Using System.IO.File, Checking if a text file contains an exact string and not a part of it

Question

So i am making a program that allows you to register and then login using console. Since a console can only store data inside of it as long as it is not closed(as far as I know) I make use of .txt files . I have written some code, when you are logging in it's supposed to check if someone else is already using the username you want but the problem is, for example if someones username is "Manly" and the next person wants their name to be "Man" it says the username already exists because "Man" exists inside (Man-"ly")"Manly" if you know what I'm saying.

This is a class inside my VS project.

using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace RegisterAndLogin
{
    class Register
    {
        public string UserName { get; set; }
        public string Password { get; set; }

        public void Registration()
        {
            Console.WriteLine("Hello user! Enter the username you want to register with.");
            UserName = Console.ReadLine();

            string UserNamesInFile = File.ReadAllText(@"C:\Users\KIRCA\Documents\C# with Denis Panjuta\Usernames.txt");
            if (UserNamesInFile.Contains(UserName))
            {
                Console.WriteLine("The username you entered is already being used. Try a new one.");
                while (UserNamesInFile.Contains(UserName))
                {
                    Console.WriteLine("Hello user! Enter the username you want to register with.");
                    UserName = Console.ReadLine();
                }
            }

            Console.WriteLine("Now enter the password you want to register with.");
            Password = Console.ReadLine();

            using (StreamWriter file = new StreamWriter(@"C:\Users\KIRCA\Documents\C# with Denis Panjuta\Usernames.txt", true))
            {
                file.WriteLine(UserName);
            }

            using (StreamWriter file1 = new StreamWriter(@"C:\Users\KIRCA\Documents\C# with Denis Panjuta\Passwords.txt", true))
            {
                file1.WriteLine(Password);
            }

            Console.WriteLine("You are succesfully registered");
            Console.WriteLine("-----------------------------------------------------------------------");   

            
        }
    }
}

I reccomend copy pasting the code into a net.framework, console c#, VS project and checking it out to understand what I'm saying.

Here is the rest of the code I have.

Class called Login:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;

namespace RegisterAndLogin
{
    class Login
    {
        public string LoginUserName { get; set; }
        public string LoginPassword { get; set; }
        public void LoginMethod()
        {
            Console.WriteLine("Hello again user! Enter your username.");
            LoginUserName = Console.ReadLine();

            string UserNameFileContents = File.ReadAllText(@"C:\Users\KIRCA\Documents\C# with Denis Panjuta\Usernames.txt");

            if (UserNameFileContents.Contains(LoginUserName))
            {
                Console.WriteLine("Username is correct. ");
            }
            else
            {
                Console.WriteLine("Username is incorrect! ");
            }

            Console.WriteLine("Now enter your password. ");
            LoginPassword = Console.ReadLine();

            string PasswordFileContents = File.ReadAllText(@"C:\Users\KIRCA\Documents\C# with Denis Panjuta\Passwords.txt");

            if (PasswordFileContents.Contains(LoginPassword))
            {
                Console.WriteLine("Password is also correct. Press any key to continue to homepage. ");
                Console.ReadLine();
            }
            else
            {
                Console.WriteLine("Password is incorrect. ");
            }
        }
    }
}

Code for Main program, program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace RegisterAndLogin
{
    class Program
    {
        static void Main(string[] args)
        {
            Register registration = new Register();
            Login login = new Login();
            Console.WriteLine("Enter 1 to register or enter 2 to login.");
            string UserNumberInput = Console.ReadLine();
            if(UserNumberInput == "1")
            {
                registration.Registration();

                Console.WriteLine("You may now login.");
                login.LoginMethod();
            }
            else if(UserNumberInput == "2")
            {
                login.LoginMethod();
            }
            else
            {
                Console.WriteLine("Enter 1 or 2 only.\nNumber or letter is invalid");
                while(UserNumberInput != "1" && UserNumberInput != "2")
                {
                    Console.WriteLine("Enter 1 to register or enter 2 to login.");
                    UserNumberInput = Console.ReadLine();
                    Console.WriteLine("--------------------------------------------------");
                }
                if (UserNumberInput == "1")
                {
                    registration.Registration();

                    Console.WriteLine("You may now login.");
                    login.LoginMethod();
                }
                else if (UserNumberInput == "2")
                {
                    login.LoginMethod();
                }
            }

            Console.ReadLine();
            
        }
    }
}```