Fix winapp new -t reactor when another template pack also registers reactor

[Alexandre Zollinger Chohfi (azchohfi)]

Description

If you install the standalone Microsoft.UI.Reactor.Templates pack alongside the WinUI templates pack, both packs register the short name reactor. winapp new -t reactor passed that name straight to dotnet new. Because the name matches templates in two packs, dotnet new refused it and scaffolding failed.

winapp new now checks which short names other installed packs also register. It creates the WinUI pack's template using the first of its aliases that no other pack claims:

• One name is shared: winapp new -t reactor scaffolds the WinUI pack's Reactor Blank App by running dotnet new reactor-blank. The template name winapp shows and reports in --json ("Template": "reactor") is unchanged.
• Every name is shared: winapp new stops before asking for a project name. It exits with code 4 and prints a message that names the conflicting pack and how to remove it.

winapp new still only installs, lists and scaffolds templates from the official WinUI pack.

Usage Example

This is the output observed with both packs installed (WinUI pack 0.0.7-alpha, .NET SDK 10.0.401):

> dotnet new list reactor
Template Name                              Short Name                             Language  Tags
-----------------------------------------  -------------------------------------  --------  ------------------------------------------
Microsoft.UI.Reactor App                   reactor                                [C#]      Windows/WinUI/Desktop/Reactor
Reactor Blank App (Experimental)           reactor,reactor-blank,winui-reactor    [C#]      Windows/WinUI/Desktop/Reactor/Experimental
...

Before (main): winapp new -t reactor --name BeforeFix --use-defaults exits with code 5.

❌ Failed to scaffold WinUI app: Unable to resolve the template, the following installed templates are conflicting:
Identity                                         Template Name       Short Name                           ...  Package
Microsoft.UI.Reactor.StandIn.CSharp              Microsoft.UI.Re...  reactor                              ...  Microsoft.UI.Reactor.Templates
Microsoft.WindowsAppSDK.Reactor.CSharp.BlankApp  Reactor Blank A...  reactor,reactor-blank,winui-reactor  ...  Microsoft.WindowsAppSDK.WinUI.CSharp.Templates

After (this PR): winapp new -t reactor --name AfterFix2 --use-defaults exits with code 0. With --verbose, the log shows dotnet new reactor-blank, and the scaffolded app builds with 0 errors.

✅ Created WinUI app AfterFix2 at ...\AfterFix2.
⚠  reactor is an experimental template. It depends on prerelease packages whose
APIs can change or be removed in a future release.
👉 Next: cd "AfterFix2" then winapp run.

When another pack claims every alias (reactor, reactor-blank and winui-reactor), winapp new exits with code 4 and creates nothing:

❌ Template 'reactor' can't be created because every short name it has (reactor, reactor-blank, winui-reactor) is also registered by another installed template pack (Microsoft.UI.Reactor.Templates), so 'dotnet new' can't tell them apart. Remove the other pack with 'dotnet new uninstall Microsoft.UI.Reactor.Templates', then re-run 'winapp new'.

With only the WinUI pack installed, winapp new -t reactor still runs dotnet new reactor, as before.

Related Issue

Fixes #948

Type of Change

• 🐛 Bug fix

Checklist

• New tests added for new functionality (if applicable)
• Tested locally on Windows
• docs/usage.md updated (if CLI commands changed)

Screenshots / Demo

N/A (CLI behavior; see the observed output above).

Additional Notes

• Where the pack list comes from: winapp new already runs dotnet new uninstall to confirm which templates the WinUI pack owns. That output lists every installed pack and its aliases, so this fix needs no extra dotnet call.
• How the conflicting pack was reproduced: Microsoft.UI.Reactor.Templates isn't published on nuget.org. The template project that microsoft/microsoft-ui-reactor removed in Use the Windows App SDK dotnet new templates and remove our own microsoft-ui-reactor#1277 used the short name reactorapp, not reactor. The repro therefore used a local pack built to match the issue exactly: package ID Microsoft.UI.Reactor.Templates, name "Microsoft.UI.Reactor App", short name reactor, and tags Windows/WinUI/Desktop/Reactor. It ran in an isolated DOTNET_CLI_HOME.
• Limitation: templates built into the .NET SDK don't appear in dotnet new uninstall output, so a collision with one of them isn't detected. None of them use WinUI or Reactor short names.
• Validation: all WinUiTemplateCatalogTests and NewCommand* tests pass (181 passed, 1 skipped), including new handler tests for both cases above. scripts\build-cli.ps1 -SkipTests -SkipMsix -SkipNpm succeeds, and the CLI schema is unchanged.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation is focused and covered by 95 passing targeted tests, a successful solution build, and NativeAOT publish.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes ambiguous Reactor template aliases by selecting a unique alias or failing with actionable guidance.

Changes:

• Detects aliases registered by other template packs.
• Scaffolds using the first unique alias.
• Adds tests and user documentation.

File	Description
WinUiTemplateCatalog.cs	Parses installed packs and identifies shared aliases.
NewCommand.cs	Uses unique aliases and reports complete conflicts.
WinUiTemplateCatalogTests.cs	Tests pack parsing and alias selection.
NewCommandHandlerTests.cs	Tests successful fallback and conflict failure.
docs/​usage.md	Documents collision handling and recovery.

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[github-actions]

Build Metrics Report

Validation passed. All required build and validation jobs succeeded.

Binary Sizes

Artifact	Baseline	Current	Delta
CLI (ARM64)	57.29 MB	57.31 MB	📈 +12.5 KB (+0.02%)
CLI (x64)	57.34 MB	57.35 MB	📈 +13.5 KB (+0.02%)
MSIX (ARM64)	23.79 MB	23.79 MB	📉 -2.4 KB (-0.01%)
MSIX (x64)	25.26 MB	25.27 MB	📈 +9.8 KB (+0.04%)
NPM Package	49.63 MB	49.65 MB	📈 +12.5 KB (+0.02%)
NuGet Package	49.74 MB	49.75 MB	📈 +8.9 KB (+0.02%)

.NET Test Results (TRX reports)

Other suites are reflected in the overall validation status above.

✅ 7886 passed, 37 skipped out of 7923 tests in 1118.0s (+7 tests, -141.5s vs. baseline)

Test Coverage

✅ 86.3% line coverage, 80.9% branch coverage · ✅ no change vs. baseline

CLI Startup Time

64ms median (x64, winapp --version) · ⚠️ +15ms vs. baseline

Try This Build

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& ([scriptblock]::Create((irm https://raw.githubusercontent.com/microsoft/winappCli/main/scripts/winapp-pr.ps1))) 949

Switching between builds often?

Put the tool on your PATH once:

& ([scriptblock]::Create((irm https://raw.githubusercontent.com/microsoft/winappCli/main/scripts/winapp-pr.ps1))) -AddToPath

Then this build is just:

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Updated 2026-09-30 21:42:51 UTC · commit bbcc652 · workflow run

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation matches the reported behavior, and the build plus 95 targeted tests passed.

Review effort: Balanced
Findings: None