frmsignin.cs

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

namespace _2018012701_CS_FIO_signin_signup
{
    public partial class frmSignIn : Form
    {
        public frmSignIn()
        {
            InitializeComponent();
        }

        private void btnSignup_Click(object sender, EventArgs e)
        {
            this.Hide();
            frmSignup frmsignup = new frmSignup();
            frmsignup.Show();
        }

        private void btnSignin_Click(object sender, EventArgs e)
        {
            string sUname = txtUname.Text.ToString();
            string sPass = txtPass.Text.ToString();
            string sFname = sUname + "reg.txt";
            string sFilePass = "";
            if (File.Exists(sFname))
            {
                StreamReader sr = new StreamReader(sFname);
                sFilePass = sr.ReadLine();
                sr.Close();
                if (sPass == sFilePass)
                {
                    MessageBox.Show("password is correct");
                }
                else
                {
                    MessageBox.Show("wrong username or password");
                }
            }
            else
            {
                MessageBox.Show("wrong username or password");
            }
        }
    }
}

frmsignup.cs

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

namespace _2018012701_CS_FIO_signin_signup
{
    public partial class frmSignup : Form
    {
        public frmSignup()
        {
            InitializeComponent();
        }

        private void btnSignup_Click(object sender, EventArgs e)
        {
            string sUname = txtUname.Text.ToString();
            string sPass = txtPass.Text.ToString();
            string sComfirm = txtComfirm.Text.ToString();
            if ((sUname != "") && (sPass != "") && (sComfirm != ""))
            {
                string sFname = sUname + "reg.txt";
                if (!File.Exists(sFname))
                {
                    StreamWriter sw = new StreamWriter(sFname);
                    sw.WriteLine(sPass);
                    sw.Close();
                    this.Hide();
                    frmSignIn frmsignin = new frmSignIn();
                    frmsignin.Show();
                }
                else
                {
                    MessageBox.Show("username already exists, please choose another username");
                }

            }
            else
            {
                MessageBox.Show("Fill all data to sign up");
            }

        }
    }
}