Solution consists of 2 dll libraries, 1 console app and 1 web app. Only the last project, the web one, uses Microsoft.Net.Compilers NuGet package. Package has been updated to version 2.3.0 now, what means C# 7.1 and VB 15.3.
source: Roslyn NuGet packages
As 3 other projects do not have Microsoft.Net.Compilers NuGet package installed, what Roslyn compiler and C# version do they use?
I use VS.Net 2017 15.2 with .NET Framework 4.7, so I would guess it shall C#7.0, but still Microsoft.Net.Compilers in version 2.2 should be installed there, but is not.
The Microsoft.Net.Compilers package overrides (by setting properties in your project file) the default version of Roslyn included with MSBuild / Visual Studio.
Regular class libraries that don't reference it are compiled using the Roslyn build included with your version of Visual Studio & MSBuild.
来源：https://stackoverflow.com/questions/45080482/roslyn-compiler-version-used-for-projects-missing-microsoft-net-compilers