The Collection classes are a group of classes designed specifically for grouping together objects and performing tasks on them. List class is a collection and defined in the System.Collections.Generic namespace and it provides the methods and properties like other Collection classes such as add, insert, remove, search etc.

The C# List < T > class represents a strongly typed list of objects that can be accessed by index and it supports storing values of a specific type without casting to or from object. 

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading;
namespace Forum
{ 
    public class ChildClass 
    {
        static void Main(string[] args)
        {
// You can add items int List collection.
            List<Customer> CustomerList = new List<Customer>();
            Customer obj = new Customer();
            obj.CustomerID = 1;
            obj.CustomerName = "Visal";
            obj.ContactNumber = "8787878787";
            CustomerList.Add(obj);
            obj = newCustomer();
            obj.CustomerID = 1;
            obj.CustomerName = "Visal";
            obj.ContactNumber = "9797979797";
            CustomerList.Add(obj);
            obj = newCustomer();
            obj.CustomerID = 1;
            obj.CustomerName = "Visal";
            obj.ContactNumber = "77986521322";
            CustomerList.Add(obj);
// You can retrieve items from List collection by using for loops.
            for (int i = 0; i < CustomerList.Count; i++)
            {
                Console.WriteLine("CustomerID: {0}", CustomerList[i].CustomerID);
                Console.WriteLine("CustomerName:{0}", CustomerList[i].CustomerName);
                Console.WriteLine("ContactNumber:{0}", CustomerList[i].ContactNumber +"\n");  
            }
                       Console.ReadLine();
        }
    }
    public class Customer
    {
        public int CustomerID { get; set; }
        public String CustomerName { get; set; }
        public string  ContactNumber { get; set; }
    }
}