using System;
using System.Collections.Generic;
using System.Text;

namespace Common
{
    class Client
    {
        private int _id;
        public int Id { get; set; }

        private string _name;
        public string Name { get; set; }

        private Common.ClientType _type;
        public Common.ClientType Type { get; set; }

        private string _identificationNumber;
        public string IdentificationNumber { get; set; }

        private string _address;
        public string Address { get; set; }

        private string _tel;
        public string Tel { get; set; }

        private string _mob;
        public string Mob { get; set; }

        private string _email;
        public string Email { get; set; }

        private string _web;
        public string Web { get; set; }

        Client() { }
        Client(string name, string identNum,Common.ClientType type,string address,string tel,string mob,string email,string web)
        {
            Name = name;
            IdentificationNumber = identNum;
            Type = type;

            Address = address;
            Tel = tel;
            Email = email;
            Web = web;
        }
    }
    }
}

============================
common
using System;
using System.Collections.Generic;
using System.Text;

namespace Common
{
    class Common
    {

        public static long? currentUserId;
        public const string connectionString = @"Data Source=10.200.86.25;Initial Catalog=LT-3;Persist Security Info=True;User ID=LT-3;Password=***********";
        public enum ClientType
        {
            Individual=0,
            Organization=1
        }
    }
}

=============================
foorm 1
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 Banking
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void loginButton_Click(object sender, EventArgs e)
        {
            string mess;
            bool res = Common.User.CheckLogin(textBox1.Text, textBox2.Text, out mess);
            if (!String.IsNullOrEmpty(mess))
            {
                MessageBox.Show(mess);
                if (res)
                {
                    this.Hide();
                    HomePage fm = new HomePage();
                    fm.Closed += (s, args) => this.Close();
                    fm.Show();
                }
            }
        }
    }
}

==================
user
using System;

namespace Common
{
    public class User
    {
        private long _id;
        public long Id { get; set; } //get i set; se kratenki isto kako cel metod da se napise
        //get{return _id;} set{_id=value;}}

        private string _name;
        public string Name { get; set; }

        private string _username;
        public string Username { get; set; }

        private string _password;
        public string Password { get; set; }

        private long? _cashAccountId;
        public long? CashAccountId { get; set; } //zatoa sto se nadvoresni klucevi se defiiniraat vaka

        private long? _clientId;
        public long? ClientId { get { return _clientId; } set { _clientId = value; Client = new Client(); } }

        public User() { } // eden prazen konstruktor

        public User(string name, string username, string password, long? cashaccId, long? clientid)
        {  //eden parametriziran
            Name = name;
            Username = username;
            Password = password;
            CashAccountId = cashaccId;
            ClientId = clientid;
        }
        public static bool CheckLogin(string username,string password,out string message)
        {
            message = String.Empty;
            if(String.IsNullOrEmpty(username) || String.IsNullOrEmpty(password))
            {
                message = "Invalid username or password!";
                return false;
            }
            try
            {
                using (SqlConnection con = new SqlConnection(Common.connectionString))
                {
                    cmd.CommandType = CommandType.StoredProcedures;
                    cmd.Parameters.AddWithValue("@username", username);
                    cmd.Parameters.AddWithValue("@password", password);

                    SqlParameter param = new SqlParameter("@res", SqlDbType.Bit);
                    param.Direction = ParameterDirection.ReturnValue;
                    cmd.Parameters.Add(param);

                    cmd.Parameters.Add("@userId", SqlDbType.BigInt).Direction = ParameterDirection.Output;
                    con.Open();
                    cmd.ExecuteNonQuery();

                    object res = param.Value;

                    if ((int)res == 1)
                    {
                        long userId = Convert.ToInt64(cmd.Parameters["@userId"].Value);
                        Common.currentUserId = userId;
                        message = "Login successful";
                        return true;
                    }
                    else
                    {
                        message = "Login failed";
                        return false;
                    }
                }
            }
            catch(Exception e)
            {
                message = e.Message;
                return false;
            }
        }
        }
}