CopyRefAssembly Could not extract the MVID from reference assembly

Following up on https://github.com/dotnet/fsharp/pull/12334, I’ve noticed that a project B (csproj) cannot consume a reference assembly of a project A (fsproj) because the mvid cannot be read from the assembly.

Project structure:

A.fsproj

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

  <PropertyGroup>
    <TargetFramework>net7.0</TargetFramework>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
    <DotnetFscCompilerPath>C:\Users\nojaf\Projects\fsharp\artifacts\bin\fsc\Debug\net6.0\fsc.dll</DotnetFscCompilerPath>
    <OtherFlags>--refout:obj\Debug\net7.0\refint\A.dll</OtherFlags>
    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Library.fs" />
  </ItemGroup>

</Project>

B.csproj

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

  <ItemGroup>
    <ProjectReference Include="..\A\A.fsproj" />
    
  </ItemGroup>

  <PropertyGroup>
    <TargetFramework>net7.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
  </PropertyGroup>

</Project>

Expected behavior

dotnet build of B.csproj will find the reference assembly of project A that was specified using the refout flag.

Actual behavior

In the MSBuild log we saw the following:

Could not extract the MVID from “obj\Debug\net7.0\refint\A.dll”. Are you sure it is a reference assembly?

This warning was logged by CopyRefAssembly,

https://github.com/dotnet/roslyn/blob/b7838e90db7c7f1de309f8ef314e0aa8bfc6dab1/src/Compilers/Core/MSBuildTask/CopyRefAssembly.cs#L56

The main problem is that the mvid guid is empty.

The current Number of Sections inside the COFF Header is 3, whereas compared to a C# reference assembly this is 4. The .mvid section is missing from the F# generation.

The MSBuild task uses this section to read that guid.

https://github.com/dotnet/roslyn/blob/b7838e90db7c7f1de309f8ef314e0aa8bfc6dab1/src/Compilers/Core/MSBuildTask/MvidReader.cs#L90-L137

This entire section could be added around

https://github.com/dotnet/fsharp/blob/36af364b538cde99d8de8df93db241af53329f22/src/Compiler/AbstractIL/ilwrite.fs#L3884-L3886

Similar to .text, .sdata, .reloc I would assume. Conditional logic could be implemented using options.referenceAssemblyOnly.

Known workarounds

Besides adding the additional section, it is possible to extract the guid currently from the Module metadata table. We were able to read it using:

#r "nuget: System.Reflection.Metadata"

open System.IO
open System.Reflection.PortableExecutable

let refDll = @"C:\Users\nojaf\Projects\reference-assemblies-sample\A\obj\Debug\net7.0\refint\A.dll"
let embeddedReader = new PEReader(File.OpenRead refDll)
let sourceReader = embeddedReader.GetMetadataReader()
let loc = sourceReader.GetModuleDefinition().Mvid
let mvid = sourceReader.GetGuid(loc)

So, from a certain point of view, the mvid is there. Just not in the way that CopyRefAssembly reads it.

Related information

Provide any related information (optional):

• Operating system: Win11
• .NET Runtime kind: .NET 7 preview 4
• local compiler, main at 5b1a3ae5d16fe93af1f8b055ab0be5bb5e47ff20

//cc @dsyme @vzarytovskii @baronfel @jaredpar

[jaredpar]

Yeah, we probably should start emitting it as part of our work of FSharp.Build support of refassemblies.

Agree. The code in CopyRefAssembly is pretty standard for reading an MVID from a PE file. If this task is erroring then likely many other tools in the .NET ecosystem will similarly fail to read the F# ref assembly.

[vzarytovskii]

Yeah, we probably should start emitting it as part of our work of FSharp.Build support of refassemblies.