Fullcode

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Premium_Manager
{
    public partial class Form4 : Form
    {

        public string username, password;
        public Form4()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.Hide();
            Form5 f5 = new Form5();
            f5.Show();
        }

        private void button2_Click(object sender, EventArgs e)
        {
            try
            {
                var sr = new System.IO.StreamReader("C:\\" + textBox1.Text + "\\login.pmid");
                username = sr.ReadLine();
                password = sr.ReadLine();
                sr.Close();

                if (username == textBox1.Text && password == textBox2.Text)
                {
                    MessageBox.Show("Your are now successfully logged in!", "Login Success", MessageBoxButtons.OK, MessageBoxIcon.Information);
                    this.Hide();
                    Form1 f1 = new Form1();
                    f1.ShowDialog();
                }
                else
                {
                    MessageBox.Show("Username or password is wrong!", "Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }

            }
            catch (System.IO.DirectoryNotFoundException)
            {
                MessageBox.Show("The user doesn't exist", "Error!", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }

        }

        private void Form4_Load(object sender, EventArgs e)
        {

        }
    }
}