Language server does not support .NET 5 SDK

[bamarsha]

Describe the bug

After installing the .NET 5 SDK, the language server can't load Q# projects:

[Info - 12:15:44] workspace folder: c:\Users\samarsha\Desktop\QSharpSandbox
[Error - 12:15:45] Error on loading project 'c:\Users\samarsha\Desktop\QSharpSandbox\Library\Library.csproj': Der SDK-Resolvertyp "WorkloadSdkResolver" konnte nicht geladen werden. Could not load file or assembly 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Das System kann die angegebene Datei nicht finden. c:\Users\samarsha\Desktop\QSharpSandbox\Library\Library.csproj.
Ignoring non-Q# project 'c:\Users\samarsha\Desktop\QSharpSandbox\Library\Library.csproj'.
[Error - 12:15:45] Error on loading project 'c:\Users\samarsha\Desktop\QSharpSandbox\App\App.csproj': Der SDK-Resolvertyp "WorkloadSdkResolver" konnte nicht geladen werden. Could not load file or assembly 'System.Runtime, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Das System kann die angegebene Datei nicht finden. c:\Users\samarsha\Desktop\QSharpSandbox\App\App.csproj.
Ignoring non-Q# project 'c:\Users\samarsha\Desktop\QSharpSandbox\App\App.csproj'.
[Info - 12:15:45] The file c:\Users\samarsha\Desktop\QSharpSandbox\App\App.qs is not associated with a compilation unit. Only syntactic diagnostics are generated.

To Reproduce

Steps to reproduce the behavior:

1. Open a Q# project in VS Code

System information

• QDK extension v0.13.20111004 for VS Code
• Windows 10, .NET 5.0.100
• VS Code 1.51.1

Additional context

The Q# project file is:

<Project Sdk="Microsoft.Quantum.Sdk/0.13.20111004">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <QscVerbosity>detailed</QscVerbosity>
  </PropertyGroup>
</Project>

The project builds and runs successfully from the command line.

[bamarsha]

Note that just having .NET 5 installed is enough to break the language server, even if I also have .NET Core 3.1 installed, and the Q# project is targeted for .NET Core 3.1. Temporarily removing the C:\Program Files\dotnet\sdk\5.0.100 folder makes the language server work again.

[bamarsha]

Possibly related issues:

• Loading net5.0 Assemblies Shouldn't Happen When Running < net5.0 MSBuildLocator#92
• Use more permissive load context for MSBuildWorkspace by default dotnet/roslyn#49248
• [.NET 5 RC2] WorkloadSdkResolver failed to load dotnet/tye#699

[bamarsha]

A workaround is to add a global.json file to your Q# project, or to any parent directory (such as your user directory), that sets the .NET SDK version to 3.1:

{
  "sdk": {
    "version": "3.1.100",
    "rollForward": "latestFeature"
  }
}

[bettinaheim]

A workaround is to add a global.json file to your Q# project, or to any parent directory (such as your user directory), that sets the .NET SDK version to 3.1:

{
  "sdk": {
    "version": "3.1.100",
    "rollForward": "latestFeature"
  }
}

@SamarSha It might make sense to just add such a global.json to our project templates. What do you think?

[bamarsha]

I think if we can't find any other solution to support .NET 5 and .NET Core 3.1 simultaneously, then adding global.json to the project template would make sense. But it seems like a last resort to me. Ideally, we would even support having only .NET 5 SDK installed (not .NET Core 3.1 SDK), since dotnet build from the command line already works in that scenario (with a Q# project targed to netcoreapp3.1).

[cgranade]

I think if we can't find any other solution to support .NET 5 and .NET Core 3.1 simultaneously, then adding global.json to the project template would make sense. But it seems like a last resort to me.

Agreed. Even if providing a global.json may be a good idea for individual projects to do in order to enable reproducible builds, it's a big step for us to require that of our users.

Ideally, we would even support having only .NET 5 SDK installed (not .NET Core 3.1 SDK), since dotnet build from the command line already works in that scenario (with a Q# project targed to netcoreapp3.1).

Agreed here ass well.