Listing 9-4 extracted from chapter Fundamental concepts of the language

Listing 9-3<     > Listing 9-5

This listing can be compiled with the command line:
csc.exe /target:exe Example_9_4.cs
Errors: 0 Warnings: 0

//#define __TRACE__
class Program {
   [System.Diagnostics.Conditional("__TRACE__")]
   public static void Trace(string s) {
      System.Console.WriteLine(s);
   }
   static void Main() {
      Trace("Hello");
      System.Console.WriteLine("Bye");
   }
}