dataset数据集，实现添加数据不能插入。请高人指点。
using System;

using System.Data;
using System.Data.Common;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Data.SqlClient;

public partial class _Default : System.Web.UI.Page 
{
   

  protected void Page_Load(object sender, EventArgs e)
  {

  }
  protected void Button1_Click(object sender, EventArgs e)
  {
  // SqlConnection conn = new SqlConnection("uid=sa;pwd=11;data source=.;database=gwdb");
  // SqlDataAdapter sda = new SqlDataAdapter("select * from users where uname=@uname and upwd=@upwd",conn);
  // sda.SelectCommand.Parameters.Add("@uname", SqlDbType.VarChar, 50, "uname");
  // //sda.SelectCommand.Parameters.Add("@upwd", SqlDbType.VarChar, 20, "upwd");
  // sda.SelectCommand.Parameters.Add("@upwd", "test");
  // sda.SelectCommand.Parameters["@uname"].Value = "test";
  //// sda.SelectCommand.Parameters["@upwd"].Value = "test";
  // DataSet ds = new DataSet();
  // sda.Fill(ds);
  // Response.Write(ds.Tables[0].Rows.Count);
   


  }
  protected void Button2_Click(object sender, EventArgs e)
  {
  DBHelper.ExecuteCommand("insert into users(uname,upwd) values('dd','dd')");
  }
  protected void Button3_Click(object sender, EventArgs e)
  {
  SqlParameter[] sql = new SqlParameter[2];
  sql[0] = new SqlParameter("@uname", SqlDbType.VarChar, 50, "uname");
  sql[1] = new SqlParameter("@upwd", SqlDbType.VarChar, 20, "upwd");
  sql[0].Value = "qq";
  sql[1].Value = "qq";
   
  DBHelper.ExecuteCommand("insert into users(uname,upwd) values(@uname,@upwd)",sql);
   
   

  }
  protected void Button4_Click(object sender, EventArgs e)
  {
  SqlDataAdapter sda = new SqlDataAdapter();
  SqlConnection conn = new SqlConnection("uid=sa;pwd=11;data source=.;database=gwdb");


  SqlCommand select = new SqlCommand("select * from users", conn);
  sda.SelectCommand = select;


  SqlCommand insert=new SqlCommand("insert into users(uname,upwd) values(@uname,@upwd)",conn);
  insert.Parameters.Add("@uname", SqlDbType.VarChar, 50, "uname");
  insert.Parameters.Add("@upwd", SqlDbType.VarChar, 20, "upwd");
  insert.Parameters["@uname"].Value = "dog";
  insert.Parameters["@upwd"].Value= "dog";
  sda.InsertCommand = insert;

  SqlCommand update = new SqlCommand("update users set upwd=@upwd",conn);
  update.Parameters.Add("@upwd", SqlDbType.VarChar, 20, "upwd");
  sda.UpdateCommand = update;

  SqlCommand delete = new SqlCommand("delete from users where uid=@id",conn);
  delete.Parameters.Add("@id", SqlDbType.Int, 4, "uid");
  sda.DeleteComm