Error List omits C# build errors until affected files are opened

_This issue has been moved from [a ticket on Developer Community](https://developercommunity.visualstudio.com/t/Error-List-omits-C-build-errors-until-a/11108120)._

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\[severity:It’s more difficult to complete my work\]
C# errors that are plainly generated by a solution build do not (consistently) show up in the Error List.
See screenshot for how a bunch of errors are missing in one but present in the other.

When I change the error list filter from Intellisense only to Build + Intellisense, two NU1903 warnings are added, but the C# errors _still_ do not appear.

When I open one of the documents with the errors in it, _then_ the errors finally appear in the error list.

In Tools->Options, I have “Run background code analysis for” _and_ “Show compiler errors and warnings for” both set to “Entire solution”.

### Repro

Git repo: https://github.com/AArnott/Nerdbank.MessagePack.git
commit: 21fdfcbbb980bc665ead0ce724941bbf98b79095

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### Original Comments

#### Feedback Bot on 6/17/2026, 02:02 AM:

Based on similar issues resolved with Microsoft in the last 6 months, the following can help resolve this issue faster.

Possible workarounds

Try opening the affected C# file(s) first, then re-check the Error List. Since the build errors appear only after the document is opened, this may temporarily force the missing errors to surface.

If not already included, please provide:

Detected: OS: 10.0.26200.0, VS: 18.9.0 Canary [11916.36.main], Project type: C#

Please share:

• Whether this happens in one specific solution or in all C# solutions.


• Exact repro steps from solution open to build completion, including whether the issue happens after every build or only intermittently.


• Whether the missing entries are build errors, warnings, or both, and if any specific error codes are missing.


• Whether the issue still happens after restarting Visual Studio or after reopening the solution.


• A short Feedback Tool recording (~30 seconds) showing the build and the Error List before and after opening an affected file.

#### Feedback Bot on 6/17/2026, 11:08 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

#### Mei Wen Chin (Centific Technologies Inc) [MSFT] on 6/25/2026, 00:41 PM:

Thanks for your sharing the detail steps, I can reproduce this issue through your shared steps and this issue has been escalated for further investigation, if there is any process, I will inform you immediately.

Start with the Nerdbank.MessagePack repository at commit 21fdfcbbb980bc665ead0ce724941bbf98b79095 and reproduce the build with Visual Studio's Error List set to Build + Intellisense. Compare the list before and after opening affected C# documents; done means build-generated C# errors appear without opening those files.

Written by the indexing model from the issue text.