Blazor CSS isolation not working with any ASPNETCORE_ENVIRONMENT other than Development

[n-develop]

Describe the bug

The CSS isolation feature in Blazor (at least for Blazor Server) is not working for any ASPNETCORE_ENVIRONMENT other than "Development". As soon as I change the environment variable, the generated CSS file is empty.

To Reproduce

The reproduction is pretty simple. Just create a new Blazor Server app using dotnet new blazorserver and change the ASPNETCORE_ENVIRONMENT setting in launchSettings.json from "Development" to anything else like "Production".

Then dotnet run the project and you can see that the CSS is missing.

You can see the minimal example in my sample repo: https://github.com/n-develop/BlazorCssIsolation

Further technical details

• OS: Tested on MacOS BigSur and Windows 10
• dotnet --info output:

.NET SDK (gemäß "global.json"):
 Version:   5.0.100
 Commit:    5044b93829

Laufzeitumgebung:
 OS Name:     Mac OS X
 OS Version:  11.0
 OS Platform: Darwin
 RID:         osx.11.0-x64
 Base Path:   /usr/local/share/dotnet/sdk/5.0.100/

Host (useful for support):
  Version: 5.0.0
  Commit:  cf258a14b7

.NET SDKs installed:
  2.1.505 [/usr/local/share/dotnet/sdk]
  2.2.106 [/usr/local/share/dotnet/sdk]
  3.1.100 [/usr/local/share/dotnet/sdk]
  3.1.101 [/usr/local/share/dotnet/sdk]
  3.1.300 [/usr/local/share/dotnet/sdk]
  3.1.301 [/usr/local/share/dotnet/sdk]
  5.0.100-rc.2.20479.15 [/usr/local/share/dotnet/sdk]
  5.0.100 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.9 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.All 2.2.4 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.9 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 2.2.4 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.1 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.4 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.5 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0-rc.2.20475.17 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.0 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.9 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.15 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.18 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.1.19 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 2.2.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.1 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.4 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0-rc.2.20475.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.0 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

[mkArtakMSFT]

Thanks for contacting us. The CSS Isolation is built on top of Static Web Assets feature, which works either in development or in published app.
To be able to use it in other environments read this comment

[n-develop]

Oh, (apparently) I did not know that. Thanks for the update. Maybe the project template should be updated to call webhost.UseStaticWebAssets() by default? Or at least it should be mentioned in the docs (https://docs.microsoft.com/en-us/aspnet/core/blazor/components/css-isolation?view=aspnetcore-5.0)
Again, thanks for the fast response.