using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Payment> A = new List<Payment>

            {

                new Payment { Period =1, Balance = 10 },

                new Payment { Period =2, Balance = 12 },

                new Payment { Period =3, Balance = 45 },

                new Payment { Period =4, Balance = 23 },

                new Payment { Period =5, Balance = 36 },

                new Payment { Period =6, Balance = 45 }

            };

​

            List<Payment> B = new List<Payment>

            {

                new Payment { Period =1, Balance = 16 },

                new Payment { Period =2, Balance = 13 },

                new Payment { Period =3, Balance = 44 },

                new Payment { Period =4, Balance = 33 },

                new Payment { Period =5, Balance = 34 },

                new Payment { Period =6, Balance = 35 }

            };

​

            List<Payment> result = A.Concat(B).GroupBy(x => x.Period)

                                    .Select(x => new Payment

                                    {

                                        Period = x.Key,

                                        Balance = x.Sum(z => z.Balance)

                                    }).ToList();

​

            List<Payment> result1 = A.Zip(B, (first, second) => new Payment

                                                                 {

                                                                     Period = first.Period,

                                                                     Balance = first.Balance + second.Balance

                                                                 }).ToList();

​

            foreach (var item in result1)

            {

                Console.WriteLine("Period : {0}", item.Period);

                Console.WriteLine("Balance : {0}", item.Balance);

                Console.WriteLine();

            }

    }

}

​

​

public class Payment

    {

        public int Period { get; set; }

        public decimal Balance { get; set; }

    }