c# datagrid checkbox multi delete

I have problem when I delete multichecked rows with this button. It only delete first row. cell_dellklient is the name of checkbox in dataGridView1 yes is the true value of checkbox

Please help

private void button2_Click(object sender, EventArgs e) { foreach (DataGridViewRow row in dataGridView1.Rows) { con.Open();

                object cell = row.Cells["cell_delklient"].Value;
                if (cell == "yes")
                {
                    SqlCommand cmd = new SqlCommand("Delete From Klienci where Nazwa ='" + row.Cells[0].Value.ToString() + "'", con);
                    cmd.ExecuteNonQuery();
                    con.Close();
                    wyczytywaniegridu();
                }
                con.Close();
            }