String Manipulator

using System;
using System.Collections.Generic;
using System.Text;

namespace String_Manipulator
{
    class Program
    {
        static void Main(string[] args)
        {
            string sb = "";

            while (true)
            {
                string text = Console.ReadLine();

                if (text == "End")
                {
                    break;
                }
                else
                {
                    if (text == "Print")
                    {
                        Console.WriteLine(sb);
                    }
                    else
                    {
                        string[] data = text.Split(' ');

                        string command = data[0];

                        if (command == "Add")
                        {
                            string currentString = data[1];
                            sb += currentString;
                        }
                        else if (command == "Upgrade")
                        {
                            char currentSymbol = char.Parse(data[1]);

                            sb = sb.Replace(currentSymbol, (char)(currentSymbol + 1));

                        }
                        else if (command == "Index")
                        {
                            char currentSymbol = char.Parse(data[1]);

                            bool isThereLookingSymbol = false;

                            for (int i = 0; i < sb.Length; i++)
                            {
                                char currentChar = sb[i];

                                if (currentSymbol == currentChar)
                                {
                                    Console.Write($"{i} ");
                                    isThereLookingSymbol = true;
                                }
                            }

                            Console.WriteLine();

                            if (isThereLookingSymbol == false)
                            {
                                Console.WriteLine("None");
                            }
                        }
                        else if (command == "Remove")
                        {
                            string stringForRemoving = data[1];

                            while (true)
                            {
                                if (sb.Contains(stringForRemoving))
                                {
                                   sb = sb.Replace(stringForRemoving, "");
                                }
                                else
                                {
                                    break;
                                }
                            }
                        }
                    }
                }
            }
        }
    }
}