using System;

​

namespace NamespaceOne {

  using ExternalNamespace;

  public class Classe1 {

    // (...

    public static void Main() {

      Console.WriteLine("Hello World");

    }

  }

}

​

namespace AnotherNamespace {

  public class Classe2 {

    // (...)

  }

}

​

// In another file .cs ... 

namespace ExternalNamespace {

  public class Classe3 {

    //(...)

  }

}