calculate the sum of ListNodes

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        
        static void Main(string[] args)
        {
            List listOfNodes = new List();
           
            Random r = new Random();
            int sum = 0;
            for (int i = 0; i < 10; i++)
            {
                listOfNodes.addObjects(r.Next(1, 100));
               
            }
            listOfNodes.DisplayList();
            foreach (object s in listOfNodes) 
               sum += (int)s;
            Console.WriteLine(sum);

            Console.ReadLine();
            
        }

        class ListNode
        {
            public object inData { get; private set; }
            public ListNode Next { get; set; }

            public ListNode(object dataValues)
                : this(dataValues, null) { }

            public ListNode(object dataValues, 
                ListNode nextNode)
            {
                inData = dataValues; Next = nextNode;
            }
        } // class ListNode-s dasasruli

        public class List : IEnumerable
        {
            private ListNode firstNode, lastNode;
            private string name;

            public List(string nameOfList)
            {
                name = nameOfList;
                firstNode = lastNode = null;
            }

            public List()
                : this("listOfNodes") { }
           
           
            public void addObjects(object inItem)
            {
                if (isEmpty())
                { firstNode = lastNode = new ListNode(inItem); }
                else { firstNode = new ListNode(inItem, firstNode); }
            }
            
            private bool isEmpty()
            {
                return firstNode == null;
            }

            public void DisplayList()
            {
                if (isEmpty())
                { Console.Write("Empty " + name); }
                else
                { 
                    Console.Write("The " + name + " is:\n");

                    ListNode current = firstNode;

                    while (current != null)
                    {
                        Console.Write(current.inData + " ");
                        current = current.Next;
                    }
                    Console.WriteLine("\n");
                }
            }

            public IEnumerator GetEnumerator()
            {
                ListNode node = firstNode;
                while (node != null)
                {
                    yield return node;
                    node = node.Next;
                }
            }
        }//end of class List
    }
}