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The "DiscoverPrecompressedAssets" task failed unexpectedly
Existing issues
For .Net 10 there is #64512 with a similar problem but the OP didn't reopen since he solved his problem in another way.
#49180 was fixed in 9.0.301, i can't tell if my problem is the same or not. The msbuild workaround in that issue doesn't get my build working.
Describe the bug
After upgrading a Blazor project (referencing another Blazor project) from .Net 8 to .Net 10 I get the following error when building (build works in .Net 8):
dotnet build .\BlazorApp\BlazorApp.csproj
Restore complete (1,1s)
BlazorPlugin net10.0 succeeded (3,1s) → BlazorPlugin\bin\Debug\net10.0\BlazorPlugin.dll
BlazorApp net10.0 failed with 1 error(s) (1,0s)
C:\Program Files\dotnet\sdk\10.0.100\Sdks\Microsoft.NET.Sdk.StaticWebAssets\targets\Microsoft.NET.Sdk.StaticWebAssets.Compression.targets(297,5): error MSB4018:
The "DiscoverPrecompressedAssets" task failed unexpectedly.
System.ArgumentException: An item with the same key has already been added. Key: C:\Users\daniel\.nuget\packages\m
icrosoft.aspnetcore.app.internal.assets\10.0.0\_framework\blazor.web.js
at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
at System.Linq.Enumerable.SpanToDictionary[TSource,TKey](ReadOnlySpan`1 source, Func`2 keySelector, IEqualityCo
mparer`1 comparer)
at System.Linq.Enumerable.ToDictionary[TSource,TKey](IEnumerable`1 source, Func`2 keySelector, IEqualityCompare
r`1 comparer)
at Microsoft.AspNetCore.StaticWebAssets.Tasks.DiscoverPrecompressedAssets.Execute()
at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
at Microsoft.Build.BackEnd.TaskBuilder.ExecuteInstantiatedTask(TaskExecutionHost taskExecutionHost, TaskLogging
Context taskLoggingContext, TaskHost taskHost, ItemBucket bucket, TaskExecutionMode howToExecuteTask)
Build failed with 1 error(s) in 5,8s
To Reproduce
Minimal repo to reproduce here: https://github.com/DanielSundberg/DiscoverPrecompressedAssets
What I did to get this error
- In an empty dir created global.json with version 8.0.100
- dotnet new blazor --use-program-main -o BlazorApp
- dotnet new blazor --use-program-main -o BlazorPlugin
- dotnet add .\BlazorApp\BlazorApp.csproj reference .\BlazorPlugin\BlazorPlugin.csproj
- dotnet build .\BlazorApp\BlazorApp.csproj
Successful build (first commit in linked repo)
- dotnet clean
- Manual delete of obj and bin directories
- Change global.json to use version 10.0.100
- Change projects to target net10.0
- dotnet build .\BlazorApp\BlazorApp.csproj
Now I get the build error above (second commit in linked repo).
Further technical details
dotnet --info
.NET SDK:
Version: 10.0.100
Commit: b0f34d51fc
Workload version: 10.0.100-manifests.4c0ca8ba
MSBuild version: 18.0.2+b0f34d51f
Runtime Environment:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.100\
.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: 10.0.0
Architecture: x64
Commit: b0f34d51fc
.NET SDKs installed:
8.0.100 [C:\Program Files\dotnet\sdk]
10.0.100 [C:\Program Files\dotnet\sdk]
.NET runtimes installed:
Microsoft.AspNetCore.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Other architectures found:
None
Environment variables:
DOTNET_CLI_TELEMETRY_OPTOUT [1]
global.json file:
C:\code\DiscoverPrecompressedAssets\global.json
Learn more:
https://aka.ms/dotnet/info
Download .NET:
https://aka.ms/dotnet/download
@javiercn does this ring a bell? Seems like there's also continued activity on https://github.com/dotnet/aspnetcore/issues/64512 which was closed.
Also reproducing this, the key seems to be a Microsoft.NET.Sdk.Web project referencing another Microsoft.NET.Sdk.Web project - which I am due to my integration test project using Microsoft.NET.Sdk.Web.
For my case, this temporary workaround sufficed:
<html><body>
<!--StartFragment-->
<Target Name="FixNetSdkDiscoverAssetsBug" BeforeTargets="ResolveBuildCompressedStaticWebAssetsConfiguration">
--
| <!-- Workaround for:
| The "DiscoverPrecompressedAssets" task failed unexpectedly.
| System.ArgumentException: An item with the same key has already been added. Key: C:\Users\[user]\.nuget\packages\microsoft.aspnetcore.app.internal.assets\10.0.0\_framework\blazor.web.js
|
| https://github.com/dotnet/sdk/issues/52089
|
| This is caused by a Microsoft.NET.Sdk.Web project (like this) referencing another project that also uses Microsoft.NET.Sdk.Web.
| It appears to be a .NET SDK bug, and we're not the only one hitting it.
| -->
|
| <ItemGroup>
| <!-- Remove any duplicate StaticWebAsset and StaticWebAssetEndpoint items -->
| <_StaticWebAsset Include="@(StaticWebAsset)" />
| <StaticWebAsset Remove="@(StaticWebAsset)" />
| <StaticWebAsset Include="@(_StaticWebAsset->Distinct())" />
| </ItemGroup>
| </Target>
<!--EndFragment-->
</body>
</html>
For my case, this temporary workaround sufficed:
-- | | | | | <_staticwebasset include="@(StaticWebAsset)"> | || |
Still doesn't build for me with the above msbuild snippet in my directory.build.props:
>dotnet build .\BlazorApp\BlazorApp.csproj
Restore complete (0,4s)
BlazorPlugin net10.0 succeeded (4,9s) → BlazorPlugin\bin\Debug\net10.0\BlazorPlugin.dll
BlazorApp net10.0 failed with 1 error(s) (1,4s)
C:\Program Files\dotnet\sdk\10.0.100\Sdks\Microsoft.NET.Sdk.StaticWebAssets\targets\Microsoft.NET.Sdk.StaticWebAssets.targets(618,5):
error Conflicting assets with the same target path 'bootstrap/bootstrap.min.css#[.{fingerprint=8inm30yfxf}]?.map.gz'.
For assets 'Identity: C:\code\DiscoverPrecompressedAssets\BlazorApp\obj\Debug\net10.0\compressed\8qkwa52ezv-{0}-8inm30yfxf-8inm30yfxf.gz,
SourceType: Discovered, SourceId: BlazorApp, ContentRoot: C:\code\DiscoverPrecompressedAssets\BlazorApp\obj\Debug\net10.0\compressed\,
BasePath: /,
RelativePath: bootstrap/bootstrap.min.css#[.{fingerprint=8inm30yfxf}]?.map.gz,
AssetKind: All,
AssetMode: All,
AssetRole: Alternative,
AssetRole: , AssetRole: ,
RelatedAsset: C:\code\DiscoverPrecompressedAssets\BlazorApp\wwwroot\bootstrap\bootstrap.min.css.map,
AssetTraitName: Content-Encoding, AssetTraitValue: gzip, Fingerprint: vhplv2fb6t, Integrity: U+unDV0au6R0diQNcEupHyJsbjJTqlVY4S8arnj+BrQ=,
FileLength: 74664,
LastWriteTime: 2025-12-16 17:30:41 +00:00,
CopyToOutputDirectory: Never,
CopyToPublishDirectory: PreserveNewest,
OriginalItemSpec: C:\code\DiscoverPrecompressedAssets\BlazorApp\wwwroot\bootstrap\bootstrap.min.css.map'
and 'Identity: C:\code\DiscoverPrecompressedAssets\BlazorPlugin\obj\Debug\net10.0\compressed\i8a4iwivx8-{0}-8inm30yfxf-8inm30yfxf.gz,
SourceType: Project,
SourceId: BlazorPlugin,
ContentRoot: C:\code\DiscoverPrecompressedAssets\BlazorPlugin\obj\Debug\net10.0\compressed\,
BasePath: /, RelativePath: bootstrap/bootstrap.min.css#[.{fingerprint=8inm30yfxf}]?.map.gz,
AssetKind: All,
AssetMode: All,
AssetRole: Alternative,
AssetRole: , AssetRole: ,
RelatedAsset: C:\code\DiscoverPrecompressedAssets\BlazorPlugin\wwwroot\bootstrap\bootstrap.min.css.map,
AssetTraitName: Content-Encoding,
AssetTraitValue: gzip,
Fingerprint: vhplv2fb6t, Integrity: U+unDV0au6R0diQNcEupHyJsbjJTqlVY4S8arnj+BrQ=,
FileLength: 74664,
LastWriteTime: 2025-12-02 20:07:32 +00:00,
CopyToOutputDirectory: Never,
CopyToPublishDirectory: PreserveNewest,
OriginalItemSpec: C:\code\DiscoverPrecompressedAssets\BlazorPlugin\wwwroot\bootstrap\bootstrap.min.css.map' from different projects.
Build failed with 1 error(s) in 7,6s