How to change this code to receive only one msgbox?

I have datagridview column with checkboxes. I want to check if there is a selected Row in this column. If its not I want to display msgbox. I can check columns by this but i'm receiving multiple msgboxes for every unchecked row. How to change it to receive only one msgbox?

Thanks

foreach (DataGridViewRow row in dataGridView1.Rows)
        {
            DataGridViewCheckBoxCell cell = row.Cells[5] as DataGridViewCheckBoxCell;

            //We don't want a null exception!
            if (cell.Value == null)
            {
                MessageBox.Show("Nie zaznaczono pola!");
            }


        }