using System;

using System.Text.RegularExpressions;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        List<Pond> ponds = new List<Pond>(){

                                            new Pond() {PondID = 1, OwnerName = "Ehsan" },

                                            new Pond() {PondID = 2, OwnerName = "Ahsan" },

                                            new Pond() {PondID = 3, OwnerName = "ShahZeb" }

                                       };

        List<Fish> fishes = new List<Fish>(){

                                            new Fish() {PondID = 1, FishName = "Star Fish" },

                                            new Fish() {PondID = 1, FishName = "Shark" },

                                            new Fish() {PondID = 2, FishName = "Star Fish" },

                                            new Fish() {PondID = 2, FishName = "Shark" },

                                            new Fish() {PondID = 3, FishName = "Dolphin" },

                                            new Fish() {PondID = 3, FishName = "Jelly Fish" }

                                       };

    var result = from p in ponds

                 join f in fishes on p.PondID equals f.PondID

                 group f by new { f.PondID,p.OwnerName } into g

                 select new { Owner = g.Key.OwnerName, Fishes = String.Join(",",g.Select(x=>x.FishName))};

    foreach(var item in result)

    {

     Console.WriteLine(String.Format("Owner : {0} and Fishses : {1}",item.Owner,item.Fishes));

    }

    }

}

​

public class Pond

    {

       public int PondID {get;set;}

       public string OwnerName {get;set;}

    }

public class Fish

    {

        public int PondID {get;set;}

        public string FishName {get;set;}

    }

​