var tmp = Enumerable.Range(0, 10000).Select(_ => Guid.NewGuid().ToString()).ToList();
            var count1 = tmp.Count;
            var count2 = tmp.Select(x => x.Length).Sum();
            var ch = new char[count2 + count1];
            int counter = 0;
            foreach (var x1 in tmp)
            {
                foreach (var x2 in x1)
                {
                    ch[counter++] = x2;
                }
                ch[counter++] = ',';
            }
            var str = new string(ch);

      static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();
            Random r = new Random();
            var data = Enumerable.Range(0, 100000).Select(_ => new string((char)r.Next(60, 100), r.Next(100, 1000))).ToList();
            sw.Restart();
            char[] chars = new char[data.Count + data.Select(x => x.Length).Sum()];
            List<Task> tList = new List<Task>();
            int begin = 0;
            
            foreach (var d in data)
            {
                int tmp = begin;
                var d1 = d;
                tList.Add(Task.Factory.StartNew(() => M1(chars, d1, tmp)));
                begin += d.Length + 1;
            }
            Task.WaitAll(tList.ToArray());
            var str = new string(chars);
            sw.Stop();
            Console.WriteLine(sw.ElapsedMilliseconds);
            sw.Start();
            str = string.Join(",", data);
            sw.Stop();
            Console.WriteLine(sw.ElapsedMilliseconds);
            Console.ReadLine();
        }
        static void M1(char[] chars, string st, int begin)
        {
            int i;
            for (i = begin; i < begin + st.Length; i++)
            {
                chars[i] = st[i - begin];
            }
            chars[i] = ',';
        }