Compile this code with -p:boo : import System interface ITest: def M() as void P as string: get event eh as EventHandler abstract class TestClass: abstract def M() as void: pass abstract P as string: get: pass event eh as EventHandler The output of BooPrinterVisitor is public interface ITest: public abstract def M() as System.Void: pass public abstract P as System.String: public abstract get: pass public abstract event eh as System.EventHandler [...] This is invalid syntax for interfaces. Fixing this is important for the C# -> Boo converter.