StackOverflow 36315788

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Xml.Serialization; // Had to add this
using Rhyous.EasyXml; // Added this from NuGet package

namespace XmlAddIncrementWithMaxId
{
    [XmlRoot("tabWyt")]
    public class TabWyt : List<Table1>
    {
        public int GetNextLp()
        {
            return Count > 0 ? this.Max(o => o.Lp) + 1 : 0;
        }

        new public void Add(Table1 t)
        {
            t.Lp = Math.Max(GetNextLp(), t.Lp);
            base.Add(t);
        }
    }

    public class Table1
    {
        [XmlElement("LP")]
        public int Lp { get; set; }
        [XmlElement("BLOCK")]
        public string Block { get; set; }
        [XmlElement("ARBPL")]
        public string Arbpl { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var myTabWyt = Serializer.DeserializeFromXml<TabWyt>(ref Xml);
            var addedTable1 = new Table1 { Block = "ABC", Arbpl = "XYZ01" };
            myTabWyt.Add(addedTable1);
            var newXmlString = new Xml(Serializer.SerializeToXml(myTabWyt));

            Console.WriteLine(newXmlString.PrettyXml);
        }

        private static string Xml = @"<?xml version=""1.0"" standalone=""yes""?>
<tabWyt>
  <Table1>
    <LP>3818326</LP>
  </Table1>
  <Table1>
    <LP>3818327</LP>
    <BLOCK>CASH</BLOCK>
    <ARBPL>JMSMF01</ARBPL>
  </Table1>
  <Table1>
    <LP>3818328</LP>
    <BLOCK>CACH</BLOCK>
    <ARBPL>JMSMF01</ARBPL>
     </Table1>
  <Table1>
    <LP>3818329</LP>
    <BLOCK>CACH</BLOCK>
    <ARBPL>JMSMF01</ARBPL>
  </Table1>
  <Table1>
    <LP>3818330</LP>
    <BLOCK>CACH</BLOCK>
    <ARBPL>JMSMF01</ARBPL>
  </Table1>
</tabWyt>";
    }
}