（急）为什么我的DropDownlist就是无法级联选择

谢谢，我现在已经可以实现级联选择了，但是选择几次后就出现以下错误该怎么办。
---------------------------------------------------------------------------------
未将对象引用设置到对象的实例。
说明: 执行当前 Web 请求期间，出现未处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。异常详细信息: System.NullReferenceException: 未将对象引用设置到对象的实例。源错误:
行 91： Cls.dbConnection=pagebase.constr;
行 92： // ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,name from kdata,teacher,kdatateacher,classdata WHERE kdata.kddataid=kdatateacher.kdataid and kdata.classdataid=classdata.classdataid and teacher.teacherid=kdatateacher.teacherid  and  kdata.classdataid=1"+jishuDown.SelectedItem.Value);
行 93： ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,teacher.name from kdata,teacher,kdatateacher,classdata WHERE kdata.kddataid=kdatateacher.kdataid and kdata.classdataid=classdata.classdataid and teacher.teacherid=kdatateacher.teacherid  and  kdata.classdataid="+jishuDown.SelectedItem.Value);
行 94：
行 95： //ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,name from kdata,teacher WHERE   classdataid=" +jishuDown.SelectedItem.Value);
源文件: d:\院系端辅助教学系统\开课管理系统（裘文彬）\kksystem（开课管理主程序）\paitext.aspx.cs    行: 93 堆栈跟踪:
[NullReferenceException: 未将对象引用设置到对象的实例。]
   kksystem.paitext.Page_Load(Object sender, EventArgs e) in d:\院系端辅助教学系统\开课管理系统（裘文彬）\kksystem（开课管理主程序）\paitext.aspx.cs:93
   System.Web.UI.Control.OnLoad(EventArgs e)
   System.Web.UI.Control.LoadRecursive()
   System.Web.UI.Page.ProcessRequestMain()
----------------------------------------------------------------------------------

接上面的源代码：
以下是源代码using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Web.Util ;
using System.Data.SqlClient;
using squishyWARE.WebComponents.squishyTREE;
using Microsoft.ApplicationBlocks.Data;
using System.Configuration;
namespace kksystem
{
/// <summary>
/// paitext 的摘要说明。
/// </summary>
public class paitext : System.Web.UI.Page
{
protected System.Web.UI.WebControls.DataGrid MyDataGrid2;
protected System.Web.UI.WebControls.Button Button1;
protected System.Web.UI.WebControls.DropDownList jishuDown;
protected System.Web.UI.WebControls.Label Label5;
protected System.Web.UI.WebControls.DropDownList zhyDown;
protected System.Web.UI.WebControls.Label Label4;
protected System.Web.UI.WebControls.Label Label3;
protected System.Web.UI.WebControls.DropDownList xiDown;
protected System.Web.UI.WebControls.DropDownList xueyDown;
protected System.Web.UI.WebControls.Label Label2; protected DataSet ds2;
protected SqlDataReader dxre2;

private void Page_Load(object sender, System.EventArgs e)
{//下拉框初始化
if(!this.IsPostBack)
{
Cxydata dx=new Cxydata();

dx.dbConnection=ConfigurationSettings.AppSettings["SQLConnectionString"];
dxre2=dx.GetDataReader("select * from xydata");
xueyDown.DataSource=dxre2;
xueyDown.DataTextField="name";
xueyDown.DataValueField="xydataid";
xueyDown.DataBind();
dxre2.Close();

if (xueyDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from xdata where xydataid="+xueyDown.SelectedItem.Value.ToString());
xiDown.DataSource=dxre2;
xiDown.DataTextField="name";
xiDown.DataValueField="xdataid";
xiDown.DataBind();
dxre2.Close();
}
if (xiDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from zydata where xdataid="+xiDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
zhyDown.DataTextField="name";
zhyDown.DataValueField="zydataid";
zhyDown.DataBind();
dxre2.Close();
} if (zhyDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from classdata where zydataid="+zhyDown.SelectedItem.Value.ToString());
jishuDown.DataSource=dxre2;
jishuDown.DataTextField="js";
jishuDown.DataValueField="classdataid";
jishuDown.DataBind();
dxre2.Close();
}

} //datagrid 初始化 Ckdata Cls=new Ckdata();
Cls.dbConnection=pagebase.constr;
// ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,name from kdata,teacher,kdatateacher,classdata WHERE kdata.kddataid=kdatateacher.kdataid and kdata.classdataid=classdata.classdataid and teacher.teacherid=kdatateacher.teacherid  and  kdata.classdataid=1"+jishuDown.SelectedItem.Value);
ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,teacher.name from kdata,teacher,kdatateacher,classdata WHERE kdata.kddataid=kdatateacher.kdataid and kdata.classdataid=classdata.classdataid and teacher.teacherid=kdatateacher.teacherid  and  kdata.classdataid="+jishuDown.SelectedItem.Value); //ds2=Cls.GetDataSet("SELECT kcname,lbid,xz,ketime,weekxs,name from kdata,teacher WHERE   classdataid=" +jishuDown.SelectedItem.Value);

MyDataGrid2.DataSource= ds2;
MyDataGrid2.DataBind();
ds2.Dispose(); // 在此处放置用户代码以初始化页面
} #region Web 窗体设计器生成的代码
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.jishuDown.SelectedIndexChanged += new System.EventHandler(this.jishuDown_SelectedIndexChanged);
this.zhyDown.SelectedIndexChanged += new System.EventHandler(this.zhyDown_SelectedIndexChanged);
this.xiDown.SelectedIndexChanged += new System.EventHandler(this.xiDown_SelectedIndexChanged);
this.xueyDown.SelectedIndexChanged += new System.EventHandler(this.xueyDown_SelectedIndexChanged);
this.Load += new System.EventHandler(this.Page_Load); }
#endregion private void xueyDown_SelectedIndexChanged(object sender, System.EventArgs e)
{
Cxydata dx=new Cxydata();

xiDown.Items.Clear();
zhyDown.Items.Clear();
jishuDown.Items.Clear();
dx.dbConnection=ConfigurationSettings.AppSettings["SQLConnectionString"];
dxre2=dx.GetDataReader("SELECT * from xdata where xydataid="+xueyDown.SelectedItem.Value.ToString());
xiDown.DataSource=dxre2;
xiDown.DataTextField="name";
xiDown.DataValueField="xdataid";
xiDown.DataBind();
dxre2.Close();
//
// if (xueyDown.SelectedIndex>=0)
// {
//
// dxre2=dx.GetDataReader("SELECT * from xdata where xydataid="+xueyDown.SelectedItem.Value.ToString());
// xiDown.DataSource=dxre2;
// xiDown.DataTextField="name";
// xiDown.DataValueField="xdataid";
// xiDown.DataBind();
// dxre2.Close();
//
// } if (xiDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from zydata where xdataid="+xiDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
zhyDown.DataTextField="name";
zhyDown.DataValueField="zydataid";
zhyDown.DataBind();
dxre2.Close();
} if (zhyDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from classdata where zydataid="+zhyDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
jishuDown.DataTextField="js";
jishuDown.DataValueField="classdataid";
jishuDown.DataBind();
dxre2.Close();
} } private void xiDown_SelectedIndexChanged(object sender, System.EventArgs e)
{
Cxydata dx=new Cxydata();
zhyDown.Items.Clear();
jishuDown.Items.Clear();
dx.dbConnection=ConfigurationSettings.AppSettings["SQLConnectionString"];
dxre2=dx.GetDataReader("SELECT * from zydata where xdataid="+xiDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
zhyDown.DataTextField="name";
zhyDown.DataValueField="zydataid";
zhyDown.DataBind();
dxre2.Close(); // if (xiDown.SelectedIndex>=0)
// {
// dxre2=dx.GetDataReader("SELECT * from zydata where xdataid="+xiDown.SelectedItem.Value.ToString());
// zhyDown.DataSource=dxre2;
// zhyDown.DataTextField="name";
// zhyDown.DataValueField="zydataid";
// zhyDown.DataBind();
// dxre2.Close();
// } if (zhyDown.SelectedIndex>=0)
{
dxre2=dx.GetDataReader("SELECT * from classdata where zydataid="+zhyDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
jishuDown.DataTextField="js";
jishuDown.DataValueField="classdataid";
jishuDown.DataBind();
dxre2.Close();
}

} private void zhyDown_SelectedIndexChanged(object sender, System.EventArgs e)
{
Cxydata dx=new Cxydata();
jishuDown.Items.Clear();
dx.dbConnection=ConfigurationSettings.AppSettings["SQLConnectionString"];
dxre2=dx.GetDataReader("SELECT * from classdata where zydataid="+zhyDown.SelectedItem.Value.ToString());
zhyDown.DataSource=dxre2;
jishuDown.DataTextField="js";
jishuDown.DataValueField="classdataid";
jishuDown.DataBind();
dxre2.Close();

} private void jishuDown_SelectedIndexChanged(object sender, System.EventArgs e)
{

}
}
}

我的一个程序，供楼主参考。
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using CraftPortal.Components;
using System.IO;namespace CraftPortal.Admin
{
/// <summary>
/// ProductAdd 的摘要说明。
/// </summary>
public class ProductAdd : System.Web.UI.Page
{
protected System.Web.UI.WebControls.Button ButtonOK;
protected System.Web.UI.WebControls.TextBox TextBoxDescriptions;
protected System.Web.UI.WebControls.RadioButtonList RadioButtonListSellState;
protected System.Web.UI.WebControls.RadioButtonList RadioButtonListPick;
protected System.Web.UI.WebControls.TextBox TextBoxVIPPrice;
protected System.Web.UI.WebControls.TextBox TextBoxMemberPrice;
protected System.Web.UI.WebControls.TextBox TextBoxMarketPrice;
protected System.Web.UI.WebControls.TextBox TextBoxPoint;
protected System.Web.UI.WebControls.TextBox TextBoxQuantity;
protected System.Web.UI.WebControls.DropDownList DropDownListGrade;
protected System.Web.UI.WebControls.DropDownList DropDownListDetail;
protected System.Web.UI.WebControls.DropDownList DropDownListCategory;
protected System.Web.UI.WebControls.TextBox TextBoxProductType;
protected System.Web.UI.WebControls.TextBox TextBoxProductName;
protected System.Web.UI.WebControls.Button ButtonClear;
protected System.Web.UI.HtmlControls.HtmlInputFile FilePicUpLoad;

private void Page_Load(object sender, System.EventArgs e)
{
if(!this.IsPostBack)
{
InitCategory();
}
InitCategoryDetail(Int32.Parse(this.DropDownListCategory.SelectedValue));
} #region Web 窗体设计器生成的代码
override protected void OnInit(EventArgs e)
{
//
// CODEGEN: 该调用是 ASP.NET Web 窗体设计器所必需的。
//
InitializeComponent();
base.OnInit(e);
}

/// <summary>
/// 设计器支持所需的方法 - 不要使用代码编辑器修改
/// 此方法的内容。
/// </summary>
private void InitializeComponent()
{
this.ButtonOK.Click += new System.EventHandler(this.ButtonOK_Click);
this.ButtonClear.Click += new System.EventHandler(this.ButtonClear_Click);
this.Load += new System.EventHandler(this.Page_Load); }
#endregion private void ButtonOK_Click(object sender, System.EventArgs e)
{
string strPicName = "nopicture.gif";
if(FilePicUpLoad.PostedFile!=null)
{
//修改成以上传时间为文件名
strPicName = DateTime.Now.ToString("yyyyMMddhhMMss")+ Path.GetExtension(FilePicUpLoad.PostedFile.FileName);
//修改路径
string strPath = HttpContext.Current.Server.MapPath(".")+"\\Data\\products\\bigpic\\";
strPath = strPath.ToLower().Replace("\\admin\\","\\");
FilePicUpLoad.PostedFile.SaveAs(strPath+strPicName);
strPath = HttpContext.Current.Server.MapPath(".")+"\\Data\\products\\smallpic\\";
strPath = strPath.ToLower().Replace("\\admin\\","\\");
FilePicUpLoad.PostedFile.SaveAs(strPath+strPicName);
}
int rst = ProductsDb.AddProduct(this.TextBoxProductName.Text,
this.TextBoxProductType.Text,
Int32.Parse(this.DropDownListGrade.SelectedValue),
Int32.Parse(this.TextBoxMarketPrice.Text),
Int32.Parse(this.TextBoxMemberPrice.Text),
Int32.Parse(this.TextBoxVIPPrice.Text),
Int32.Parse(this.TextBoxQuantity.Text),
Int32.Parse(this.RadioButtonListSellState.SelectedValue),
Int32.Parse(this.TextBoxPoint.Text),
strPicName,
this.TextBoxDescriptions.Text,
Int32.Parse(this.DropDownListCategory.SelectedValue),
Int32.Parse(this.DropDownListDetail.SelectedValue),
Int32.Parse(this.RadioButtonListPick.SelectedValue));
//保存成功，清空已输入的信息
if(rst!=-1)
{
ClearTextBox();
}
}
private void ClearTextBox()
{
TextBoxDescriptions.Text = "";
TextBoxVIPPrice.Text = "";
TextBoxMemberPrice.Text = "";
TextBoxMarketPrice.Text = "";
TextBoxPoint.Text = "";
TextBoxQuantity.Text = "";
TextBoxProductType.Text = "";
TextBoxProductName.Text = "";
}
private void InitCategory()
{
DataSet ds = (DataSet)HttpContext.Current.Cache["Category"];
this.DropDownListCategory.DataSource = ds.Tables["Categories"];
this.DropDownListCategory.DataTextField = "CategoryName";
this.DropDownListCategory.DataValueField = "CategoryId";
this.DropDownListCategory.DataBind();
} private void InitCategoryDetail(int categoryId)
{
DataSet ds = (DataSet)HttpContext.Current.Cache["Category"];
ds.Tables["CategoryDetails"].DefaultView.RowFilter = "CategoryId = "+categoryId;
this.DropDownListDetail.DataSource = ds.Tables["CategoryDetails"].DefaultView;
this.DropDownListDetail.DataTextField = "DetailName";
this.DropDownListDetail.DataValueField = "DetailId";
this.DropDownListDetail.DataBind();
} private void ButtonClear_Click(object sender, System.EventArgs e)
{
ClearTextBox();
}
}
}

前台部分
<%@ Page language="c#" Codebehind="ProductAdd.aspx.cs" AutoEventWireup="false" Inherits="CraftPortal.Admin.ProductAdd" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<HTML>
<HEAD>
<title>ProductAdd</title>
<link href="../ASPNETPortal.css" type="text/css" rel="stylesheet" >
</HEAD>
<body>
<form id="Form1" method="post" runat="server">

<TABLE id="Table1" cellSpacing="1" cellPadding="1" width="600" align="center" border="1">
<TR>
<TD class="ProductEditTdHeight" width="100">商品名称</TD>
<TD class="ProductEditTdHeight" width="200">
<asp:TextBox id="TextBoxProductName" runat="server"></asp:TextBox></TD>
<TD class="ProductEditTdHeight" width="100">商品型号</TD>
<TD width="200">
<asp:TextBox id="TextBoxProductType" runat="server"></asp:TextBox></TD>
</TR>
<TR>
<TD>所属大类</TD>
<TD>
<asp:DropDownList id="DropDownListCategory" runat="server" AutoPostBack="True"></asp:DropDownList></TD>
<TD>所属小类</TD>
<TD>
<asp:DropDownList id="DropDownListDetail" runat="server"></asp:DropDownList></TD>
</TR>
<TR>
<TD>商品等级</TD>
<TD>
<asp:DropDownList id="DropDownListGrade" runat="server">
<asp:ListItem Value="1">一星</asp:ListItem>
<asp:ListItem Value="2">二星</asp:ListItem>
<asp:ListItem Value="3">三星</asp:ListItem>
<asp:ListItem Value="4">四星</asp:ListItem>
<asp:ListItem Value="5" Selected="True">五星</asp:ListItem>
</asp:DropDownList></TD>
<TD>上货数量</TD>
<TD>
<asp:TextBox id="TextBoxQuantity" runat="server"></asp:TextBox></TD>
</TR>
<TR>
<TD>赠送积分</TD>
<TD>
<asp:TextBox id="TextBoxPoint" runat="server"></asp:TextBox></TD>
<TD>市场价格</TD>
<TD>
<asp:TextBox id="TextBoxMarketPrice" runat="server"></asp:TextBox></TD>
</TR>
<TR>
<TD>会员价格</TD>
<TD>
<asp:TextBox id="TextBoxMemberPrice" runat="server"></asp:TextBox></TD>
<TD>VIP价格</TD>
<TD>
<asp:TextBox id="TextBoxVIPPrice" runat="server"></asp:TextBox></TD>
</TR>
<TR>
<TD>商品等级</TD>
<TD>
<asp:RadioButtonList id="RadioButtonListPick" runat="server" RepeatDirection="Horizontal" Width="152px">
<asp:ListItem Value="1" Selected="True">普通商品</asp:ListItem>
<asp:ListItem Value="0">经典商品</asp:ListItem>
</asp:RadioButtonList></TD>
<TD>是否销售</TD>
<TD>
<asp:RadioButtonList id="RadioButtonListSellState" runat="server" RepeatDirection="Horizontal" Width="152px">
<asp:ListItem Value="1" Selected="True">是</asp:ListItem>
<asp:ListItem Value="0">否</asp:ListItem>
</asp:RadioButtonList></TD>
</TR>
<TR>
<TD>商品图片</TD>
<TD colSpan="3"><INPUT id="FilePicUpLoad" style="WIDTH: 370px; HEIGHT: 22px" type="file" size="42" name="FilePicUpLoad"
runat="server"></TD>
</TR>
<TR>
<TD>商品简介</TD>
<TD></TD>
<TD></TD>
<TD></TD>
</TR>
<TR>
<TD align="center" colSpan="4">
<asp:TextBox id="TextBoxDescriptions" runat="server" Width="544px" Height="248px" TextMode="MultiLine"></asp:TextBox></TD>
</TR>
<TR>
<TD></TD>
<TD></TD>
<TD></TD>
<TD></TD>
</TR>
<TR>
<TD></TD>
<TD></TD>
<TD>
<asp:Button id="ButtonClear" runat="server" Text="清除" CssClass="admininput"></asp:Button></TD>
<TD>
<asp:Button id="ButtonOK" runat="server" Width="48px" Text="添加"></asp:Button></TD>
</TR>
</TABLE>

</form>
</body>
</HTML>

看看这个
http://www.metabuilders.com/Tools/ListLink.aspx

调试易

（急）为什么我的DropDownlist就是无法级联选择

解决方案 »