SecretChat-ExamPreparation

using System;
using System.Text;

namespace SecretChat
{
    class Program
    {
        static void Main(string[] args)
        {
            string concealedMessage = Console.ReadLine();
            StringBuilder sbMessage = new StringBuilder(concealedMessage);
            string command = string.Empty;
            while ((command = Console.ReadLine()) != "Reveal")
            {
                string[] commandArray = command.Split(":|:");
                if (commandArray[0] == "InsertSpace")
                {
                    int index = int.Parse(commandArray[1]);
                    sbMessage.Insert(index, " ");
                    Console.WriteLine(sbMessage);
                }

                else if (commandArray[0] == "Reverse")
                {
                    string substring = commandArray[1];
                    if (sbMessage.ToString().Contains(substring))
                    {
                        int startIndex = sbMessage.ToString().IndexOf(substring);
                        int substringLength = substring.Length;
                        sbMessage.Remove(startIndex, substringLength);
                        string reverseMessage = string.Empty;
                        for (int i = substring.Length - 1; i >= 0; i--)
                        {
                            reverseMessage += substring[i];
                        }

                        sbMessage.Append(reverseMessage);
                        Console.WriteLine(sbMessage);
                    }

                    else
                    {
                        Console.WriteLine("error");
                    }
                }

                else if (commandArray[0] == "ChangeAll")
                {
                    string substring = commandArray[1];
                    string replacement = commandArray[2];
                    if (sbMessage.ToString().Contains(substring))
                    {
                       sbMessage.Replace(substring, replacement);
                        Console.WriteLine(sbMessage);
                    }
                }
            }

            Console.WriteLine($"You have a new text message: {sbMessage}");
        }
    }
}