Listing 9-1 extracted from chapter Fundamental concepts of the language

Listing 8-17<     > Listing 9-2

This listing can be compiled with the command line:
csc.exe /target:exe Example_9_1.cs
Errors: 0 Warnings: 2

using B;
namespace A{
   class Program{
      static void Main() {}
      ClassFoo f; // Here, you can use the ClassFoo class.
   } 
}
namespace B{
   using A;
   class ClassFoo{Program p;} // Here, you can use the Program class.
}