var req_item = new Req_shipment.Req_Item() ;
            List<Req_shipment.Req_Item> req_items; req_items = new List<Req_shipment.Req_Item>(); 

                while (sqlResult.Read())
                { i++;
                   req_item.itemIndex = Convert.ToInt32(sqlResult.GetString(2)); req_item.offerId = sqlResult.GetString(3); // Всосали очередную запись 
                   Console.WriteLine($"req_item.itemIndex = {req_item.itemIndex} \t req_item.offerId = {req_item.offerId} "); // Глянули что считали
                   req_items.Add(req_item); // Добавили новую строку в список
                }
 

...
while (sqlResult.Read())
...

            public class Req_shipment
            {
                public class Req_Item
                {
                    public int itemIndex;
                    public string offerId;
                };
                public int shipmentId;
                public List<Req_Item> items;
            };

var req_items = new List<Req_shipment.Req_Item>();

while (sqlResult.Read())
{
    req_items.Add(new Req_shipment.Req_Item { itemIndex = sqlResult.GetString(2), offerId = sqlResult.GetString(3) });
}

var req_items = new List<Req_shipment.Req_Item>();

while (sqlResult.Read())
{
    req_items.Add(new Req_shipment.Req_Item { itemIndex = sqlResult.GetString(2), offerId = sqlResult.GetString(3) });
}

using System;
using System.Collections.Generic;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            var list = new List<int>(3);
            int item;

            list.Add(item = 1);
            list.Add(item = 2);
            list.Add(item = 3);
            item = 4;

            for (var i = 0; i < list.Count; i++)
            {
                Console.WriteLine(list);
            }

            Console.ReadKey();
        }
    }
}

private static void Main(string[] args)
{
    var list = new List<Action>();
    for (var i = 0; i < 10; i++)
    {
        list.Add(() => Console.WriteLine(i));
    }
    foreach (var act in list)
    {
        act();
    }
}