using System;

using System.Linq;

​

 public class ClassA

 {

     public string ID { get; private set; }

​

     public class ClassB

     {

         public string Setter() 

         {

             var g = new ClassA();

             g.ID = "11";

             return g.ID;

         }

     }

 }

​

public class Program

{

    public static void Main()

    {

        var h = new ClassA.ClassB();

        Console.WriteLine(h.Setter());

    }

}