How use custom serialization?

Question

Hello everybody i`m new to c#. Can you help me with understanding custom XML serialization.

Here is my code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Collections;
using System.Text;
using System.Runtime.Serialization.Formatters.Binary;
using System.Text.RegularExpressions;
using System.IO;
using System.Xml.Serialization;
using System.Runtime.Serialization;
using System.Security.Permissions;


namespace ConsoleApplication3
{   
    public class ShoppingCartItem : ISerializable
    {
        public int productID;
        public decimal price;
        public int quantity;
       
         [NonSerialized]
        public decimal total;

        public ShoppingCartItem():this(0,0,0){}      
        public ShoppingCartItem(int productID, decimal price, int quantity)
        {
            this.price = price;
            this.productID = productID;
            this.quantity = quantity;
            total = price * quantity;
        }
        protected ShoppingCartItem(SerializationInfo info, StreamingContext sc)
        {
            this.price = info.GetDecimal("Price");
            this.productID = info.GetInt32("Product ID");
            this.quantity = info.GetInt32("Quantity");           
        }

        //[SecurityPermissionAttribute(SecurityAction.Demand, SerializationFormatter = true)]
        public virtual void GetObjectData(SerializationInfo info, StreamingContext sc)
        {
            info.AddValue("Price", price);
            info.AddValue("Product ID", productID);
            info.AddValue("Quantity", quantity);           
        }
        [OnSerialized]
        public void Calc(StreamingContext sc)
        {
            total = price * quantity;
        }
        [OnDeserialized]
        public void Check(StreamingContext sc)
        {
            if ( total == 0 ) Calc(sc);
        }
    }

    class Program
    {
        public static void Main()
        {
            ShoppingCartItem s = new ShoppingCartItem(5510, 1236, 12);
            XmlSerializer xs = new XmlSerializer(typeof(ShoppingCartItem));
            FileStream fs = new FileStream("new.xml", FileMode.Create);            
            xs.Serialize(fs, s);
            fs.Close();
        }
    }
}

After serialization new.xml contains
<?xml version="1.0"?>
<ShoppingCartItem xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <productID>5510</productID>
  <price>1236</price>
  <quantity>12</quantity>
  <total>14832</total>
</ShoppingCartItem>

Why tags are not that which i had specified and why total is serialized?
Thanks.