using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

​

public class Program

{

 public static void Main(string[] args)

        {

          //  var firstId = new Guid("44905b64-6e66-4814-9cc7-68c3af684d6a");

         //   var secondId = new Guid("3d45ee1e-6fb8-40a7-bb88-b661e6c8dc8b");

​

            var connectionId =GetConcatenatedId("16c396b7-78e3-46a9-bda2-3570d44e451a","f2377e1b-75c0-43ed-bd66-f2525adbd1be");

            var connectionId2 =GetConcatenatedId("16c396b7-78e3-46a9-bda2-3570d44e451a/f2377e1b-75c0-43ed-bd66-f2525adbd1be");

            var parsed = Parse("8cO-Dw1Mu0C-T8TIdkTTu-MO7wkxgt5LsUlmpSOc7cY");

            var parsedFull = ParseFull("8cO-Dw1Mu0C-T8TIdkTTu-MO7wkxgt5LsUlmpSOc7cY");

​

            Console.WriteLine("concated " + connectionId);

            Console.WriteLine("concated from full " + connectionId2);

            Console.WriteLine("parsed " + parsed);

            Console.WriteLine("parsed full line " + parsedFull);

        }

        static string GetConcatenatedId(string fullString)

            {

            var items = fullString.Split('/');

            return Convert.ToBase64String(

                Guid.Parse(items[0]).ToByteArray()

                    .Concat(Guid.Parse(items[1]).ToByteArray()).ToArray())

                        .Replace('+', '-').Replace('/', '_').TrimEnd('=');

            }

        static string GetConcatenatedId(string parentItemId, string itemId)

            {

            return Convert.ToBase64String(

                Guid.Parse(parentItemId).ToByteArray()

                    .Concat(Guid.Parse(itemId).ToByteArray()).ToArray())

                        .Replace('+', '-').Replace('/', '_').TrimEnd('=');

            }

        static (Guid first, Guid second) Parse(string concatenatedId)

            {

            var decodedConcatenatedId = concatenatedId.Replace('-', '+').Replace('_', '/');

            switch(decodedConcatenatedId.Length % 4) {

                    case 2: decodedConcatenatedId += "=="; break;

                    case 3: decodedConcatenatedId += "="; break;

                }

​

            var byteArray = Convert.FromBase64String(decodedConcatenatedId);

            var first = new Guid(byteArray.Take(16).ToArray());

            var second = new Guid(byteArray.Skip(16).Take(16).ToArray());

​

            return (first, second);

            }

        static string ParseFull (string concatenatedId)

            {

            var (first,second) = Parse(concatenatedId);

​

            return first.ToString()+"/"+ second.ToString();

            }

    }

​