if (e.Exception.Message == "DataGridViewComboBoxCell value is not valid.")
            {
                object value = dataGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Value;
                if (!((DataGridViewComboBoxColumn)dataGridView1.Columns[e.ColumnIndex]).Items.Contains(value))
                {
                    ((DataGridViewComboBoxColumn)dataGridView1.Columns[e.ColumnIndex]).Items.Add(value);
                    e.ThrowException = false;
                }
            }