Unable to build project using VS 17.12.3: “DiscoverPrecompressedAssets” task failed unexpectedly

Is there an existing issue for this?

I have searched the existing issues

Describe the bug

I've found a similar issue, but the exception there isn't the same (System.ArgumentException there and System.InvalidOperationException here) and that issue was closed without an answer, so I've decided to create a new one.

Basically, I'm unable to build an existing project that uses Blazor.BrowserExtension.Template. It used to work fine with version 17.11.35219.272.

Expected Behavior

Simply put, the project build should work without errors, as it used to do.

Steps To Reproduce

I've created a minimalistic project to showcase the issue. To reproduce the error, all that has to be done is to build the project.

Exceptions (if any)

Here's the error I get whenever I try to build the project using VS 17.12.3:

The "DiscoverPrecompressedAssets" task failed unexpectedly.
System.InvalidOperationException: Fingerprint for '...\ExtensionTest\ExtensionTest\obj\Debug\net8.0\browserextension\build\BackgroundWorker.js' is not defined.
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.StaticWebAsset.Validate()
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.StaticWebAsset.FromTaskItem(ITaskItem item)
   at System.Linq.Enumerable.WhereSelectArrayIterator`2.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at Microsoft.AspNetCore.StaticWebAssets.Tasks.DiscoverPrecompressedAssets.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()

.NET Version

9.0.101

Anything else?

UPDATE: If I use SDK 8.0.404 and specify the version by adding a global.json to the solution folder, it actually works. Still no way to work with 9.0.101, though.

I have the same issue building my maui app, your workaround is working (as I did not update my projects to dotnet 9):

global.json at root directory:

{
  "sdk": {
    "version": "8.0.206"
  }
}

Hitting a similar error after upgrading from 8 to 9:

Error MSB4018 : The "DiscoverPrecompressedAssets" task failed unexpectedly.
System.ArgumentException: An item with the same key has already been added. Key: [path]\wwwroot\app.css
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   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)

In csproj, I have several files linked as:

<Content Include="..\[path]\wwwroot\**\app.scss">
    <Link>wwwroot\%(RecursiveDir)%(Filename)%(Extension)</Link>
    <CopyToPublishDirectory>PreserveNewest</CopyToPublishDirectory>
    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
</Content>

If I remove these links, the app builds but I can't use assets from the web project in the sln. This works as expected on 8.0.X

This is caused by:
https://github.com/dotnet/sdk/blob/1d3bf3b00bd92bc9eef62d56e97072425ea8d87f/src/StaticWebAssetsSdk/Tasks/Compression/DiscoverPrecompressedAssets.cs#L35C9-L35C106

ToDictionary() crashes if source contains duplicate keys.

+1

bump!!!

This issue has persisted since SDK > 8.0.403 and I'm also looking forward to a fix.

As Github Copilot is being removed in 17.11, this is now beginning to be an issue as we cannot update without breaking our builds. The fix is in fact quite simple and already 2 PRs are open to fix it!

bump

Hi, I'm hitting the same issue. VS Version 17.12.3. Project did not change for several months but suddenly it cannot be build.

The "DiscoverPrecompressedAssets" task failed unexpectedly. System.ArgumentException: An item with the same key has already been added. at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) at System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add)
at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable1 source, Func2 keySelector, Func2 elementSelector, IEqualityComparer1 comparer)
at Microsoft.AspNetCore.StaticWebAssets.Tasks.DiscoverPrecompressedAssets.Execute()
at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
at Microsoft.Build.BackEnd.TaskBuilder.d__26.MoveNext()`

+1

We have also encountered this issue when attempting to update to .NET 9, based on Microsoft's recommendation that we do so in dotnet/maui#28475 to resolve the BlazorWebView component never being cleaned up when no longer needed in .NET 8.

I'm on VS17.14.6 and dotnet 9.0.301 now.
The bug seems the have been resolved

Edit: VS17.4.7 🤗

@aminmashayekhi your issue seems different.

Could you file a separate issue and provide a repro project so that we can investigate further?

Switch the calendar type to Georgian resolved the bug!

I'm using Visual Studio 22 and Dotnet 7. I resolved this issue by switching from the current branch to the main branch and rebuilding the API. When I returned to my current branch, the build succeeded.

Maybe the problem was a build cache.🤔

We believe these issues were addressed by #48952 and #49020 that are available in 9.0.301.

The issue was that a cache was not being correctly recomputed, and we were introducing duplicates into the cache that caused issues in later stages of the pipeline.

The original issue was addressed by deduplicating Content items before processing them as part of the pipeline.

If you still experience issues after upgrading to .NET SDK 9.0.302, please file a new issue to let us know.

Just encountered this problem when upgrading from .NET 9.0.307 SDK to .NET 10.0.100 under VS2022 Community (64-bit) Version 17.14.20, also removed all obj and bin and cleaned but it's still there,

@1944GPW

@1944GPW .NET 10 is only supported with VS 2026.

Still encountering this problem on both VS2026 and Rider 2025.3 on a .NET 10 project.

@frazambi

Still encountering this problem on both VS2026 and Rider 2025.3 on a .NET 10 project.

@frazambi I think they want a new issue to be submitted.

#59625 (comment)

@1944GPW .NET 10 is only supported with VS 2026.

That's interesting, I managed to build other solutions upgraded to .NET 10 on VS2022 17.14.20 without any problems. :)
That aside, I installed VS2026, and still have the same issue as I posted above, and like others here.
Also - just reopen this issue please as there seems to be no reason not to.

Encountering this problem on VS2026 going from .Net 8 to .Net 10. Mine appears to be saying that blazor.web.js is the duplicate file, but I can't find where it's duplicated and none of the other fixes for the error work.

I was able to reproduce the error when a RCL and Web Project has the same package.
Here's example: https://github.com/LeeStevens318/csharp-discover-precompressed-assets

Which worked fine on .net 8, but caused an issue when upgrading to 9/10.

Microsoft Visual Studio Community
January 2026 Feature Update
Channel: Stable
Version: 18.2.0

@LeeStevens318

@LeeStevens318 thank you! Unfortunately, I had already checked this and in my case, the only shared package is: Microsoft.AspNetCore.App.Internal.Assets.

In my working .Net 8 version, I have 7 top-level packages shared by the project that won't build in .Net 10 (and over 271 that are Transitive). After upgrading to .Net 10, I have only one shared package (Microsoft.AspNetCore.App.Internal.Assets, which doesn't exist in my .Net 8 projects) and only 71 Transitive packages. The main project builds and runs, but this other project fails to build because of the "An item with the same key has already been added" error.

I can't remove Microsoft.AspNetCore.App.Internal.Assets (a note reads: "Implicitly referenced by an SDK. To update the package, update the SDK to which it belongs."). Any ideas?

@tlevey01

@tlevey01 This issue is closed, so if you're still having this problem, please open a new one so that can triage it for investigation.

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Unable to build project using VS 17.12.3: “DiscoverPrecompressedAssets” task failed unexpectedly #59625

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Unable to build project using VS 17.12.3: “DiscoverPrecompressedAssets” task failed unexpectedly #59625

Description

Is there an existing issue for this?

Describe the bug

Expected Behavior

Steps To Reproduce

Exceptions (if any)

.NET Version

Anything else?

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MarcoMedrano commented on Dec 24, 2024

lofcz commented on Jan 10, 2025

evgenyvalavin commented on Jan 29, 2025

TechnoZamb commented on Feb 10, 2025

SirBirne commented on Feb 11, 2025

Lionel-Renaux commented on Feb 14, 2025

LeeStevens318 commented on Feb 17, 2025

MiroslavJuhasCamea commented on Feb 19, 2025

tbaggett-ebsco commented on Mar 27, 2025

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lbrpsoftware commented on Jun 24, 2025

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1944GPW commented on Nov 12, 2025

danroth27 commented on Nov 12, 2025

frazambi commented on Nov 13, 2025

bwinklesky commented on Nov 13, 2025

1944GPW commented on Nov 14, 2025

tlevey01 commented on Jan 19, 2026

LeeStevens318 commented on Jan 20, 2026

tlevey01 commented on Jan 21, 2026

danroth27 commented on Jan 22, 2026

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