using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    class Posting

    {

        public int TransactionId { get; set; }

        public int PostingState { get; set; }

        public DateTime CreateDate { get; set; }

        public int CreateUser { get; set; }

    }

​

    class Transaction

    {

        public int TransactionId { get; set; }

        public int PostingCount { get; set; }

        public int TransactionState { get; set; }

        public DateTime CreateDate { get; set; }

        public int CreateUser { get; set; }

    }

    public static void Main()

    {

        List<Posting> postings = new List<Posting>

            {

                new Posting{ TransactionId = 1 },

                new Posting{ TransactionId = 1 },

                new Posting{ TransactionId = 2 },

                new Posting{ TransactionId = 2 },

​

            };

​

            var transactions = postings

            .GroupBy(p => p.TransactionId, (key, g) => new { TransactionId = key, Postings = g })

            .Select(x => new Transaction

            {

                TransactionId = x.TransactionId,

                PostingCount = x.Postings.Count(),

                TransactionState = x.Postings.First().PostingState,

                CreateDate = x.Postings.First().CreateDate,

                CreateUser = x.Postings.First().CreateUser

            });

​

            Console.WriteLine(transactions.Count());

    }

}