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;
using System.Data.OleDb;

namespace vjezba_baza
{
    public partial class Form1 : Form
    {
        User odabraniUser;
        public Form1()
        {
            InitializeComponent();
        }
        OleDbConnection con = new OleDbConnection("Provider=OraOLEDB.Oracle; OLEDB.NET=True; DATA SOURCE = veza1; PERSIST SECURITY INFO=True;USER ID = MHRANJ; PASSWORD = DfhI51aB461");
        string userid;
        private void LoginBtn_Click(object sender, EventArgs e)
        {
            con.Open();
            if (UsernameText.Text != "")
            {

                string querry = "select * from SHOP_USER";
                OleDbDataAdapter adapter = new OleDbDataAdapter(querry, con);
                DataTable dt = new DataTable();
                adapter.Fill(dt);
                bool pristupi = false;
                foreach (DataRow row in dt.Rows)
                {
                    if (row[1].ToString() == UsernameText.Text)
                    {
                        userid = row[0].ToString();
                        odabraniUser = new User(userid);
                        pristupi = true;
                    }
                }
                if (pristupi)
                {
                    Form2 f2 = new Form2(odabraniUser);
                    f2.Show();
                }
                else MessageBox.Show("Invalid username");
            }
            else MessageBox.Show("Input username");
            con.Close();
        }
    }
}