MSBuildWorkspace: Unable to build hello world .netstandard2.0 class library anymore

[ceztko]

Issue Description

Using MSBuild.Locator, Workspaces.MSBuild and the Roslyn infrastructure I'm unable to compile an hello world netstandard2.0 class library. The same library compile just fine when targeting .Net framework 4.5.2. I noticed the issue after I installed Visual Studio 16.8. Some releases before the same scenario was working.

Steps to Reproduce

1. Unpack the following test project:
   TestMSBuildWorkspaceCompilation.zip

2. Open TestMSBuildWorkspaceCompilation.sln;

3. Compile and run TestMSBuildWorkspaceCompilation project. When trying to compile the netstandard2.0 project an exception should trigger and compilation errors are printed. More explanations are in the code there.

Expected Behavior

No compilation errors should the issued when compiling the project targeting netstandard2.0.

Actual Behavior

A bunch of errors are printed, as the build system is unable to load the netstandard2.0 base library when compiling the project.

TestClassLibrary\obj\Debug\netstandard2.0\.NETStandard,Version=v2.0.AssemblyAttributes.cs[82..88)), CS0400: The type or namespace name 'System' could not be found in the global namespace (are you missing an assembly reference?)
TestMSBuildWorkspaceCompilation\TestClassLibrary\obj\Debug\netstandard2.0\TestClassLibrary.AssemblyInfo.cs[421..427)), CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)
TestMSBuildWorkspaceCompilation\TestClassLibrary\obj\Debug\netstandard2.0\TestClassLibrary.AssemblyInfo.cs[497..503)), CS0246: The type or namespace name 'System' could not be found (are you missing a using directive or an assembly reference?)
[...]

Analysis

I'm currently using VS 16.9.2 but the same scenario was working some releases before 16.8.

Versions & Configurations

msbuild 16.9.0.11203
VisualStudio 16.9.2
.NET SDK 5.0.201

[ceztko]

Any confirm on reproduction of this?

[ceztko]

This issue seems to be more related to roslyn infrastructure (tracking issue dotnet/roslyn#52293). Feel free to close anytime if you're already sure it's not pertinent to msbuild, or I will do it myself later.

[Forgind]

Sorry I was a little slow in getting to this, but I agree with your analysis—the GetCompilationAsync call is what's finding the errors, and MSBuild doesn't have anything to do with that. Roslyn sounds like a likely candidate.