using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var settlmentChannels = new List<RegistrySettlementChannel> {

            new RegistrySettlementChannel { ComponentSerialNumbers = "ABC", ChannelNumber = 1, RegisterContentCode = "UN" },

            new RegistrySettlementChannel { ComponentSerialNumbers = "ABC", ChannelNumber = 2, RegisterContentCode = "EG" },

            new RegistrySettlementChannel { ComponentSerialNumbers = "ABC", ChannelNumber = 3, RegisterContentCode = "UN" }

        };

        var meteringChannels = new List<IcpMeterChannel> {

            new IcpMeterChannel { Serial = "ABC", ContentCode = "UN", ChannelNumber = 1 },

            new IcpMeterChannel { Serial = "ABC", ContentCode = "CN", ChannelNumber = 2 },

            new IcpMeterChannel { Serial = "ABC", ContentCode = "CN", ChannelNumber = 3 }

        };

        var matches = from s in settlmentChannels

                      join m in meteringChannels on 

                        new {Serial = s.ComponentSerialNumbers, Channel = s.ChannelNumber, Content = s.RegisterContentCode} 

                        equals new {Serial = m.Serial, Channel = m.ChannelNumber, Content = m.ContentCode} 

                            into jointTable

                      from p in jointTable.DefaultIfEmpty()

                      select new { SettlementRegister = s.ComponentSerialNumbers, MeteringChannel = p == null ? "(No Matches)" : p.Serial };    

        foreach(var match in matches)

        {

            Console.WriteLine("{0} {1} ", match.SettlementRegister, match.MeteringChannel);

        }

    }

}

​

    public class RegistrySettlementChannel

    {

        public string ComponentSerialNumbers { get; set; }

        public int ChannelNumber { get; set; } 

        public string RegisterContentCode { get; set; }

    }

​

    public class IcpMeterChannel

    {

        public int ChannelNumber { get; set; } 

        public string Serial { get; set; } 

        public string ContentCode { get; set; } 

    }