using System;

using System.Collections.Generic;

using System.Xml;

using System.Xml.Serialization;

using System.IO;

public class Program

{

    public static void Main()

    {

        var partes = new Partes{

            new Parte{

                CodLista = 1,

                NroContratoInterno = "BA-300-2019",

                CUIT = 33333333333,

                Sucursal = new Sucursal{

                    CodLista = 5

                }

            }

        };

        var serializer =  new XmlSerializer(typeof(Partes));

        using(StringWriter textWriter = new StringWriter())

        {

            serializer.Serialize(textWriter, partes);

            Console.WriteLine(textWriter.ToString());

        }

    }

}

​

[XmlRootAttribute("Partes",IsNullable = false)]

public class Partes:List<Parte>{

​

}

​

public class Parte{

    [XmlAttribute("CodLista")]

    public int CodLista {get;set;}

    public string NroContratoInterno {get;set;}

    public long CUIT {get;set;} 

    public Sucursal Sucursal {get;set;}

}

​

public class Sucursal{

    [XmlAttribute("CodLista")]

    public int CodLista{get;set;}

}