ProduceOnlyReferenceAssembly=true fails with `Do not use refout when using refonly`

The following project fails to compile:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <ProduceOnlyReferenceAssembly>true</ProduceOnlyReferenceAssembly>
  </PropertyGroup>

</Project>

The above project will fail to compile with error

CSC : error CS8308: Do not use refout when using refonly.

This used to work with target netstandard2.0 instead.

The workaround is to set the following to false:

    <ProduceReferenceAssembly>false</ProduceReferenceAssembly>

However it’s rather counter-intuitive that to generate a reference assembly - but only a reference assembly - you actually have to turn off producing a reference assembly.

It is likely related to this bit and explains why netstandard2.0 works but net5.0+ doesn’t: https://github.com/dotnet/sdk/blob/a30e465a2e2ea4e2550f319a2dc088daaafe5649/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.TargetFrameworkInference.targets#L233

[tannergooding]

Something else in the build system now decided to specify the other one.

Right, the SDK added a new option specifying it by default for .NET 5+ (it’s the Microsoft.NET.TargetFrameworkInference.targets I linked a couple posts up).

My suggested fix was one the SDK could take to ensure it remains correct by default if the user specifies ProduceOnlyReferenceAssembly as you did.

[jcouv]

Sounds like a good idea. I don’t see any impact on the compiler and this would make it easier to deal with the implicit default from the SDK.

Linking this issue to main ref assemblies umbrella issue: https://github.com/dotnet/roslyn/issues/2184