Linq: group by and string.join, C#

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

namespace Rextester
{
    public class UserClient
    {
        public string UserName { get; set; }
        public string ClientName { get; set; }
    }
    public class Program
    {
        public static void Main(string[] args)
        {
            List<UserClient> list = new List<UserClient> (){
                new UserClient(){
                UserName = "User1",
                    ClientName = "Client1"
                },
                    new UserClient(){
                UserName = "User1",
                    ClientName = "Client2"
                },
                    new UserClient(){
                UserName = "User2",
                    ClientName = "Client1"
                },
            };
            
            var userClients = from c in list
              group c by c.UserName into u 
              select new {
                  UserName = u.First().UserName,
                  ClientName = string.Join(",", (from n in u select n.ClientName).ToArray()) 
              };
            
            foreach(var item in userClients)
            Console.WriteLine(item);
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,2 sec, absolute running time: 0,14 sec, cpu time: 0,12 sec, average memory usage: 16 Mb, average nr of threads: 3, absolute service time: 0,39 sec