using System;

class B {
  
  public static void Main(){
    A a = new A(5);   
    double d;

    a[1] = 6; a[2] = 7.0; a[3] = 8.0;  

    d = a[1] + a[2];   
                       

    Console.WriteLine("a: {0}, d: {1}", a, d);
  }
}

 
 
 
 
 
a refers to an instance of class A: anAObject.
 
 
Setting d, e, and f via the indexer. 
 
Corresponds to d = a.d + a.e 
in case d and e had been public.