ARM64</Platforms>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <UseWinUI>true</UseWinUI>
    <WindowsPackageType>None</WindowsPackageType>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.UI.Reactor" Version="0.0.0-local" />
    <PackageReference Include="Microsoft.WindowsAppSDK" Version="2.1.3" />
  </ItemGroup>
</Project>

In selfhost the version is 0.0.0-local (produced by mur pack-local — see "Which mode are you in?" above). Outside the source clone, replace it with whatever Microsoft.UI.Reactor version you depend on.

After dotnet new reactorapp -n <Name>, the workspace contains exactly two source files: App.cs (entry point + initial component) and <Name>.csproj. There is no Program.cs and no GlobalUsings.cs — modify App.cs in place. The .csproj does not enable implicit usings; App.cs has its own using directives at the top — the canonical set (System + Reactor + Reactor.Core + Reactor.Layout + Xaml + Xaml.Controls + static Factories) — which is the only place you add new namespaces (e.g. using System.Linq; when you reach for .Select(...)). 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.

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.

nuget.config (selfhost only — sibling of the .csproj)

If your .csproj lives outside the Reactor clone, add a nuget.config next to it pointing at the clone's local-nupkgs/ (absolute path):

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <packageSources>
    <add key="reactor-local" value="C:\path\to\reactor2\local-nupkgs" />
    <add key="nuget.org"     value="https://api.nuget.org/v3/index.json" protocolVersion="3" />
  </packageSources>
</configuration>

Inside the clone you don't need this — the repo-level nuget.config already configures the feed.

WindowsPackageType MUST be None (unpackaged, no App.xaml). UseWinUI MUST be true. No XAML files of any kind.

Required imports

using Microsoft.UI.Reactor;
using Microsoft.UI.Reactor.Core;
using Microsoft.UI.Reactor.Layout;   // FlexDirection, FlexJustify, ... (if using Flex)
using Microsoft.UI.Xaml;             // Thickness, HorizontalAlignment, VerticalAlignment
using Microsoft.UI.Xaml.Controls;    // Orientation, InfoBarSeverity, etc.
using static Microsoft.UI.Reactor.Factories;   // TextBlock(), Button(), VStack() bare calls

App entry point

// Component root
ReactorApp.Run<MyRoot>("Title", width: 1024, height: 768);

// Inline render function
ReactorApp.Run("Title", ctx =>
{
    var (msg, setMsg) = ctx.UseState("Hello!");
    return VStack(TextBlock(msg), Button("Change", () => setMsg("Changed!")));
});

Components

Class component (primary pattern)

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);
});

Embedding & props

// Embed:
VStack(Component<MyWidget>(), Component<AnotherWidget>())

// Typed props — use records for free structural equality:
record UserCardProps(string Name, string Role);
class UserCard : Component<UserCardProps> { ... }
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

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

Hooks

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

UseState / UseReducer

var (count, setCount) = UseState(0);
var (items, updateItems) = UseReducer(new List<Todo>());

// List mutation via UseReducer (UseState with List<T> won't re-render on mutate!):
updateItems(list => [.. list, new Todo("New", false)]);

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");

// Provide:
VStack(...).Provide(ThemeCtx, "dark")

// Consume:
var theme = UseContext(ThemeCtx);

DSL — the essentials

For the complete catalog (every factory, modifier, enum) see skills/dsl-reference.md. The 90% cases:

// Text + layout — prefer FlexRow/FlexColumn for linear layout; they use CSS Flexbox
// semantics (grow/shrink/gap/wrap, justify-content, align-items) so the model matches
// the web. VStack/HStack remain available when you specifically want StackPanel's
// shrink-wrap behavior.
FlexColumn(children...)         FlexRow(children...)
VStack(spacing, children...)    HStack(spacing, children...)
TextBlock("hi")  Heading("Title")    SubHeading("Section")  Caption("note")
// WinUI 3 type-ramp factories — map 1:1 to TitleTextBlockStyle etc.
Title("Page")    Subtitle("Group")   Body("paragraph")      BodyStrong("bold")  BodyLarge("intro")
TitleLarge("Feature")                // 40px — primary title on a feature/landing page
Display("Hero")                      // 68px — hero banner, at most one per page
// Card(child) factory bakes in CardBackground + 1px CardStroke + 8 radius + 16 padding.
Card(child)
Border(child).CornerRadius(8).Background(Theme.CardBackground).Padding(16)
ScrollView(VStack(...))      // modern InteractionTracker-backed; ScrollViewer(...) is the classic control if you need attached props / parallax
Grid(columns: [GridSize.Star(), GridSize.Px(200)],
     rows:    [GridSize.Auto,   GridSize.Star()],
    childA.Grid(row: 0, column: 0), childB.Grid(row: 1, column: 1))
TitleBar("App") with { Subtitle = "Home", Content = ..., RightHeader = ... }

// Controls
Button("Click", () => ...)      TextBox(value, setValue, placeholder)
CheckBox(isChecked, setChecked) ToggleSwitch(on, setOn)
Slider(v, 0, 100, setV)         ComboBox(items, index, setIndex)

// Strings auto-convert to TextBlockElement: VStack("A", "B") works.

Conditional rendering

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))
// Or LINQ: VStack(items.Select(i => TextBlock(i.Name)).ToArray())

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 use 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.
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 (e.g. ListView with many items) overflows the container and Yoga shrinks every sibling proportionally — heading/buttons/inputs all collapse. Pass .Flex(grow: 1, basis: 0) (matches CSS flex: 1) or add .Flex(shrink: 0) to each fixed-size sibling. See skills/dsl-reference.md.

Starter template

using Microsoft.UI.Reactor;
using Microsoft.UI.Reactor.Core;
using Microsoft.UI.Xaml;
using static Microsoft.UI.Reactor.Factories;

ReactorApp.Run<App>("My App", width: 1200, height: 800);

class App : Component
{
    public override Element Render()
    {
        var (page, setPage) = UseState("Home");

        return Grid(
            columns: [GridSize.Star()],
            rows: [GridSize.Auto, GridSize.Star()],
            Border(
                HStack(12,
                    Heading("My App").VAlign(VerticalAlignment.Center),
                    NavBtn("Home", page, setPage),
                    NavBtn("Settings", page, setPage))
            ).Background("#f0f0f0").Padding(horizontal: 24, vertical: 12).Grid(row: 0),

            Border(page switch
            {
                "Home"     => Component<HomePage>(),
                "Settings" => Component<SettingsPage>(),
                _ => TextBlock("Not found")
            }).Padding(24).Grid(row: 1));
    }

    static Element NavBtn(string label, string current, Action<string> set) =>
        Button(label, () => set(label)).IsEnabled(!(label == current));
}

Testing

Run the suite that matches what you changed.

# Unit tests — fast, no UI window (~3s)
dotnet test tests/Reactor.Tests

# Selfhost tests — real WinUI controls, in-process (~2 min)
dotnet test tests/Reactor.SelfTests

# Packaged selfhost tests — the same fixtures under MSIX package identity (~5 min).
# Needed for anything that reads ms-appx:, Package.Current, MRT, or package identity.
# Requires Developer Mode; fails rather than skips without it.
dotnet test tests/Reactor.PackagedTests -p:Platform=x64

# E2E — cross-process UI Automation via winapp ui (~30s, needs the winapp CLI)
dotnet test tests/Reactor.AppTests --filter "ClassName=Microsoft.UI.Reactor.AppTests.Tests.InteractiveTests"

# Everything (includes the packaged tier, so Developer Mode is required)
dotnet test Reactor.slnx

samples/Reactor.TestApp is the interactive demo, not a test runner.

Single-file apps with dotnet run

For lightweight demos, skip the .csproj entirely. Add a file-level header:

#:package Microsoft.UI.Reactor@0.0.0-local
#:package Microsoft.WindowsAppSDK@2.0.1
#:property OutputType=WinExe
#:property TargetFramework=net10.0-windows10.0.22621.0
#:property UseWinUI=true
#:property WindowsPackageType=None

using Microsoft.UI.Reactor;
using static Microsoft.UI.Reactor.Factories;

ReactorApp.Run("Hello", ctx =>
{
    var (count, setCount) = ctx.UseState(0);
    return VStack(TextBlock($"Count: {count}"), Button("+1", () => setCount(count + 1)));
});

Run with dotnet run MyApp.cs -p:Platform=ARM64 (or x64). In selfhost the version is 0.0.0-local — run mur pack-local first if the package isn't found. Outside the clone, replace the version with the published release you depend on.

Always capture dotnet run output. Build errors exit with code 1. Read compiler output, fix, retry. Don't assume success without checking. Note: single-file builds do not load analyzers — for analyzer coverage (REACTOR_DSL_001, REACTOR_HOOKS_*, etc.), use a .csproj.

Comparison to React