Fix CS0006 errors when NuGet cache cleared during file-based app recompilation by Copilot · Pull Request #51609 · dotnet/sdk

Copilot · 2025-11-07T09:34:20Z

Clearing NuGet cache (dotnet nuget locals all -c) breaks recompilation of file-based apps when using cached CSC arguments. The optimized CSC path references DLLs from NuGet cache (e.g., ILLink.RoslynAnalyzer.dll) that no longer exist after cache clearing, causing CS0006 metadata file errors.

Changes

Reactive fallback on CS0006 errors

Detect CS0006 errors in CSC compilation output
Set fallbackToNormalBuild = true to trigger existing MSBuild fallback
Report errors only to verbose output, not standard output
Error code "CS0006" is language-independent; only the message text varies by locale

Test coverage

Added CscOnly_AfterMSBuild_NuGetCacheCleared test following existing test patterns
Includes proper XML doc comments with issue reference
Documents testing limitations (MSBuild restore recreates deleted packages, making negative testing challenging)

Original prompt

This section details on the original issue you should resolve

<issue_title>Clearing NuGet package local cache breaks re-compiling file-based apps</issue_title>
<issue_description>### Describe the bug
Clearing local NuGet package cache breaks re-compiling file-based apps.

To Reproduce

Add NuGet package source (at least until 10.0.1 packages exist on nuget.org):
dotnet nuget add source https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet10/nuget/v3/index.json -n dotnet10

Create a whatever.cs file with execute (chmod +x whatever.cs) with below contents.

Execute it to compile and run the first time:
./whatever.cs

Clear NuGet package local cache:
dotnet nuget locals all -c

At this point, executing ./whatever.cs still works fine (cached binary)

Make any slight tweak to the code, e.g. the string passed to WriteLine: Console.WriteLine("I broke it!");

Now, trying to rebuild / execute will fail:
./whatever.cs

Error:

error CS0006: Metadata file '/Users/bruck/.nuget/packages/microsoft.net.illink.tasks/10.0.0-rtm.25513.102/analyzers/dotnet/cs/ILLink.CodeFixProvider.dll' could not be found
error CS0006: Metadata file '/Users/bruck/.nuget/packages/microsoft.net.illink.tasks/10.0.0-rtm.25513.102/analyzers/dotnet/cs/ILLink.RoslynAnalyzer.dll' could not be found

The build failed. Fix the build errors and run again.

Code:
#!/usr/bin/env dotnet
#:property TargetFramework=net10.0
#:property RuntimeFrameworkVersion=10.0.0-rtm.25513.102
Console.WriteLine("Hello from a .NET script!");
Exceptions (if any)

Further technical details

.NET SDK:
Version: 10.0.101
Commit: e17b0d08de
Workload version: 10.0.100-manifests.c57ac48b
MSBuild version: 18.0.3+e17b0d08d

Runtime Environment:
OS Name: Mac OS X
OS Version: 15.7
OS Platform: Darwin
RID: osx-arm64
Base Path: /usr/local/share/dotnet/sdk/10.0.101/

.NET workloads installed:
There are no installed workloads to display.
Configured to use workload sets when installing new manifests.
No workload sets are installed. Run "dotnet workload restore" to install a workload set.

Host:
Version: 10.0.1
Architecture: arm64
Commit: e17b0d08de

.NET SDKs installed:
8.0.407 [/usr/local/share/dotnet/sdk]
8.0.411 [/usr/local/share/dotnet/sdk]
10.0.101 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.14 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.17 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.14 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.17 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
</issue_description>

<agent_instructions>If the optimized file-based app compilation fails with CS0006, let's fallback to full msbuild (and avoid reporting the original errors; just perhaps to the verbose reporter). Also add a test for this (running a simple app; clearing nuget cache; changing .cs file; rerunning again - should fail without the fix - the app in this test doesn't even need to reference packages because there are some referenced implicitly due to PublishAot=true being the default; but if that ever changes consider adding also a test where we specifically reference some package and it can have PublishAot=false).</agent_instructions>

Comments on the Issue (you are @copilot in this section)
@jjonescz Thanks for reporting this. There is an optimization to recompile using C# compiler only (skipping MSBuild, so also restore) on subsequent re-runs of file-based apps. You can force full recompilation using `dotnet run app.cs --no-cache` (or `dotnet build app.cs`).
I don't think the NuGet command can reliably find and clear the file-based app cache. Instead, the optimization logic could check that the referenced DLLs still exist and fallback to full build if not (we could also do that only if the optimized compilation fails; to not slow down the fast path).</comment_new>

Fixes Clearing NuGet package local cache breaks re-compiling file-based apps #51597

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

- Detect CS0006 errors in CSC compilation output - Fallback to full MSBuild when CS0006 detected - Report errors only to verbose output when falling back - Add test for fallback mechanism when artifacts cleared Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

- Check NuGet cache files exist before reusing cached CSC arguments - Avoid attempting CSC compilation when files are known to be missing - Provides better performance by skipping CSC attempt when it will fail Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

- Use StringComparison.Ordinal for CS0006 error check - Add more detailed comments explaining the check - Clarify test comments about PublishAot and analyzers Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

- Add comment explaining CS0006 error code is locale-independent - Clarify that only error message text varies by locale - Error code format is consistent across all locales Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs

src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

- Update comment to clarify CS0006 can affect libraries, not just analyzers - Remove preemptive NuGet cache check as suggested (not needed if PublishAot=false) - Add CscOnly_AfterMSBuild_NuGetCacheCleared test with doc comments - Use proper test structure following file conventions Note: The test has limitations in verifying the fix because MSBuild's restore recreates deleted packages, making it pass both with and without the fix. The test validates that the fallback mechanism works correctly. Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

The test cannot easily verify the fallback by deleting NuGet cache files because MSBuild's restore recreates them before CSC is attempted. Simplified to test the basic CSC flow and document the fix intent. Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

Copilot

Pull Request Overview

This PR adds a fallback mechanism to handle CS0006 errors (missing metadata files) during fast CSC-only compilation, ensuring that the build falls back to full MSBuild when NuGet packages are missing. This addresses scenarios where the NuGet cache is cleared between builds.

Key Changes:

Added CS0006 error detection in the fast compilation path that triggers a fallback to full MSBuild
Relocated the binary logger warning message to display only after successful CSC compilation
Added comprehensive test coverage for NuGet cache clearing scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs	Implements CS0006 error detection logic to trigger fallback to MSBuild when metadata files are missing
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Moves binary logger warning to display only after successful CSC compilation, avoiding misleading messages when falling back to MSBuild
test/dotnet.Tests/CommandTests/Run/RunFileTests.cs	Adds test methods to verify correct behavior when NuGet cache is cleared, and extends the Build helper method to support custom command configuration

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs

jjonescz · 2025-11-12T17:26:51Z

@RikkiGibson @333fred @MiYanni for reviews, thanks

…e-issue

jjonescz · 2025-12-01T18:27:23Z

@RikkiGibson for another review, thanks

jjonescz · 2025-12-05T17:22:23Z

@RikkiGibson for another review, thanks

jjonescz · 2025-12-12T10:58:45Z

@RikkiGibson for another review, thanks

…e-issue

RikkiGibson · 2026-01-06T17:47:27Z

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs

+
+                    // Check if the compilation failed with CS0006 error (metadata file not found).
+                    // This can happen when NuGet cache is cleared and referenced DLLs (e.g., analyzers or libraries) are missing.
+                    if (completed.ReturnCode != 0 && completed.Output.Contains("error CS0006:", StringComparison.Ordinal))


Is it correct to assume this will work in any locale?

Yes, the error prefix is not translated. For example, this is what I get when building an invalid program under Czech locale:

invalid.cs(1,1): error CS0103: Název x v aktuálním kontextu neexistuje.

RikkiGibson

Done review pass, just had one question.

…e-issue

Initial plan

fb103fc

Copilot AI assigned Copilot and jjonescz Nov 7, 2025

Copilot started work on behalf of jjonescz November 7, 2025 09:34 View session

Copilot AI and others added 4 commits November 7, 2025 09:45

Address code review comments

c70b64d

- Use StringComparison.Ordinal for CS0006 error check - Add more detailed comments explaining the check - Clarify test comments about PublishAot and analyzers Co-authored-by: jjonescz <3669664+jjonescz@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix NuGet package local cache breaking re-compiling of apps~~ Fix CS0006 errors when NuGet cache cleared during file-based app recompilation Nov 7, 2025

Copilot AI requested a review from jjonescz November 7, 2025 10:18

Copilot finished work on behalf of jjonescz November 7, 2025 10:18

jjonescz reviewed Nov 7, 2025

View reviewed changes

Copilot started work on behalf of jjonescz November 7, 2025 10:56 View session

Copilot AI and others added 2 commits November 7, 2025 11:17

Copilot AI requested a review from jjonescz November 7, 2025 11:26

Copilot finished work on behalf of jjonescz November 7, 2025 11:26

Add proper tests

7ff3523

jjonescz approved these changes Nov 7, 2025

View reviewed changes

jjonescz marked this pull request as ready for review November 7, 2025 14:40

jjonescz requested review from a team and Copilot November 7, 2025 14:40

Copilot AI reviewed Nov 7, 2025

View reviewed changes

src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs Show resolved Hide resolved

This was referenced Nov 7, 2025

Tests Failing in Template Engine: Failed to update search cache. (The SSL connection could not be established, see inner exception.) #51147

Open

SSL Template Issue Causes Missing OSX Templates #51198

Open

jjonescz added the Area-run-file Items related to the "dotnet run <file>" effort label Nov 7, 2025

333fred approved these changes Nov 12, 2025

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MiYanni approved these changes Nov 19, 2025

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jjonescz requested a review from RikkiGibson November 19, 2025 09:10

jjonescz mentioned this pull request Nov 20, 2025

Skip obj→bin copy on build failure in CSharpCompilerCommand #51614

Merged

Merge branch 'release/10.0.2xx' into copilot/fix-nuget-cache-recompil…

fd57d00

…e-issue

jjonescz added 3 commits January 6, 2026 14:23

Merge branch 'release/10.0.2xx' into copilot/fix-nuget-cache-recompil…

627498b

…e-issue

Fixup after merge

aaea8b4

Improve formatting

ab0adcc

This was referenced Jan 6, 2026

Could Not Resolve SDK - repo testfx issue #51530

Open

[failing test] Microsoft.NET.Build.Tests.GivenThatWeWantToBuildAWindowsRuntimeComponent.ManagedWinRTComponentCanBeReferenced #52131

Closed

RikkiGibson reviewed Jan 6, 2026

View reviewed changes

jjonescz requested a review from RikkiGibson January 7, 2026 10:34

RikkiGibson approved these changes Jan 7, 2026

View reviewed changes

Merge branch 'release/10.0.2xx' into copilot/fix-nuget-cache-recompil…

eddfe57

…e-issue

jjonescz enabled auto-merge (squash) January 8, 2026 10:33

jjonescz merged commit adbd99b into release/10.0.2xx Jan 8, 2026
23 of 26 checks passed

jjonescz deleted the copilot/fix-nuget-cache-recompile-issue branch January 8, 2026 11:36

dotnet-maestro bot mentioned this pull request Jan 9, 2026

[release/10.0.2xx] Source code updates from dotnet/sdk dotnet/dotnet#4114

Merged

This was referenced Jan 28, 2026

[main] Source code updates from dotnet/sdk dotnet/dotnet#4471

Closed

[main] Source code updates from dotnet/sdk dotnet/dotnet#4646

Merged

This was referenced Feb 12, 2026

[release/10.0.3xx] Source code updates from dotnet/sdk dotnet/dotnet#4826

Merged

[release/10.0.2xx] Source code updates from dotnet/sdk dotnet/dotnet#4889

Merged

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To Reproduce

Error:

Exceptions (if any)

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Copilot AI commented Nov 7, 2025 •

edited by jjonescz

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