private void btnDel_Click(object sender, EventArgs e) { GradeList grades = GradeList.getInstance(); int idGrade = 0; foreach (DataGridViewCell cell in _dgv.SelectedCells) { int.TryParse(_dgv.Rows[cell.RowIndex].Cells[0].Value.ToString(), out idGrade); grades.Delete(idGrade); } loadGrade(); }