在DataGridView中增加合計行，可使用下面的程式碼：

DataGridViewRow row = (DataGridViewRow)dgv.Rows[0].Clone();
var index = dgv.Rows.Add();
dgv.Rows[index].Cells["ID_"].Value = 0;
dgv.Rows[index].Cells[colName].Value = amount;
dgv.Rows[index].ReadOnly = true;
dgv.Rows[index].DefaultCellStyle.BackColor = Color.LightGray;

DataRow row = dt.NewRow();
row["ID_"] = 0;
row[txtName] = "合計";
row[colName] = amount;
dt.Rows.Add(row.ItemArray);
dt.AcceptChanges();
dgv.DataSource = dt;
dgv.Rows[dgv.Rows.Count - 1].ReadOnly = true;
dgv.Rows[dgv.Rows.Count - 1].DefaultCellStyle.BackColor = Color.LightGray;