Может ли кто-нибудь предложить, как объединить элемент, клонировать элемент itemtostring()

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<pre>
Items.cs
class Items
    {
        public int ItemNumber { get; set; }
        public DateTime ItemDate { get; set; }
        //public int Quantity { get; set; }
        //public double Cost { get; set; }
        public List<ItemsLine> ItemsLines { get; set; } = new List<ItemsLine>();
        //public List<ItemsLine> sourceItem;


        public void AddItemLine(ItemsLine itemLine)
        {
            ItemsLines.Add(itemLine);
        }

        public void RemoveItemLine(int SOMEID)
        {
            ItemsLines.RemoveAt(1);
        }

        /// GetTotal should return the sum of (Cost * Quantity) for each line item
        public decimal GetTotal()
        {
            //return Convert.ToInt32(this.Cost) * this.Quantity;  
            return ItemsLines.Sum(i => Convert.ToInt32(i.Cost) * i.Quantity);

        }

        /// MergeItem appends the items from the sourceItem to the current invoice
        /// </summary>
        /// <param name="sourceItem">Item to merge from</param>
        public void MergeItem(Items sourceItem)
        {
            //IEnumerable<ItemsLine> items = new List<ItemsLine>();

            IEnumerable<Items> items = 
            
            
            //throw new NotImplementedException();
        }

        /// Creates a deep clone of the current item (all fields and properties)
        /// </summary>
        //public Items Clone()
        //{
            
            
            
        //    //throw new NotImplementedException();
        //}


        /// Outputs string containing the following (replace [] with actual values):
        /// Invoice Number: [ItemNumber], ItemDate: [dd/MM/yyyy], ItemsLineCount: [Number of items in LineItems]
        public override string ToString()
        {
            string result = "Item Number\tQuantity\tCost\n";
            foreach (ItemsLine item in ItemsLines)
                result += (item.ItemLineId + "\t\t" + item.Quantity + "\t\t" + item.Cost.ToString() + /*item.Total.ToString()+*/ "\n");

            return result;
        }
    }

class ItemsLine
    {

        public int ItemLineId { get; set; }
        public int Quantity { get; set; }
        public double Cost { get; set; }

       // public double Total { get { return this.Cost * this.Quantity; } }

        public ItemsLine(int itemLineId, int quantity, double cost)
        {
            this.ItemLineId = itemLineId;
            this.Quantity = quantity;
            this.Cost = cost;
        }

        public ItemsLine()
        {

        }
    }

<pre lang="c#">
<pre>
class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Welcome to ItemBilling Appliation....");

            CreateItemWithOneItem();
            CreateItemWithMultipleItemsAndQuantities();
            RemoveItem();
            //MergeItem();
            //CloneItem();
            //ItemToString();
        }


        private static void CreateItemWithOneItem()
        {
            var item = new Items();

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 1,
                Cost = 6.99,
                Quantity = 1,
                
            });
            Console.WriteLine(item.ToString());
            Console.WriteLine(item.GetTotal());
        }

        private static void CreateItemWithMultipleItemsAndQuantities()
        {
            var item = new Items();

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 1,
                Cost = 10.21,
                Quantity = 4,
            });

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 2,
                Cost = 5.21,
                Quantity = 1,
                
            });

            Console.WriteLine(item.ToString());
            Console.WriteLine(item.GetTotal());
        }

        private static void RemoveItem()
        {
            var item = new Items();

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 1,
                Cost = 5.21,
                Quantity = 1,
                
            });

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 2,
                Cost = 10.99,
                Quantity = 4,
                
            });

            item.RemoveItemLine(1);
            Console.WriteLine(item.ToString());
            
            Console.WriteLine(item.GetTotal());
        }

        private static void MergeItem()
        {
            var item1 = new Items();

            item1.AddItemLine(new ItemsLine()
            {
                ItemLineId = 1,
                Cost = 10.33,
                Quantity = 4,
                
            });

            var item2 = new Items();

            item2.AddItemLine(new ItemsLine()
            {
                ItemLineId = 2,
                Cost = 5.22,
                Quantity = 1,
                
            });

            item2.AddItemLine(new ItemsLine()
            {
                ItemLineId = 3,
                Cost = 6.27,
                Quantity = 3,
                
            });

            item1.MergeItem(item2);
            Console.WriteLine(item1.GetTotal());
        }

        private static void CloneItem()
        {
            var item = new Items();

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 1,
                Cost = 6.99,
                Quantity = 1,

            });

            item.AddItemLine(new ItemsLine()
            {
                ItemLineId = 2,
                Cost = 6.27,
                Quantity = 3,

            });

            var clonedItem = item.Clone();
            Console.WriteLine(clonedItem.GetTotal());
        }

        private static void ItemToString()
        {
            var item = new Items()
            {
                ItemDate = DateTime.Now,
                ItemNumber = 1000,
                ItemsLines = new List<ItemsLine>()
                {
                    new ItemsLine()
                    {
                        ItemLineId = 1,
                        Cost = 6.99,
                        Quantity = 1,

                    }
                }
            };

            Console.WriteLine(item.ToString());
        }

    }

Что я уже пробовал:

Я немного запутался , как объединить, клонировать и элемент в Tostring

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