Duplicate TargetFrameworkAttribute errors in .NET 9.0 Blazor projects with VS Code

[Mark-Phillipson]

Is there an existing issue for this?

• I have searched the existing issues

Describe the bug

Description

When working with .NET 9.0 Blazor WebAssembly projects in Visual Studio Code (both regular and Insiders versions), duplicate 'TargetFrameworkAttribute' errors are appearing during build, despite having a correct project configuration.

Environment

• .NET Version: 9.0.5
• Project Type: Blazor WebAssembly
• IDE: Visual Studio Code with C# Dev Kit
• Operating System: Windows
• Project SDK: Microsoft.NET.Sdk.BlazorWebAssembly

Project Configuration

<Project Sdk="Microsoft.NET.Sdk.BlazorWebAssembly">
  <PropertyGroup>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
    <RootNamespace>BlazorApp.Client</RootNamespace>
    <ServiceWorkerAssetsManifest>service-worker-assets.js</ServiceWorkerAssetsManifest>
  </PropertyGroup>
  
  <ItemGroup>
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly" Version="9.0.5" />
    <PackageReference Include="Microsoft.AspNetCore.Components.WebAssembly.DevServer" Version="9.0.5" PrivateAssets="all" />
  </ItemGroup>
</Project>

Expected Behavior

The project should build without duplicate attribute errors, as the project file configuration is correct and follows the standard template structure.

Steps To Reproduce

Steps to Reproduce

1. Create a new Blazor WebAssembly project targeting .NET 9.0
2. Open the project in VS Code
3. Build the project
4. Observe build errors in the Problems panel

Exceptions (if any)

No response

.NET Version

9.0.204

Anything else?

Additional Information

• The issue persists even after:
  • Cleaning and rebuilding the solution
  • Deleting bin/obj folders
  • Restoring NuGet packages
  • Using different versions of VS Code
• This appears to be specific to .NET 9.0 Blazor projects when using VS Code
• The same project builds correctly in Visual Studio 2022

Workarounds Attempted

• Manually deleting the generated AssemblyAttributes.cs file (regenerates with same issue)
• Adding <GenerateAssemblyInfo>false</GenerateAssemblyInfo> (impacts other necessary assembly attributes)
• Various combinations of project file settings related to assembly info generation

Terminal Last Command:

dotnet build
PS C:\Users\MPhil\source\repos\Words\cautious-robot\Client> dotnet build
Restore complete (0.5s)
  Shared succeeded (0.3s) → C:\Users\MPhil\source\repos\Words\cautious-robot\Shared\bin\Debug\netstandard2.0\Shared.dll
  CodeleLogic succeeded (0.5s) → C:\Users\MPhil\source\repos\Words\cautious-robot\CodeleLogic\bin\Debug\net8.0\CodeleLogic.dll
  Client failed with 8 error(s) (0.4s)
    C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\.NETCoreApp,Version=v9.0.AssemblyAttributes.cs(4,12): error CS0579: Duplicate 'global::System.Runtime
.Versioning.TargetFrameworkAttribute' attribute                                                                                                                                       C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(13,12): error CS0579: Duplicate 'System.Reflection.AssemblyCompanyAttribute' a
ttribute                                                                                                                                                                              C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(14,12): error CS0579: Duplicate 'System.Reflection.AssemblyConfigurationAttrib
ute' attribute                                                                                                                                                                        C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(15,12): error CS0579: Duplicate 'System.Reflection.AssemblyFileVersionAttribut
e' attribute                                                                                                                                                                          C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(16,12): error CS0579: Duplicate 'System.Reflection.AssemblyInformationalVersio
nAttribute' attribute                                                                                                                                                                 C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(17,12): error CS0579: Duplicate 'System.Reflection.AssemblyProductAttribute' a
ttribute                                                                                                                                                                              C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(18,12): error CS0579: Duplicate 'System.Reflection.AssemblyTitleAttribute' att
ribute                                                                                                                                                                                C:\Users\MPhil\source\repos\Words\cautious-robot\Client\obj\Debug\net9.0\Client.AssemblyInfo.cs(19,12): error CS0579: Duplicate 'System.Reflection.AssemblyVersionAttribute' a
ttribute                                                                                                                                                                          
Build failed with 8 error(s) in 1.6s

[javiercn]

@Mark-Phillipson thanks for contacting us.

Is this issue specific to VS code? (Does it work if you build from the command line with dotnet build? Could you provide a minimal repro project that demonstrates the issue?

[Mark-Phillipson]

This is happening in Visual Studio proper as well as VSCode/VSCode Insiders.

After several hours with copilot current workaround was to add the following into the *.csproj file:

    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
    <GenerateTargetFrameworkAttribute>false</GenerateTargetFrameworkAttribute>

[Mark-Phillipson]

This is potentially fixed but here is the repo: https://github.com/Mark-Phillipson/cautious-robot

[javiercn]

@Mark-Phillipson thanks for the additional details.

This helps us get more info to route it to the appropriate teams (in this repo we only deal with runtime issues)

[javiercn]

@Mark-Phillipson

I was going to move the issue here but it recommends that you file the issue through the Report an Issue dialog in VS Code (see instructions there), so I think it's better if you follow those instructions to make sure the issue doesn't get lost.

[Mark-Phillipson]

Okay thanks will do.