用户代码未处理NullReferenceexception/OleDbException……问题

一
using System;
using System.Data;
using System.Configuration;
using System.Collections;
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 .OleDb ;

public partial class ReadNews : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        int newsid = Convert.ToInt32(Request.QueryString["id"].ToString());//出错…用户代码未处理NullReferenceexception 未将对象引用设置到对象的实例

        String strConnection = "Provider=Microsoft.Jet.OleDb.4.0;";
        strConnection +=@"Data Source=E:\Newweb\Database\News.mdb";
        OleDbConnection myconn = new OleDbConnection(strConnection);
        myconn.Open();
        String sql = "select A.Title,A.Content,A.AddTime,A.Author,A.ReadNumber, B.ClassName from News A,NewClass B where B.ClassID=A.ClassID and A.ID=" + newsid;
        OleDbCommand mycmd = new OleDbCommand(sql, myconn);
        OleDbDataReader dr = mycmd.ExecuteReader();//出错……………………用户代码未处理OleDbException 标准表达式中数据类型不匹配
        if (dr.Read())
        {
            lbl_class.Text = dr["ClassName"].ToString();
            lbl_content.Text = dr["Content"].ToString();
            lbl_title.Text = dr["Title"].ToString();
            lbl_info.Text = "作者:" + dr["Author"].ToString() + "   时间:" + dr["AddTime"].ToString() + "   阅读:" + dr["ReadNumber"].ToString() + "次";
        }
        int number = Convert.ToInt32(dr["ReadNumber"].ToString()) + 1;
        dr.Close();
        myconn.Close();
        UpdateNews(newsid,number);
    }
    protected void UpdateNews(int id, int n)
    {
        String strConnection = "Provider=Microsoft.Jet.OleDb.4.0;";
        strConnection += @"Data Source=E:\Newweb\Database\News.mdb";
        OleDbConnection myconn = new OleDbConnection(strConnection);
        myconn.Open();
        String sql = "Update News set ReadNumber=" + n + " where ID=" + id;
        OleDbCommand mycmd = new OleDbCommand(sql, myconn);
        mycmd.ExecuteNonQuery();
        myconn.Close();
    }
}

二
protected void BtnOK_Click(object sender, EventArgs e)
    {
        String action = Request.QueryString["Action"];
        String strConnection = "Provider=Microsoft.Jet.OleDb.4.0;";
        strConnection += @"Data Source=E:\Newweb\Database\News.mdb";
        OleDbConnection myconn = new OleDbConnection(strConnection);
        myconn.Open();

        if (action == "Add")
        {
            int id = ddl.SelectedIndex+1;
            String sql = "insert into News(Title,Author,ClassID,Content) values('" + TxtTitle.Text + "','" + TxtAuthor.Text  + "','" + id + "','" + TxtContent.Text + "')";
            //Response.Write(sql);
            OleDbCommand mycmd = new OleDbCommand(sql, myconn);
            OleDbDataReader result = mycmd.ExecuteReader();//有错…用户代码未处理OleDbException 索引或主关键字不能包含一个空（Null）值。
            result.Close();
            myconn.Close();
            Bind_Gridview();

        }
        else if (action == "Edit")
        {
            int  classid = Convert.ToInt32(ddl.SelectedValue) ;
            int id = Convert.ToInt32(Request.QueryString["ID"]);
           // Response.Write(id);
            String sql = "Update News set Title='" + TxtTitle.Text + "',Author='" + TxtAuthor.Text + "',ClassID=" + classid + ",Content='" + TxtContent.Text + "' where ID="+id;
            OleDbCommand mycmd = new OleDbCommand(sql, myconn);
            OleDbDataReader result = mycmd.ExecuteReader();
            result.Close();
            myconn.Close();
            Bind_Gridview();

        }
    }
出错在：
int newsid = Convert.ToInt32(Request.QueryString["id"].ToString());//出错…用户代码未处理
OleDbDataReader dr = mycmd.ExecuteReader();//出错……………………用户代码未处理OleDbException 标准表达式中数据类型不匹配
OleDbDataReader result = mycmd.ExecuteReader();//有错…用户代码未处理OleDbException 索引或主关键字不能包含一个空（Null）值。
急求帮助，万分感激！ --------------------编程问答-------------------- 先判断Request.QueryString["id"]存在不？ --------------------编程问答-------------------- 朋友，能不能再具体些呢 --------------------编程问答-------------------- int newsid = Convert.ToInt32(Request.QueryString["id"].ToString());//出错…用户代码未处理NullReferenceexception 未将对象引用设置到对象的实例

加判断
int newsid;
if(Request.QueryString["id"].ToString()==null)
{
//处理逻辑
}
else
{
newsid=Convert.ToInt32(Request.QueryString["id"].ToString());
} --------------------编程问答-------------------- int newsid;
if(Request.QueryString["id"]==null)
{
//处理逻辑
}
else
{
newsid=Convert.ToInt32(Request.QueryString["id"].ToString());
}

--------------------编程问答-------------------- 断点看一下newsid有值不? --------------------编程问答-------------------- 帮顶 --------------------编程问答-------------------- 1.
int newsid=0;
if(Request.QueryString["id"] != null)
{
   newsid = Convert.ToInt32(Request.QueryString["id"].ToString());
}
else
{

}
2.String action = Request.QueryString["Action"];也要同样处理:
  String action;
  if(Request.QueryString["Action"]!= null)
{
    action = Request.QueryString["Action"].ToString();
  }
else
{
}

int id = ddl.SelectedIndex+1; 也要判断一下ddl.SelectedIndex是不是存在?

断点判断一下,你的SQL语句到底是什么?//String sql ...就这句

补充：.NET技术 ,  C#