Strange diagnostics errors, prefedined type System… is not defined or imported

Question

I am getting Roslyn diagnostics errors when parsing a very basic .NET 4.6 application. The solution files can be downloaded from there https://github.com/dotnet/roslyn/file

Answer 1

So this finally solved it:

1. Added the microsoft.build redirects as suggested by @GeorgeAlexandria http://github.com/Microsoft/msbuild/issues/2369#issuecomment-353674937

2. Cleared the Microsoft.Build.* from the output bin folder

3. Added the Microsoft.Build.Locator as a reference

4. Added the line MSBuildLocator.RegisterDefaults() above the workspace code.

Source: https://github.com/dotnet/roslyn/issues/26029#issuecomment-380164421