linq sort, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Conversation
    {
        public int userID;
        public string lastMessage;
        public List<Message> messages;
        public int messageCount;
    }
    
    public class Message
    {
        public string message;      
        public DateTime messageDate;           
    }
    
    public class Program
    {
        static List<Conversation> conversations=null;
        
        public static void AddConversation(int userID, string messageToAdd)
        {
            Conversation conversation=conversations.Where(u=>u.userID==userID).SingleOrDefault();
            
            System.Threading.Thread.Sleep(200);
            
            if(conversation==null)
            {
                conversation=new Conversation();
                conversation.userID=userID;
                conversation.messages=new List<Message>();            
                conversation.messages.Add(new Message {message=messageToAdd, messageDate=DateTime.Now.AddDays(new Random().Next(1,50))} ); 
                conversations.Add(conversation); 
            }
            else
            {
                conversation.messages.Add(new Message {message=messageToAdd, messageDate=DateTime.Now.AddDays(new Random().Next(1,50))} ); 
            }
        }
        
        public static void Main(string[] args)
        {
            
            conversations= new List<Conversation>();
            AddConversation(1,"message 1.1");          
            AddConversation(2,"message 2.1");
           
            AddConversation(2,"message 2.2");
            AddConversation(1,"message 1.2");
            AddConversation(1,"message 1.3");
            AddConversation(1,"message 1.4");
            AddConversation(1,"message 1.5");
           
            AddConversation(1,"message 1.6");
            AddConversation(1,"message 1.7");
            AddConversation(1,"message 1.8");
            AddConversation(1,"message 1.9");
            AddConversation(1,"message 1.10");
            
            conversations = conversations.OrderBy(c=>c.userID).Select(x=> new Conversation() {userID=x.userID,messageCount=x.messages.Count, lastMessage=x.messages.OrderByDescending(m=>m.messageDate).FirstOrDefault().message, messages=x.messages.OrderByDescending(m=>m.messageDate).ToList()}).ToList();
            foreach (var conversation in conversations)
             {      
                  Console.WriteLine("UserID# {0} \tTotal Messages# {1} \tLast Message # {2}", conversation.userID,conversation.messageCount, conversation.lastMessage);

foreach (var message in conversation.messages)
                  {
                      Console.WriteLine("\t\t{0} :\t{1}", message.messageDate,message.message);
                  }
             }
            
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,17 sec, absolute running time: 2,58 sec, cpu time: 0,11 sec, average memory usage: 24 Mb, average nr of threads: 5, absolute service time: 2,8 sec