using System;

namespace LoginFunction
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("How many users will you register? ");

            // You can move this declaration to the " Convert.ToInt32(Console.ReadLine());" line instead. There's no need to have it initialised before that.
            int users;

            // isFound is never used
            bool loop = true, isFound = false;

            // These vars are only used in the for-loop and should be declared there.
            string[] username, password;

            // No need to declare this here. It's only used in the while loop and should be declared in that scope.
            string tempusername, temppassword;

            // What happens if I try to register 11 users? You should move this to after you ask how many users you want and use that number for the index.
            // The names are bad because they should be plurar. It's usernames and passwords.
            username = new string[10];
            password = new string[10];

            // This will fail if you try and give it anything but a valid number. -1 is a valid number and will break your code.
            users = Convert.ToInt32(Console.ReadLine());

            for (int x = 0; x < users; x++)
            {
                Console.WriteLine("Please enter the user's name: ");
                username[x] = Console.ReadLine();

                Console.WriteLine("Please enter the user's password:");
                password[x] = Console.ReadLine();

                Console.WriteLine("User {0} has been registered at index of {1}", username[x], x);
            }

            Console.WriteLine("Please test a few of the user accounts?");
            // The users > 0 should be an if-statement but you should just not allow for this situation by checking the amount of users earlier on.
            while (users > 0 && loop)
            {
                Console.WriteLine("Enter the user's name:");
                tempusername = Console.ReadLine();
                Console.WriteLine("Enter the user's password:");
                temppassword = Console.ReadLine();

                // See how calling your arrays username and password is confusing now?
                if (Program.DoubleArrayChecker(username, password, tempusername, temppassword))
                {
                    Console.WriteLine("Pass correct");
                }
                else
                {
                    Console.WriteLine("Pass incorrect");
                }

                Console.WriteLine("try again? 1 - Yes, 2 - No");
                // Why recycle the temppassword var for this? This is a very bad practice!
                temppassword = Console.ReadLine();

                // The spacing between your ifs is confusing.
                if (temppassword == "2")
                {
                    loop = false;
                }

                else if (temppassword == "1")
                {
                    // Loop is true at this point regardless.
                    loop = true;
                }

                else
                {
                    // Why is the default to break?
                    loop = false;
                }
            }
        }

        public static bool DoubleArrayChecker(string[] users, string[]passes, string username, string password)
        {
            var index = 0;
            // I check this because if you give two inequal arrays the code would fail when one array runs out of values before the other.
            // Substract 1 because arrays start at 0.
            var smallestArray = Math.Min(users.Length, passes.Length) - 1;

            do
            {
                if (users[index] == username && passes[index] == password)
                {
                    return true;
                }

                index++;

            } while (index <= smallestArray);

            return false;
        }
    }
}