private void btnAdd_Click(object sender, EventArgs e)
        {
            int   idModel = Convert.ToInt32(cbModel.SelectedValue);
            Model model   = ModelList.getInstance().getItem(idModel);

            Grade_AddEdit aeG = new Grade_AddEdit(new Grade(model));

            if (aeG.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                loadGrade();
            }
        }
        private void _dgv_CellMouseDoubleClick(object sender, DataGridViewCellMouseEventArgs e)
        {
            if ((e.ColumnIndex < 0) || (e.RowIndex < 0))
            {
                return;
            }

            int idGrade = Convert.ToInt32(_dgv.Rows[e.RowIndex].Cells[0].Value);

            GradeList grades = GradeList.getInstance();
            Grade     grade  = grades.getItem(idGrade);

            Grade_AddEdit aeG = new Grade_AddEdit(grade);

            if (aeG.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                loadGrade();
            }
        }