C#嵌套命名空间

using System;
using first_space;
using first_space.second_space;

namespace first_space {

   class abc {

      public void func() {
         Console.WriteLine("Inside first_space");
      }
   }

   namespace second_space {

      class efg {

         public void func() {
            Console.WriteLine("Inside second_space");
         }
      }
   }   
}

class TestClass {

   static void Main(string[] args) {
      abc fc = new abc();
      efg sc = new efg();
      fc.func();
      sc.func();
      Console.ReadKey();
   }
}