Add MSBuild conditional support for .slnx files

[Copilot]

Summary

This PR enables .slnx files (XML-based solution files) to be recognized as MSBuild files, allowing them to support MSBuild Condition attributes in addition to XML comment-style conditionals.

Problem

Currently, .slnx files only support XML comment-style conditionals:

<!--#if (IncludeWebProject) -->
<Project Path="src\WebApp\WebApp.csproj" />
<!--#endif -->

However, MSBuild Condition attributes were not recognized and would be ignored:

<!-- This was IGNORED in .slnx files -->
<Project Path="src\Api\Api.csproj" Condition="'$(IncludeApiProject)' == 'true'" />

This is inconsistent with other MSBuild files (.csproj, .targets, .props) which support both conditional styles.

Solution

Updated the configuration to treat .slnx files as MSBuild files by:

• Changing ConditionalType from Xml to MSBuild
• Changing EvaluatorType from CPP to MSBuild
• Changing flag prefix from "#-" to "<!--/" to match other MSBuild files
• Updating documentation to include .slnx in the MSBuild file extensions list

Result

.slnx files now support both conditional styles:

<!-- XML comment-style conditionals (backward compatible) -->
<!--#if (IncludeWebProject) -->
<Project Path="src\WebApp\WebApp.csproj" />
<!--#endif -->

<!-- MSBuild Condition attributes (NEW) -->
<Project Path="src\Api\Api.csproj" Condition="'$(IncludeApiProject)' == 'true'" />

Benefits

• Consistency: .slnx files now behave like other MSBuild files
• Backward Compatible: Existing templates using <!--#if--> syntax continue to work
• More Options: Template authors can choose the most appropriate conditional style
• Cleaner Templates: MSBuild Condition attributes are often more concise than multi-line XML comments

Fixes dotnet#9370

Warning

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• 7devsblobprodcus323.vsblob.vsassets.io
  • Triggering command: /home/REDACTED/work/templating/templating/.dotnet/dotnet msbuild /m /nologo /clp:Summary /v:minimal /nr:true /warnaserror /p:TreatWarningsAsErrors=true /p:ContinuousIntegrationBuild=false /home/REDACTED/work/templating/templating/artifacts/toolset/restore.proj /t:__WriteToolsetLocation /clp:ErrorsOnly;NoSummary /p:__ToolsetLocationOutputFile=/home/REDACTED/work/templating/templating/artifacts/toolset/11.0.0-beta.25479.108.txt (dns block)
  • Triggering command: /usr/share/dotnet/dotnet build /home/REDACTED/work/templating/templating/src/Microsoft.TemplateEngine.Orchestrator.RunnableProjects/Microsoft.TemplateEngine.Orchestrator.RunnableProjects.csproj --no-restore (dns block)

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add support for slnx files</issue_title>
<issue_description>### Is your feature request related to a problem? Please describe.

Current templating requires conditionals specifed as //#if inside of slx files rather than treating them like MSBuild files and using `

Describe the solution you'd like.

I want slnx files recognized as MSBuild files

Additional context

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1

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