Analyzer host process fails to load analyzers correctly when building in parallel

[jnm2]

Version Used: SDK 5.0.201

This has always worked when running msbuild, but it does not work using dotnet build. The only workaround I have at the moment is to put /p:BuildInParallel=false in the dotnet build line which is not great because I want people to be able to walk up and type dotnet build multiple times. <BuildInParallel>false</BuildInParallel> didn't seem to have any effect on the repro.

What would it take to get from here to things just working for consumers of analyzers that have dependencies?

Steps to Reproduce:

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

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="PropertyChangedAnalyzers" Version="3.2.1" PrivateAssets="all" />
    <PackageReference Include="ReflectionAnalyzers" Version="0.1.22-dev" PrivateAssets="all" />
  </ItemGroup>

</Project>

using System.ComponentModel;
using System.Runtime.CompilerServices;

public sealed class C : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    private string property1;
    public string Property1 { get => property1; set => Set(ref property1, value); }

    private bool Set<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
    {
        if (RuntimeHelpers.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }

    private void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Run this (rebuild is only to simulate edit + build during development):

for ($i = 0; $i -lt 10; $i++) { dotnet build /t:rebuild }

Expected Behavior:

No warnings.

Actual Behavior:

37 warnings appear, all analyzer crashes due to not being able to load an assembly:

CSC : warning AD0001: Analyzer 'PropertyChangedAnalyzers.PropertyDeclarationAnalyzer' threw an exception of type 'System.IO.FileLoadException' with message 'Could not load file or assembly 'Gu.Roslyn.Extensions, Version=0.12.9.0, Culture=neutral, PublicKeyToken=4b04740f2fd5868f'. Could not find or load a specific file.

[sharwell]

.NET Core relies on Assembly Load Context (ALC) to load more than one version of the same assembly side-by-side. Currently the compiler doesn't support ALC for analyzer references, so if more than one analyzer in the project is providing the same dependency, they all need to have the same version. This applies even to the case where strong names can distinguish between the assemblies, which would work side-by-side for .NET Framework builds.

[jnm2]

Does that make this a duplicate of work that is already tracked, or is new work still needed to achieve a "just working" analyzer ecosystem?

[jaredpar]

Think we need to just follow the pattern that the MSBuild team laid out here for how they isolated Task in different AssemblyLoadContext:

• Load plugins in an AssemblyLoadContext msbuild#4916

One complication we have to consider though is that analyzers and there dependencies are passed with the /analyzer: flag. Essentially consider if you have a NuPkg which has the following layout:

• analyzers
  • cs
    • Analyer.dll
    • Util.dll

The standard tasks and targets will cause the compiler command line to include both /analyzer:Analyzer.dll and /analyzer:Util.dll. There are a couple of reasons for this but the simplest one is because these are all inputs to the compiler and hence should be expressed on the command line.

This means that for the purpose of isolating in an AssemblyLoadContext we can't just isolate every single /analyzer argument. Instead we need to group them based on the containing directory path. Essentially put all of the analyzer and generator DLLs from the same directory into the same AssemblyLoadContext.

Details on how AssemblyLoadContext works are here