PR Summary

• Make applicable private Guid fields readonly.
• Use C# 7.2 in-modifier for passing a readonly Guid instead of ref-modifier (to fix compiler error).

PR Context

Follow-up to #13966.

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
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• Breaking changes
  • None
  • OR
  • [NA] Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
  • OR
  • [NA] Documentation needed
    • [NA] Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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  • [NA] Make sure you've added a new test if existing tests do not effectively test the code changed
• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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  • [NA] I have considered the user experience from a tooling perspective and enumerated concerns in the summary. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
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