谁能帮我看下为什么更新不了数据库，DataTable数据已经确认了被修改，可是数据库里对应的数据却并没有被修

using System;using System.Collections.Generic;using System.ComponentModel;using System.Data;using System.Drawing;using System.Linq;using System.Text;using System.Windows.Forms;using System.Data.SqlClient;namespace 数据绑定{ public partial class Form1 : Form { private SqlConnection cn; private SqlDataAdapter da; private DataSet ds; public Form1() { InitializeComponent(); ds = new DataSet(); cn = new SqlConnection("Data Source=PC-201012280020;Initial Catalog=teaching;Integrated Security=True"); da = new SqlDataAdapter("select * from class", cn); da.Fill(ds, "class"); DataView dv = new DataView(ds.Tables["class"]); DataTable dt = ds.Tables["class"]; this.dataGridView1.DataSource = dt; } private void dataGridView1_CellValueChanged(object sender, DataGridViewCellEventArgs e) { //获得绑定的数据表 DataTable dt=(DataTable)this.dataGridView1.DataSource;//这里必须对应DataSource=DataTable才可以 int x=this.dataGridView1.CurrentCellAddress.X;//获取列标 int y = this.dataGridView1.CurrentCellAddress.Y;//获取行标 this.dataGridView2.DataSource = dt; dt.Rows[y][x] = this.dataGridView1.CurrentCell.Value.ToString(); //table.Rows[2][1]就是第3行第2列的单元格 //dr[1]该行的第2列 //构造命令 SqlCommandBuilder custDB = new SqlCommandBuilder(da); //更新数据库 da.Update(dt); //更新数据表 dt.AcceptChanges(); } }}