C#使用access数据库对数据表进行update

private void Button12_Click(object sender, EventArgs e)
      {
          OleDbCommand aa = new OleDbCommand();
          OleDbConnection ab = new OleDbConnection();
          ab.ConnectionString = Data.mystr;
          ab.Open();
          if (TextBox26.Text == "")
          {
              MessageBox.Show("请输入需要修改的学号");
              ab.Close();
          }
          else
          {
            string ac = "update xxb set xm='" + TextBox25.Text.Trim() + "',mz='" + ComboBox8.Text.Trim() + "',xb='" + ComboBox7.Text.Trim() + "',dh='" + TextBox24.Text.Trim() + "',ss='" + TextBox23.Text.Trim() + "' ,sfzh='" + TextBox22.Text.Trim() + "' ,nj='" + TextBox21.Text.Trim() + "' ,zzmm='" + ComboBox6.Text.Trim() + "' ,zy='" + ComboBox5.Text.Trim() + "' ,bj='" + TextBox20.Text.Trim() + "' ,jtzz='" + TextBox19.Text.Trim() + "' ,yb='" + TextBox18.Text.Trim() + "' ,jzxm='" + TextBox17.Text.Trim() + "',jzdh='" + TextBox16.Text.Trim() + "' ,bz='" + TextBox15.Text.Trim() + "' where xh=" + TextBox26.Text.Trim();
            aa.Connection = ab;
            aa.CommandText = ac;
            aa.ExecuteNonQuery();
            int ni = aa.ExecuteNonQuery();
            if (ni > 0)
            {
                MessageBox.Show("修改成功");
            }
            else
            {
                MessageBox.Show("修改失败");
            }
          }
      }