Solution 1: https://www.reddit.com/r/csharp/comments/83dvub/can_someone_help_me_on_this_homework_i_have/

Program.cs

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

namespace LoginFunction
{
    class Program
    {
        static void Main(string[] args)
        {
            int numberOfUsers;

            Console.WriteLine("How many users will you register? ");

            // Makes sure the output is both valid and a number over zero. 
            while (!int.TryParse(Console.ReadLine(), out numberOfUsers) || numberOfUsers <= 0)
                Console.WriteLine("Enter a valid number");
            
            // Made it a dict for easier comparing
            // The key will be the username, the value will be the pass. In practice this will be a database record ID and a model with a collection of values
            var users = new Dictionary<string, string>();
           
            // Use the actual length of the user dictionary to better reflect the amount of users we have
            // as we sometimes skip a user if we already have them registered.
            while(users.Count < numberOfUsers)
            {
                // Ask for the inputs
                Console.WriteLine("Please enter the user's name: ");
                var username = Console.ReadLine();

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

                // Check if we already have a user with that name
                if (users.Keys.Contains(username))
                {
                    Console.WriteLine("User already exists. Skipping.");
                }
                else
                {
                    users.Add(username, password);
                    Console.WriteLine("User {0} has been registered.", username);
                }
            }
            
            Console.WriteLine("Please test a few of the user accounts?");

            // default value of bool is true
            bool keepChecking;

            // Showing off a different type of while loop. Doesn't add much value but it's cool for you to see anyways. 
            do
            {
                Console.WriteLine("Please enter the user's name: ");
                var username = Console.ReadLine();

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

                // Short-hand expression. 
                // value = expression ? valueIfTrue : ValueIfFalse
                // I use the return of theh check password function. 
                // For now we say the pass is wrong if we don't have the username stored. You could check that seperately. 
                Console.WriteLine(Program.CheckPassword(users, username, password) ? "Valid pass" : "Invalid pass");

                // Ask if we want to continue (uppercase value should be the default in console apps)
                Console.WriteLine("Keep checking? (Y:n)");
                // Hence we check if the value is "n" and only stop when it is instead of checking if the value is "Y"
                keepChecking = Console.ReadLine() != "n";
            } while (keepChecking);

        }

        public static bool CheckPassword(Dictionary<string, string> users, string username, string password)
        {
            // See if we have the user at all. You'll get a key error when this isn't the case and you try to read the password anyways. 
            if (!users.Keys.Contains(username))
            {
                return false;
            }

            // Check if the password matches
            if (users[username] == password)
            {
                return true;
            }

            // Always return false if we haven't hit the other cases.
            return false; 
        }
    }
}