using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var input = new List<Datatype>

        {

            new Datatype

            {

                ConnectionType = ConnectionType.COM,

                ComPort = "COM1",

                TcpPort = null,

                ConnectionName = "CONN1"

            },

            new Datatype

            {

                ConnectionType = ConnectionType.COM,

                ComPort = "COM1",

                TcpPort = null,

                ConnectionName = "CONN2"

            },

            new Datatype

            {

                ConnectionType = ConnectionType.COM,

                ComPort = "COM2",

                TcpPort = null,

                ConnectionName = "CONN3",

            },

            new Datatype

            {

                ConnectionType = ConnectionType.IP,

                ComPort = null,

                TcpPort = "1234",

                ConnectionName = "CONN4"

            },

            new Datatype

            {

                ConnectionType = ConnectionType.IP,

                ComPort = null,

                TcpPort = "1234",

                ConnectionName = "CONN5"

            },

            new Datatype

            {

                ConnectionType = ConnectionType.IP,

                ComPort = null,

                TcpPort = "4321",

                ConnectionName = "CONN6"

            }

        };

        var result = input.GroupBy(x => x.ConnectionType == ConnectionType.IP ? x.TcpPort : x.ComPort).Where(x => x.Count() > 1).Select(x => x.Key + "\n" + string.Join("\n",x.Select(y => "\t" + y.ConnectionName)));

        Console.WriteLine(string.Join("\n", result));

    }

}

​

public class Datatype

{

    public ConnectionType ConnectionType {get;set;}

    public string ComPort {get;set;}

    public string TcpPort {get;set;}

    public string ConnectionName {get;set;}

}

​

public enum ConnectionType { COM, IP }