products.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
    public static void Main () {
        Product p1 = new Product ();
        Console.WriteLine (p1.ID);

        Product p2 = new Product ();
        Console.WriteLine (p2.ID);
        Console.WriteLine (p1.ID);

        p1.SetPrice (2000);
        p2.SetPrice (200000);

    }

    public class Product {

        private static int _counter;
        private int _id;

        public Product () {
            if (_counter == null) {
                _counter = 1;
            } else {
                _counter++;
            }

            _id = _counter;
        }

        public int ID {
            get {
                return _id;
            }
        }

        private int _price;
        public int Price {
            get { return _price; }
        }

        public void SetPrice (int price) {
            if (price >= 10 && price <= 10000) {
                _price = price;
            } else {
                throw new ArgumentOutOfRangeException (nameof (price));
            }
        }

        ~Product () {
            _counter--;
            Console.WriteLine ($"Counter decreased ({_counter.ToString()})");
        }

    }

}