Roslyn provides open-source C# and Visual Basic compilers with rich code analysis APIs. It enables building code analysis tools with the same APIs that are used by Visual Studio. It is the default C# compiler that ships with Visual Studio and has been open sourced by Microsoft.
To clone or download, visit here
If you receive Rosyln missing errors in Asp.Net web applications, open the Package Manager Console in Visual Studio and run the following command to reinstall the compiler,
Update-Package Microsoft.CodeDom.Providers.DotNetCompilerPlatform -r
While you are here, it is worth reading on how Microsoft rewrote its C# compiler in C# and made it open source. Visit the following link for more information,