Razor Class Library project cannot be referenced by Blazor WASM project: The type or namespace name 'ApplicationPartAttribute' does not exist in the namespace 'Microsoft.AspNetCore.Mvc.ApplicationParts' (are you missing an assembly reference?)

[zxyao145]

Describe the bug

I'm trying to migrate the blade project to NET 5.0 RC1, but after adding a reference to the Razor Class Library project to the WASM project, the compiler prompts an error:

obj\Debug\net5.0\XXXX\XXXX.MvcApplicationPartsAssemblyInfo.cs(14,54,14,78): error CS0234: The type or namespace name 'ApplicationPartAttribute' does not exist in the namespace 'Microsoft.AspNetCore.Mvc.ApplicationParts' (are you missing an assembly reference?).

When change the SDK Microsoft.NET.Sdk.BlazorWebAssembly to Microsoft.NET.Sdk.Web (similar to using NET Core 3.1), I can build the project success, but It's not generate as the Blazor WASM project. Then I found the solution is in issues 24171:

Add <GenerateMvcApplicationPartsAssemblyAttributes>false</GenerateMvcApplicationPartsAssemblyAttributes> to project props.

And It's helpful. The issue has been closed, and I think this may be a bug, please fix it, thanks a lot!

To Reproduce

Exceptions (if any)

Further technical details

• Include the output of dotnet --info

.NET SDK (reflecting any global.json):
 Version:   5.0.100-rc.1.20452.10
 Commit:    473d1b592e

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19041
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\5.0.100-rc.1.20452.10\

Host (useful for support):
  Version: 5.0.0-rc.1.20451.14
  Commit:  38017c3935

.NET SDKs installed:
  2.1.810 [C:\Program Files\dotnet\sdk]
  3.1.402 [C:\Program Files\dotnet\sdk]
  5.0.100-rc.1.20452.10 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-rc.1.20451.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.22 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-rc.1.20451.14 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 3.1.8 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.0-rc.1.20452.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

• The IDE (VS / VS Code/ VS4Mac) you're running on, and it's version
  VS 2019 16.8.0 Preview 3.2

[javiercn]

@zxyao145 thanks for contacting us.

Most likely you created a razor class library with support for pages and views or referenced a library that targets ASP.NET Core. As such, that is not going to work in Blazor Webassembly.

I suggest you look at your dependencies to identify what is bringing on the ASP.NET Core framework and remove it or factor it out of your library.

[zxyao145]

@javiercn
Yes, my application has two modes: Blazor Server and Blazor WASM, I reuse the razor pages in Blazor Server project and Blazor WASM project, and I think this situation may be more common. Is there any deficiency in the current scheme, or is there a better solution?

Thank you very much!

[javiercn]

No, this is just how dependencies work. You need to factor your app to avoid bringing in server dependencies on the wasm payload.

[zxyao145]

@javiercn OK, thank you very much!

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