using System;

using System.Collections.Generic;

using System.Linq;

       public class Summary {

            public int itemNo { get; set; }

            public string buyerNo { get; set; }

            public string sellerNo { get; set; }

​

            public string itemDesc { get; set; }

        }

public class Program

{

    public static void Main()

    { List<Summary> list = new List<Summary>()

                {

                    new Summary() {

                        itemNo =1,

                        buyerNo = "B1",

                        sellerNo ="S1",

                        itemDesc = "Item1"

                    }, new Summary() {

                        itemNo =2,

                        buyerNo = "B2",

                        sellerNo ="S2",

                        itemDesc = "Item2"

                    }, new Summary() {

                        itemNo =2,

                        buyerNo = "B3",

                        sellerNo ="S3",

                        itemDesc = "Item2"

                    }, new Summary() {

                        itemNo =3,

                        buyerNo = "B4",

                        sellerNo ="S4",

                        itemDesc = "Item3"

                    }, new Summary() {

                        itemNo =3,

                        buyerNo = "B5",

                        sellerNo ="S5",

                        itemDesc = "Item3"

                    },

                };

​

                var result = from summary in list

                group summary by new

                {

                    itemNo = summary.itemNo,

                    itemDesc = summary.itemDesc

​

                } into grp

                select new

                {

                    itemNo = grp.Key.itemNo,

                    buyerNo = string.Join(",", grp.Select(i => i.buyerNo)),

                    sellerNo = string.Join(",", grp.Select(i => i.sellerNo)),

                    itemDesc = grp.Key.itemDesc

                };

        foreach(var item in result)

            Console.WriteLine(string.Format("{0} {1} {2} {3}",item.itemNo,item.buyerNo,item.sellerNo,item.itemDesc));

    }

}