using(SampleContext db = new SampleContext())
            {
                var user = db.Customers.Where(u => u.Name == textBox1.Text);
                foreach(Customer u in user)
                {
                    if(u.Password == textBox2.Text)
                    {
                        string lg = textBox1.Text;
                        katalog k = new katalog();
                        k.log = lg;
                        k.Show();
                        this.Hide();
                    }else
                    {
                        MessageBox.Show("Error");
                    }
                }
            }