Почему не работает проблема заключается в обновлении оператора из строки

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<pre>private void prijavaAction()
        {
            if (textBox1.Text.All(char.IsDigit))

            {
                string queryString = "SELECT [ime_prezime]  FROM dbo.radnici WHERE bar_kod='" + textBox1.Text + "'";// pronaci radnika u bazi
                using (SqlConnection connection = new SqlConnection(cs))
                {
                    SqlCommand command = new SqlCommand(queryString, connection);
                    connection.Open();
                    SqlDataReader reader = command.ExecuteReader();
                    try
                    {
                        if (reader.Read())
                        {
                            notificationPanel.BackColor = System.Drawing.ColorTranslator.FromHtml("#FFE11A");
                            messageLabel.Text = (reader["ime_prezime"].ToString()) + "  je prijavljen u \n " + DateTime.Now.ToString("dd.MM.yyyy HH:mm:ss");
                            readerresultTextbox.Text = (reader["ime_prezime"].ToString());
                         }
                        else
                        {
                            notificationPanel.BackColor = System.Drawing.ColorTranslator.FromHtml("#FD7400");
                            messageLabel.Text = "Radnik ne postoji u bazi! \nProverite vašu karticu ili probajte ponovo!";
                            textBox1.Text = "";
                        }
                    }
                    finally
                    {

                        string prijava = "SELECT count (*) from dbo.prijava_radnika where vrijemeprijave is not null and vrijemeodjave is null and brojprijavnice2 is NULL and radnik=" + readerresultTextbox.Text;
                        SqlCommand provjeraprijaveradnika = new SqlCommand(prijava);
                        //con.Open();




                        if ((prijava != null)) //radi
                            //if ((prijava !))
                        {
                            String saveStaff = "INSERT INTO dbo.prijava_radnika (vrijemeprijave, status, radnik) VALUES(@vrijemeprijave,'prijavljen',@radnik)";


                            using (SqlConnection openCon2 = new SqlConnection(cs))
                            {

                                using (SqlCommand querySaveStaff = new SqlCommand(saveStaff))
                                {
                                    querySaveStaff.Connection = openCon2;
                                    querySaveStaff.Parameters.Add("@vrijemeprijave", SqlDbType.DateTime).Value = DateTime.Now;
                                    querySaveStaff.Parameters.Add("@radnik", SqlDbType.VarChar, 255).Value = readerresultTextbox.Text;
                                    openCon2.Open();
                                    querySaveStaff.ExecuteNonQuery();
                                    openCon2.Close();
                                }


                            }

                        }

                        else

                        {
                            String updateStaff = "UPDATE dbo.prijava_radnika SET vrijemeprijave=@vrijemeodjave, status='odjavljen' WHERE radnik=" + provjeraprijaveradnika.ToString();


                            using (SqlConnection openCon3 = new SqlConnection(cs))
                            {

                                using (SqlCommand queryupdateStaff = new SqlCommand(updateStaff))
                                {
                                    queryupdateStaff.Connection = openCon3;
                                    queryupdateStaff.Parameters.Add("@vrijemeodjave", SqlDbType.DateTime).Value = DateTime.Now;
                                    openCon3.Open();
                                    queryupdateStaff.ExecuteNonQuery();
                                    openCon3.Close();
                                }


                            }

                           

                        }

                    // Always call Close when done reading.
                    reader.Close();
                    }
                
                  }
            }
            else
            {
                notificationPanel.BackColor = System.Drawing.ColorTranslator.FromHtml("#FD7400");
                messageLabel.Text = "Bar kod nije važeci ili nije bar kod";
                textBox1.Text = "";


            }
        }

Что я уже пробовал:

Проблема в обновлении, вставка работает нормально. Необходимо найти в dbo.prijava_radnika столбец radnik из строки querystring и обновить, если он существует. если не существует, то вставьте

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