Implement Aspire Single-File AppHost template with CLI feature flag integration

[Copilot]

This PR implements the Aspire Single-File AppHost template as requested in the original design discussions. The template provides a minimal, single-file approach to creating Aspire AppHost applications for scenarios where a full project structure is not needed.

Key Features

Template Structure:

• Creates only a single apphost.cs file with complete minimal distributed application setup using the #:sdk directive format
• No .csproj, appsettings.json, or launchSettings.json files (as single-file hosts don't need them)
• No Visual Studio integration (ide.host.json removed since VS doesn't support single-file scenarios)

Version and Feature Gating:

• Only available for Aspire 9.5+ and .NET 10+
• Gated behind the singlefileAppHostEnabled CLI feature flag
• Template only appears when the feature is explicitly enabled

CLI Integration:

• Template name: aspire-apphost-singlefile
• Localized description: "Single-file AppHost (experimental)"
• Seamless integration with existing Aspire CLI template system

Usage

# Enable the experimental feature
aspire config set features:singlefileAppHostEnabled true

# Create a new single-file AppHost
aspire new aspire-apphost-singlefile -n MySingleFileHost

This creates a minimal apphost.cs file with complete SDK directives:

#:sdk Microsoft.NET.Sdk
#:sdk Aspire.AppHost.Sdk@aspireVersion
#:package Aspire.Hosting.AppHost@aspireVersion
#:property PublishAot=false

var builder = DistributedApplication.CreateBuilder(args);

builder.Build().Run();

Implementation Details

The template follows the existing Aspire template patterns:

• Located in src/Aspire.ProjectTemplates/templates/aspire-apphost-singlefile/
• Includes proper version symbols and source conditions
• Auto-generated localization files for international support
• Integrated with the DotNetTemplateFactory using the existing feature flag infrastructure

Changes Made

• Template Files: Complete template structure with version 9.5 support
• CLI Integration: Added feature flag check in DotNetTemplateFactory.cs
• Localization: Template description properly localized using existing TemplatingStrings infrastructure
• Test Infrastructure: Updated CliTestHelper.cs to support the new IFeatures dependency injection
• File Format: Updated template to use lowercase filename (apphost.cs) with complete SDK directive format including:
  • Microsoft.NET.Sdk - Base .NET SDK
  • Aspire.AppHost.Sdk@aspireVersion - Aspire AppHost SDK
  • Aspire.Hosting.AppHost@aspireVersion - Required Aspire hosting package
  • PublishAot=false - Disable AOT for compatibility

The implementation is minimal and focused, providing exactly what was requested: a single-file AppHost template that integrates cleanly with the existing Aspire ecosystem while being appropriately gated for experimental use.

Created from VS Code via the GitHub Pull Request extension.

Created from VS Code via the GitHub Pull Request extension.

Created from VS Code via the GitHub Pull Request extension.

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[mitchdenny]

@DamianEdwards would be good to get your eyes on the template changes here, I am way out of my depth with template.json :)

[davidfowl]

Single file supports launch settings via {app}.run.json where {app}.cs is the file. That would let us mimic what we have today with normal projects.

[mitchdenny]

So maybe we don't automatically set the environment variables inside the CLI? This would preserve the ability to do dotnet new apphost-singlefile and have it work?

[DamianEdwards]

This isn't working for me when I try it locally. I think file-based apps get this property explicitly written to their virtual project file which would override this. We might have to move this to our targets file instead. I'm trying that in #11445

[mitchdenny]

If it works can you apply to this PR?

[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 -- 11393

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 11393"

[davidfowl]

testeed