Source generator Newtonsoft.Json FileNotFoundException

[onetsmr]

How can I use Newtonsoft.Json lib in the source generator project?

My generator project

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

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <LangVersion>preview</LangVersion>
    <UserSecretsId>...</UserSecretsId>
    <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
    <IncludeBuildOutput>false</IncludeBuildOutput>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.8.0" PrivateAssets="all" />
    <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="3.3.2" PrivateAssets="all" />
    <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
  </ItemGroup>

  ... missing ...

</Project>

I get

Exception was of type 'FileNotFoundException' with message 
'Could not load file or assembly 'Newtonsoft.Json, Version=12.0.0.0, 
Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' 
or one of its dependencies.

What I already tried:

1. GeneratePackageOnBuild true/false
2. IncludeBuildOutput true/false
3. Add Newtonsoft.Json 12.0.3 into the main project, which use generator project (it is .net 5.0)
4. Make as described here https://github.com/dotnet/roslyn/blob/master/docs/features/source-generators.cookbook.md#use-functionality-from-nuget-packages

Nothing works. I keep getting 'Could not load file or assembly' exception.

If I comment code line which calls Newtonsoft.Json.JsonConvert class and go to the debugger - then I see Newtonsoft.Json lib in the context.Compilation.ReferencedAssemblyNames array.

[NealCallaghan]

Checkout the csproj here https://github.com/dotnet/roslyn-sdk/blob/master/samples/CSharp/SourceGenerators/SourceGeneratorSamples/SourceGeneratorSamples.csproj also to get to debug, in your execute method of the generator write Debugger.Launch() then you can debug. VS will ask you which instance etc but you will get it.