Error FilterStaticWebAssetEndpoints on build after updating to net10 (CSS?)

[rwb196884]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

I have a Blazor project that I'm trying to upgrade from net8 to net10.
It was all working in net8, but in net10 I now get this error on building the Blazor app.

C:\Program Files\dotnet\sdk\10.0.102\Sdks\Microsoft.NET.Sdk.StaticWebAssets\targets\Microsoft.NET.Sdk.StaticWebAssets.ScopedCss.targets(397,3): error MSB4018:
      The "FilterStaticWebAssetEndpoints" task failed unexpectedly.
      System.ArgumentException: An item with the same key has already been added. Key: C:\Work\blah\Whatever.Web.Client\obj\Debug\net10.0\scopedcss\projectbundle\Whatever.Web.Client.bundle.scp.css
         at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
         at Microsoft.AspNetCore.StaticWebAssets.Tasks.StaticWebAsset.ToAssetDictionary(ITaskItem[] candidateAssets, Boolean validate)
         at Microsoft.AspNetCore.StaticWebAssets.Tasks.FilterStaticWebAssetEndpoints.Execute()
         at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
         at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(TaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExe
      cutionMode howToExecuteTask)

I thought that this was something to do with having my API controllers in a different project and copying the generated swagger file about, but this seems to be something to do with CSS.

Any ideas what's going on? Is it fixable? Do I need to stop on net8?

Expected Behavior

No response

Steps To Reproduce

No response

Exceptions (if any)

System.ArgumentException: An item with the same key has already been added.

.NET Version

10.0.102

Anything else?

> dotnet --info
.NET SDK:
 Version:           10.0.102
 Commit:            4452502459
 Workload version:  10.0.100-manifests.6d969a7e
 MSBuild version:   18.0.7+445250245

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19045
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\10.0.102\

.NET workloads installed:
 [android]
   Installation Source: VS 18.2.11415.280, VS 17.14.36717.8
   Manifest Version:    36.1.2/10.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\10.0.100\microsoft.net.sdk.android\36.1.2\WorkloadManifest.json
   Install Type:              Msi

 [maui-windows]
   Installation Source: VS 18.2.11415.280, VS 17.14.36717.8
   Manifest Version:    10.0.1/10.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\10.0.100\microsoft.net.sdk.maui\10.0.1\WorkloadManifest.json
   Install Type:              Msi

 [maccatalyst]
   Installation Source: VS 18.2.11415.280, VS 17.14.36717.8
   Manifest Version:    26.1.10502/10.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\10.0.100\microsoft.net.sdk.maccatalyst\26.1.10502\WorkloadManifest.json
   Install Type:              Msi

 [ios]
   Installation Source: VS 18.2.11415.280, VS 17.14.36717.8
   Manifest Version:    26.1.10502/10.0.100
   Manifest Path:       C:\Program Files\dotnet\sdk-manifests\10.0.100\microsoft.net.sdk.ios\26.1.10502\WorkloadManifest.json
   Install Type:              Msi

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:      10.0.2
  Architecture: x64
  Commit:       4452502459

.NET SDKs installed:
  9.0.308 [C:\Program Files\dotnet\sdk]
  10.0.102 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.12 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 10.0.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.12 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 10.0.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.23 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.12 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 10.0.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  DOTNET_CLI_UI_LANGUAGE                   [en-US]
  DOTNET_GCHeapCount                       [2]
  DOTNET_GCNoAffinitize                    [1]
  DOTNET_MULTILEVEL_LOOKUP                 [0]
  DOTNET_TC_CallCountThreshold             [1000]
  DOTNET_ThreadPool_UnfairSemaphoreSpinLimit [0]
  DOTNET_gcConcurrent                      [0]
  Detected COMPlus_* environment variable(s). Consider transitioning to DOTNET_* equivalent.

global.json file:
  Not found

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

Download .NET:
  https://aka.ms/dotnet/download

[github-actions]

Triage Summary

Area: area-blazor — The error occurs in Microsoft.NET.Sdk.StaticWebAssets during Blazor build, specifically in the FilterStaticWebAssetEndpoints task related to scoped CSS bundling (bundle.scp.css). This is part of Blazor's static web asset build pipeline.

Type: bug — A build error that was not present on .NET 8 and now occurs after upgrading to .NET 10, with a clear exception (ArgumentException: An item with the same key has already been added).

Regression Info

• Previously working version: .NET 8
• Broken since: .NET 10 (10.0.102 SDK)
• The FilterStaticWebAssetEndpoints MSBuild task throws an ArgumentException due to a duplicate key in the asset dictionary for Whatever.Web.Client.bundle.scp.css. This did not occur when the project targeted net8.

Potential Duplicates

• duplicated ProjectReferences to razor class libraries lead to builds failing because of FilterStaticWebAssetEndpoints #58354 — "Duplicated ProjectReference causes FilterStaticWebAssetEndpoints to throw" (similarity: high — identical exception, same task, same .bundle.scp.css key duplication pattern). That issue was triggered by a duplicated <ProjectReference>, which may apply here too.

Notes

• The root cause in duplicated ProjectReferences to razor class libraries lead to builds failing because of FilterStaticWebAssetEndpoints #58354 was a duplicated <ProjectReference> entry in the .csproj, which caused the same key to appear twice in the asset dictionary. The author of this issue should check whether there are any duplicate <ProjectReference> entries (directly or via Directory.Build.props) in Whatever.Web.Client.csproj.
• Steps to reproduce were not provided, which makes it harder to verify if this is a new/different cause or the same issue as duplicated ProjectReferences to razor class libraries lead to builds failing because of FilterStaticWebAssetEndpoints #58354. More information (minimal repro or .binlog) would help narrow it down.
• The error may also indicate that the deduplication logic that was present or needed for .NET 10 is missing/broken in FilterStaticWebAssetEndpoints.

Generated by Issue Triage Agent for dotnet/aspnetcore for issue #66599 · ● 277K · ◷

[rwb196884]

The file Project.Client.bundle.scp.css contains the css that seems to have been edited from a component in the client side project that has a css file.
If I remove this css file then the build still fails but with a different error:

error MSB4018:
      The "DiscoverPrecompressedAssets" task failed unexpectedly.
      System.ArgumentException: An item with the same key has already been added. Key: C:\blah\Project.Client\bin\Debug\ne
      t10.0\wwwroot\_framework\BlazorBootstrap.zo6lifkmy7.wasm
         at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
         at System.Linq.Enumerable.SpanToDictionary[TSource,TKey](ReadOnlySpan`1 source, Func`2 keySelector, IEqualityComparer`1 comparer)
         at System.Linq.Enumerable.ToDictionary[TSource,TKey](IEnumerable`1 source, Func`2 keySelector, IEqualityComparer`1 comparer)
         at Microsoft.AspNetCore.StaticWebAssets.Tasks.DiscoverPrecompressedAssets.Execute()
         at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
         at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(TaskExecutionHost taskExecutionHost, TaskLoggingContext taskLoggingContext, TaskHost
       taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask)

If I remove Bootstrap then I get the same error but for the file ...bin\Debu g\net10.0\wwwroot\_framework\Microsoft.AspNetCore.Authorization.1l4onz0xjo.wasm

[rwb196884]

This is the cause: the project that won't build has a reference to a web project (because I want to copy some files from its wwwroot).

  <ProjectReference Include="..\Project.Api\Project.Api.csproj">
    <ExcludeAssets>contentFiles</ExcludeAssets>
    <PrivateAssets>contentFiles</PrivateAssets>
    <!--These don't work to exclude the content files appsettings*.json. 
       Therefore there is a workaround hack in Project.Api.
       https://github.com/dotnet/sdk/issues/50170 -->
  </ProjectReference>

Project.Api is a Sdk="Microsoft.NET.Sdk.Web".

Commenting out this references fixes DiscoverPrecompressedAssets but not FilterStaticWebAssetEndpoints.