Implement feature flag validation and cleanup for single file AppHost support in RunCommand

[Copilot]

This PR completes the single file AppHost implementation in the RunCommand by adding proper feature flag validation and cleaning up redundant logic.

Changes Made

Feature Flag Validation

• Added validation that the singlefileAppHostEnabled feature flag is enabled when a .cs file is detected as an AppHost
• Returns a clear error message when the feature is disabled but a single file AppHost is attempted
• Maintains consistency with the ProjectLocator which already checks this feature flag

Code Cleanup

• Removed redundant !isSingleFileAppHost check in the build logic (was checked at lines 130 and 142)
• Streamlined the build skip logic for single file AppHosts to avoid unnecessary nested conditions

Error Handling

• Added SingleFileAppHostFeatureNotEnabled error string to resources with proper localization support
• Updated both .resx and .Designer.cs files following the project's resource management patterns

Test Coverage

• Added test case RunCommand_WhenSingleFileAppHostAndFeatureDisabled_ReturnsNonZeroExitCode to verify proper error handling
• All existing tests continue to pass (358/358)

Behavior

Before: The RunCommand would attempt to process .cs files returned by ProjectLocator without validating if the feature was enabled, potentially leading to inconsistent behavior.

After: The RunCommand now properly validates the feature flag and provides clear feedback:

# Feature disabled (default)
$ aspire run --project apphost.cs
❌ Single file AppHost feature is not enabled. To use .cs AppHost files, enable the feature using configuration.

# Feature enabled
$ features__singlefileAppHostEnabled=true aspire run --project apphost.cs
# Proceeds with single file AppHost execution

Configuration

The feature can be enabled via:

• Environment variable: features__singlefileAppHostEnabled=true
• JSON configuration in .aspire/settings.json:

{
  "features": {
    "singlefileAppHostEnabled": true
  }
}

This change ensures the RunCommand implementation is fully consistent with the ProjectLocator's feature flag handling while maintaining backward compatibility and providing clear user feedback.

Created from VS Code via the GitHub Pull Request extension.

---

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[mitchdenny]

If anyone wants to following along at home, here is an apphost.cs file that will work with this change:

#:sdk Microsoft.NET.Sdk
#:sdk Aspire.AppHost.Sdk@9.5.0-preview.1.25460.2
#:package Aspire.Hosting.AppHost@9.5.0-preview.1.25460.2
#:property PublishAot=false

var builder = DistributedApplication.CreateBuilder(args);

builder.Build().Run();

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 11397

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 11397"

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 11397

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 11397"

[davidfowl]

@mitchdenny why do we need both sdks?

[mitchdenny]

Yeah without it it doesn't understand PackageReference at this point.

[Copilot]

Pull Request Overview

This PR completes the single file AppHost implementation in the RunCommand by adding proper feature flag validation and cleaning up redundant logic. The changes ensure that when a .cs file is detected as an AppHost, the feature flag is properly validated before proceeding with execution.

Key changes:

• Added feature flag validation with clear error messaging when single file AppHost is disabled
• Cleaned up redundant build logic checks and streamlined control flow
• Added comprehensive localization support for the new error message

Reviewed Changes

Copilot reviewed 18 out of 19 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Aspire.Cli/Commands/RunCommand.cs	Main implementation adding feature flag validation and logic cleanup for single file AppHost support
src/Aspire.Cli/DotNet/DotNetCliRunner.cs	Updated CLI argument construction to handle single file AppHost execution
src/Aspire.Cli/Resources/ErrorStrings.resx	Added new error string for single file AppHost feature validation
src/Aspire.Cli/Resources/xlf/*.xlf	Added localized translations for the new error message across multiple languages
tests/Aspire.Cli.Tests/Commands/RunCommandTests.cs	Added test coverage for feature flag validation error scenario
src/Aspire.Cli/Utils/AppHostHelper.cs	Minor whitespace cleanup

Files not reviewed (1)

• src/Aspire.Cli/Resources/ErrorStrings.Designer.cs: Language not supported