Enhance CS0012 documentation with troubleshooting steps for when assemblies appear to be referenced

[Copilot]

The current CS0012 compiler error documentation doesn't address scenarios where the assembly appears to be referenced in the project but the compiler error persists. This enhancement adds a dedicated troubleshooting section to help developers resolve CS0012 errors that occur after NuGet package operations or when references seem to exist but are not properly resolved.

Changes Made

Added a new "When the assembly appears to be referenced" subsection to the CS0012 documentation that provides step-by-step troubleshooting guidance:

1. Package restoration - Run dotnet restore after package operations
2. Cache clearing - Clear NuGet package caches and restore
3. Version conflict detection - Check for binding redirect warnings
4. Clean rebuild - Remove stale references through clean/build cycle
5. Package integrity verification - Reinstall packages that may be corrupted

Problem Addressed

This addresses user feedback about encountering CS0012 errors for System.ServiceModel.ClientBase<> after NuGet uninstall/reinstall operations, where the dependency appeared to be present but the compiler couldn't resolve it. The enhanced documentation now provides concrete steps to resolve these common post-package-operation scenarios.

Example Scenario

A developer receives this error after NuGet operations:

The type 'ClientBase<>' is defined in an assembly that is not referenced. 
You must add a reference to assembly 'System.ServiceModel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

With the updated documentation, they now have clear steps to resolve the issue even when the assembly appears to be referenced.

Fixes #28000.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

---

Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/assembly-references.md	Resolve errors and warnings related to assembly references

[BillWagner]

This LGTM and is ready for final review.

[learn-build-service-prod]

PoliCheck Scan Report

The following reports lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 issues. Other issues are also a high priority. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a comment that you post to the PR. The feedback helps reduce false positive instances in future scans.

✅ No issues found

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.