using static System.Console;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    private static List<ListaBid> _lista = new List<ListaBid>

    {

        new ListaBid { Partnumber = "A01", SaldoPartnumberSap = 10 },

        new ListaBid { Partnumber = "A02", SaldoPartnumberSap = 05 },

        new ListaBid { Partnumber = "A01", SaldoPartnumberSap = 11 },

        new ListaBid { Partnumber = "A02", SaldoPartnumberSap = 15 }

    };

    public static void Main()

    {

        var itensPedido = (from l in _lista

                           select new 

                           {

                               CodigoProduto = l.Partnumber,

                               SaldoPartnumberSap = l.SaldoPartnumberSap,

                               Saldo = l.Saldo,

                           }).ToList();

        var groupped = itensPedido.GroupBy(x => x.CodigoProduto)

                                  .Select(g => new 

                                          { 

                                              Chave = g.Key, 

                                              Itens = g.ToList(),

                                              Total = g.Count(),

                                              Soma = g.Sum(x => x.SaldoPartnumberSap),

                                          });

        foreach(var g in groupped)

        {

            WriteLine($"{g.Chave} - Total: {g.Total} - Soma: {g.Soma}");

        }

    }

}

​

class ListaBid 

{

    public string Partnumber { get; set; }

    public int SaldoPartnumberSap { get; set; }

    public int Saldo { get; set; }  

}