.NET Framework 4.8 build with SGen.exe fails when referencing a .NET Standard 2.0 project

[MaaadMax]

Describe the bug

After I have ported some class libraries to .NET Standard 2.0, I get the following error message when building a .NET Framework 4.8 project which references a .NET Standard 2.0 project:
The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

After some investigation I found out that the build only fails when GenerateSerializationAssemblies is set to On and SGenUseProxyTypes is set to false in the .NET Framework project. When I set SGenUseProxyTypes to true or remove it the build succeeds.

I hope this is the right place for this bug report.

To Reproduce

• Create a new class library with .NET Framework 4.8 and rename the default class to NetFramework
• Add the following two lines to the *.csproj file of the .NET 4.8 project (to the first PropertyGroup)
  <GenerateSerializationAssemblies>On</GenerateSerializationAssemblies>
<SGenUseProxyTypes>false</SGenUseProxyTypes>
• Create a new class library with .NET Standard 2.0 and rename the default class to NetStandard
• In the .NET 4.8 Project add a reference to the .NET Standard 2.0 class library
• Create a public property in the .NET 4.8 class library with the type NetStandard
public NetStandard.NetStandard NetStandard { get; set; }
• Build the solution in visual studio -> build fails

Attached you'll find a small repro with both projects.

Exceptions

CS0012 The type 'System.Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'

Further technical details

• dotnet --info:

.NET SDK (reflecting any global.json):
Version: 6.0.101
Commit: ef49f6213a

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19042
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.101\

Host (useful for support):
Version: 6.0.1
Commit: 3a25a7f1cc

• I am using Visual Studio Enterprise 2022 (17.0.4
  DotNetStandard_SGenUseProxyTypes.zip
  )

[KalleOlaviNiemitalo]

The error happens regardless of whether the project targeting .NET 4.8 is a legacy project or a .NET SDK project.

MSBuild runs C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\sgen.exe with a /reference option that includes C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.8\Facades\netstandard.dll, but sgen.exe then runs csc.exe with a series of /R options that does not include /R:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.8\Facades\netstandard.dll". If I run csc.exe directly, with the same command line plus that option, then it succeeds.

I think this would need a fix in sgen.exe, but is that even open source?

[KalleOlaviNiemitalo]

dotnet/sdk#10255 (comment) suggests using Microsoft.XmlSerializer.Generator instead of sgen.exe. If I remove the GenerateSerializationAssemblies MSBuild property, and add both a PackageReference and a DotNetCliToolReference as instructed in https://docs.microsoft.com/dotnet/core/additional-tools/xml-serializer-generator, then the build succeeds and the generated serializer does read the Class1.NetStandard property.

It's a bit surprising though; I thought DotNetCliToolReference was being deprecated (dotnet/sdk#3115) and replaced with tool manifests. dotnet/docs#26253 has been filed.

[MaaadMax]

#10255 (comment) suggests using Microsoft.XmlSerializer.Generator instead of sgen.exe. If I remove the GenerateSerializationAssemblies MSBuild property, and add both a PackageReference and a DotNetCliToolReference as instructed in https://docs.microsoft.com/dotnet/core/additional-tools/xml-serializer-generator, then the build succeeds and the generated serializer does read the Class1.NetStandard property.

It's a bit surprising though; I thought DotNetCliToolReference was being deprecated (#3115) and replaced with tool manifests. dotnet/docs#26253 has been filed.

With the XML serializer generator it actually works.
It isn't the best solution because I have to update 20+ projects but if SGEN.exe won't be ported to .NET Core, it may be the only solution.

Thanks for your help!

[marcpopMSFT]

Old issue triage: not sure who own SGEN but our best guess is Roslyn since the modern core way of doing this would be a generator.