/in/foo.c:7:1: error: unknown type name 'using'
 using System;
 ^~~~~
/in/foo.c:9:1: error: unknown type name 'class'
 class Program {
 ^~~~~
/in/foo.c:9:15: error: expected '=', ',', ';', 'asm' or '__attribute__' before '{' token
 class Program {
               ^