csharp zero to hero week 3

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;




namespace Lesson_3
{
    class MyProgram
    {

        static bool validInput;
        static int userSelection;
        static string[] userOptions = new string[3];
        static string file = @".\Users.txt";
        static public void Main(string[] args)
        {
            userOptions[0] = "Login";
            userOptions[1] = "Register";
            userOptions[2] = "Exit";

            Console.WriteLine("Grettings user! Welcome to the week 3 lesson exercise.");

            Start();

        }

        static void Login()
        {
            //Stores user input and checks against filestream and grants access
            string userName;
            string password;
            int index = 0;
            List<string> lines = new List<string>();
            StreamReader reader;

            //Check if File exists and catches error if it doesn't
            try
            {
                reader = new StreamReader(file);
                reader.Close();
            }

            catch (System.IO.FileNotFoundException)
            {
                Console.WriteLine("Authentication credentials unavailable. Contact your system administrator for assistance. Program will now exit.");
                Console.ReadLine();
                Exit();
            }

            //Stores File Stream contents and displays it for testing. Remove foreach loop before deployment.
            reader = new StreamReader(file);
            string contents = reader.ReadToEnd();
            string[] contentsArray = contents.Split(new string[] { "\r" }, StringSplitOptions.None);
            reader.Close();

            for (int i = 0; i < contentsArray.Length; i++)
            {
                contentsArray[i] = contentsArray[i].Trim();
            }
            foreach (var line in contentsArray)
            {
                Console.WriteLine(line);
            }

            //Records username and confirms it exists
            Console.WriteLine("Hello user. Enter your Username and press enter. . .");
            userName = $"username:{Console.ReadLine()}";

                if (contentsArray.Contains(userName))
                {
                    Console.WriteLine("User account located.");
                }
                else
                {
                    Console.WriteLine("User name not found. please try again or use the register option to create a new account.");
                    Start();
                }


            //Locates password for username and asks for user to enter
            int v = Array.IndexOf(contentsArray, userName) + 1;
            index = v;
            Console.WriteLine(v);
            Console.WriteLine($"Please enter your password and press enter. . .");
            password = $"password:{Console.ReadLine()}";

            if (contentsArray[v] == password)
            {
                Console.WriteLine("Access granted.");
            }
            else
            {
                Console.WriteLine("Password incorrect. Please try again or use the register option to create a new account");
            }


            Console.ReadLine();
            Start();

        }

        //User landing interface. Gives user 3 options to choose from.
        static int Start()
        {
            Console.WriteLine("Please choose one of the following options. . .");
            foreach (var option in userOptions)
            {
                Console.WriteLine($"{Array.IndexOf(userOptions, option) + 1}). {option}");
            }

            validInput = int.TryParse(Console.ReadLine(), out int output);

            if ((validInput == true) && (output > 0) && (output < 4))
            {
                output--;
                userSelection = output;
                switch (userOptions[userSelection])
                {
                    case "Login":
                        Login();
                        break;
                    case "Register":
                        Register();
                        break;
                    case "Exit":
                        Exit();
                        break;
                }
            }

            else
            {
                Console.WriteLine("Invalid Selection. Please try again.");
                Start();
            }

            return userSelection;



        }

        //Adds a new user account
        static void Register()
        {
            string userName;
            string password;

            List<string> lines = new List<string>();
            StreamReader reader;
            try
            {
                reader = new StreamReader(file);
                reader.Close();
            }

            catch (System.IO.FileNotFoundException)
            {
                Console.WriteLine("Authentication credentials unavailable. Contact your system administrator for assistance. Program will now exit.");
                Console.ReadLine();
                Exit();
            }

            reader = new StreamReader(file);
            string contents = reader.ReadToEnd();
            string[] contentsArray = contents.Split(new string[] { "\r" }, StringSplitOptions.None);
            reader.Close();

            for (int i = 0; i < contentsArray.Length; i++)
            {
                contentsArray[i] = contentsArray[i].Trim();
            }

            //displays user account info for debuging. remove foreach loops before deployment
            foreach (var line in contentsArray)
            {
                Console.WriteLine(line);
            }

            Console.WriteLine("Enter your desired Username and press enter. . .");
            userName = $"username:{Console.ReadLine()}";

            // checks if user already exists
            if (contentsArray.Contains(userName))
            {
                Console.WriteLine("User account already exists. Press enter and try again");
                Console.ReadLine();
                Register();
            }


            Console.WriteLine($"Please enter your desired password.");
            password = $"password:{Console.ReadLine()}";

            //writes new user account into file.
            using (StreamWriter writer = new StreamWriter(file))
            {
                foreach (var line in contentsArray)
                {
                    writer.WriteLine(line);
                }
                writer.WriteLine(userName);
                writer.WriteLine(password);
            }

            Console.WriteLine("Account created! Press enter to return to main menu.");
            Console.ReadLine();
            Start();
        }

        static void Exit()
        {
            Console.WriteLine("Goodbye!");
            Console.ReadLine();
            Environment.Exit(0);
        }
    }
}