Yes, we can make a program having structure or class under structure.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
 
namespace ConsoleApplication14
{
    struct demo
    {
     
        public
void fun1()
        {
            demo2
d2 = new demo2();
            demo3
d3 = new demo3();
       
           
d2.fun2();
           
Console.WriteLine("structure under structure is implemented
successfully");
           
d3.fun2();
           
Console.WriteLine("structure under class  is implemented successfully");
        }
        struct
demo2 
        {
           
public void fun2()
            {
               
Console.WriteLine("Testing Comment");
            }
        }
        class
demo3
        {
           
public void fun2()
            {
               
Console.WriteLine("Testing Comment");
            }
        }
        
       
    }
    class Program
    {
 
        static
void Main(string[] args)
        {
            demo
d1 = new demo();
           
d1.fun1();
            demo2
d2 = new demo2();
            demo3
d3 = new demo3();
 
           
d2.fun2();
           
Console.WriteLine("Class under structure is implemented
successfully");
           
d3.fun2();
           
Console.WriteLine("Class under class is implemented
successfully");
 
           
Console.ReadLine();
            
        }
        struct
demo2
        {
           
public void fun2()
            {
               
Console.WriteLine("Testing Comment");
            }
        }
        class
demo3
        {
           
public void fun2()
            {
               
Console.WriteLine("Testing Comment");
            }
        }
        
    }
}