Form1

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 WindowsFormsApp3
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            CreateUsers();
            InitializeComponent();

        }
        AdminPanel _AdminPanel;
        UserPanel _UserPanel;

        List<User> users = new List<User>();

        private void CreateUsers()
        {

            for (int i =0; i<7; i++)
            {
                User x = new User();
                x.user = "user" + i.ToString();
                x.password = "1234";
                x.isAdmin = false;
                users.Add(x);
            }
            for (int i = 0; i < 3; i++)
            {
                User x = new User();
                x.user = "admin" + i.ToString();
                x.password = "1234";
                x.isAdmin = true;
                users.Add(x);
            }
        }


        private void LoginButton_Click(object sender, EventArgs e)
        {
            if(txt_login.Text.Length == 0)
            {
                txt_login.Focus();
            }

            if(txt_password.Text.Length == 0)
            {
                txt_password.Focus();
            }
            Auth(txt_login.Text, txt_password.Text);
        }

        private void CloseButton_Click(object sender, EventArgs e)
        {
            Close();
        }

        private void Auth(string user, string password)
        {
            foreach(User y in users)
            {
                if(user== y.user && y.passiscorrect(password) == true && y.isAdmin == true)
                {
                    if ((_AdminPanel = (AdminPanel)checkifwinopen(typeof(AdminPanel))) == null)
                    {
                        _AdminPanel = new AdminPanel();
                        _AdminPanel.Show();
                    }
                    else _AdminPanel.BringToFront();
                    break;
                }

                if (user == y.user && y.passiscorrect(password) == true && y.isAdmin == false)
                {
                    if ((_UserPanel = (UserPanel)checkifwinopen(typeof(UserPanel))) == null)
                    {
                        _UserPanel = new UserPanel();
                        _UserPanel.Show();
                    }
                    else _UserPanel.BringToFront();
                    break;
                }
            }

        }
        private Form checkifwinopen(Type FormType)
        {
            foreach(Form OpenForm in Application.OpenForms)
            {
                if (OpenForm.GetType() == FormType) return OpenForm;
            }
            return null;
        }
    }

}