unload or exit application

Ok Sir, i just change the close() method to Hide() on if the username and password is correct and it working and of course I add the code that you suggest . Thank you Sir. hehehehe

Sorry Sir, but when i try to login and then the password and username is correct so obviously the login form only will close but the whole application will all close. This is my code sir.

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 MySql.Data.MySqlClient;

namespace VetirenaryInformationSystem
{
    public partial class PasswordForm : Form
    {
        private string conn = "SERVER = localhost; DATABASE = veterinary_db; UID = root; PASSWORD = ;";
        public PasswordForm()
        {
            InitializeComponent();

        }


        private void button1_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void PasswordForm_Load(object sender, EventArgs e)
        {
            TXTusername.CharacterCasing = CharacterCasing.Upper;
            TXTpassword.CharacterCasing = CharacterCasing.Upper;
          
           
        }

        private void BTNLOGIN_Click(object sender, EventArgs e)
        {
            if (loging(TXTusername.Text, TXTpassword.Text) == true)
            {
                MessageBox.Show("Successfully login","Comfirm message",MessageBoxButtons.OK,MessageBoxIcon.Information);
                this.Close();
            }
            else
            {
                MessageBox.Show("Username and password not match. Please try again!","Error message",MessageBoxButtons.OK,MessageBoxIcon.Error);
                TXTpassword.Text = "";
                TXTusername.Text = "";
               
            }
           
 
        }

        public bool loging(string username, string password)
        {
            string login = "SELECT username, userPass from userrecord WHERE username = '" + TXTusername.Text + "' AND userPass ='" + TXTpassword.Text + "'";
            MySqlConnection connect = new MySqlConnection(conn);
            MySqlCommand cmd = new MySqlCommand("SELECT username, userPass from userrecord WHERE username = '" + username + "' AND userPass ='" + password + "'");
            cmd.Connection = connect;
            connect.Open();
            MySqlDataReader read = cmd.ExecuteReader();
            if (read.Read() != false)
            {
            if(read.IsDBNull(0) == true)
            {
                cmd.Connection.Close();
                read.Dispose();
                cmd.Dispose();
                return true;
            }
                else
            {
                cmd.Connection.Close();
                read.Dispose();
                cmd.Dispose();
                return true;
            }
          
            }
            else
            {
                return false;
            }
        }

        private void PasswordForm_FormClosed(object sender, FormClosedEventArgs e)
        {
                Application.Exit();

        }
       
    }
}

Thank you very much Sir. Its working.

When the user closes the login form, then its FormClosed event will fire.

So, if you add a handler for that (go to the form's Properties Box and double click on the FormClosed event) and insert Application.Exit() within the handler, then it should do what you want.

Sorry Sir, what i mean is if the user will close the login form or click the  X red button on the upper right (close) it should be the whole application will  close.

Thank you Sir.

If you call Application.Exit() from the login form, then it should close the whole application.