数据无法删除

各位大哥好：
    小弟学习中遇到如下错误：选中一行数据，点击删除按钮却数据无法删除。调试在 82行 cmd.ExecuteNonQuery ();语句出现异常：DELETE 语句与 REFERENCE 约束"FK_Order_Details_Products"冲突。该冲突发生于数据库"Northwind"，表"dbo.Order Details", column 'ProductID'。
语句已终止。
后台代码如下using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.IO;
using System.Data .SqlClient ;public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if(!Page .IsPostBack )
        {
            bandexcel();
        }
    }
    protected void bandexcel()
    {
        SqlConnection con=new SqlConnection (ConfigurationManager .ConnectionStrings ["dbexcel"].ConnectionString );
        con.Open ();
        string cmdtxt="select * from Products";
        SqlDataAdapter sda=new SqlDataAdapter (cmdtxt ,con );
        DataSet ds=new DataSet ();
        sda.Fill (ds);
        this.GridView1.DataSource =ds;
        this.GridView1 .DataBind ();
        con.Close ();
    }
    protected void btnout_Click(object sender, EventArgs e)
    {
        Response.ContentType = "application/vnd.ms-excel";
        Response.Charset = "gb2312";
        EnableViewState = false;
        StringWriter tw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(tw);
        GridView1.RenderControl(hw);
        Response.Write(tw.ToString());
        Response.End();
    }
    public override void VerifyRenderingInServerForm(Control control)
    {
        //base.VerifyRenderingInServerForm(control);
    }    protected void CheckBox2_CheckedChanged(object sender, EventArgs e)
    {
        foreach (GridViewRow gr in GridView1.Rows)
        {
            CheckBox cbox = (CheckBox)gr.FindControl("CheckBox1");
            if (cbox.Checked == false)
            {
                cbox.Checked = true;
            }
            else
            {
                cbox.Checked = false;
            }
        }    }
    protected void btndel_Click(object sender, EventArgs e)
    {
        btndel.Attributes.Add("onclick", "javasctript:return confirm('确认删除吗？')");
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbexcel"].ConnectionString);
        con.Open();
        for(int i=0;i<=GridView1.Rows .Count -1;i++)
        {
            CheckBox check=(CheckBox )GridView1 .Rows [i].FindControl ("CheckBox1");
            if(check.Checked ==true)
            {
                string sqlstr="delete from Products where ProductID='"+ GridView1 .DataKeys[i].Value+"'";
                SqlCommand cmd=new SqlCommand (sqlstr,con);
                cmd.ExecuteNonQuery ();
            }
            con .Close ();
            bandexcel ();
        }
    }
    protected void btncancel_Click(object sender, EventArgs e)
    {
        for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
        {
            CheckBox check = (CheckBox)GridView1.Rows[i].FindControl("CheckBox1");
            check.Checked = false;
        }
    }
}
前台代码如下：
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>无标题页</title>
    <style type="text/css">
        .style1
        {
            width: 626px;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
     <table style="width: 999px" >
       <tr>
        <td class="style1" >
    
        <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
            BackColor="White" BorderColor="#336666" BorderStyle="Double" BorderWidth="3px" 
            CellPadding="4" GridLines="Horizontal" Width="995px" Height="186px" DataKeyNames ="ProductID"
                onselectedindexchanged="CheckBox2_CheckedChanged">
            <RowStyle BackColor="White" ForeColor="#333333" />
            <Columns>
                <asp:BoundField DataField="ProductID" HeaderText="商品ID号" />
                <asp:BoundField DataField="ProductName" HeaderText="商品名称" />
                <asp:BoundField DataField="UnitPrice" HeaderText="商品单价" />
                <asp:BoundField DataField="SupplierID" HeaderText="商品数量" />
                <asp:BoundField DataField="UnitPrice" HeaderText="商品总价" />
                <asp:TemplateField>
                    <HeaderTemplate>
                        <asp:CheckBox ID="CheckBox2" runat="server" AutoPostBack="True" Font-Size="9pt" 
                            oncheckedchanged="CheckBox2_CheckedChanged" />
                    </HeaderTemplate>
                    <ItemTemplate>
                        <asp:CheckBox ID="CheckBox1" runat="server" />
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>
            <FooterStyle BackColor="White" ForeColor="#333333" />
            <PagerStyle BackColor="#336666" ForeColor="White" HorizontalAlign="Center" />
            <SelectedRowStyle BackColor="#339966" Font-Bold="True" ForeColor="White" />
            <HeaderStyle BackColor="#336666" Font-Bold="True" ForeColor="White" />
        </asp:GridView></td></tr>
     <tr>   
    <td class="style1">
    
    <asp:Button ID="btnout" runat="server" OnClick ="btnout_Click" Text="导出成EXCEL文件格式" />
        <br />
        <asp:Button ID="btndel" runat="server" onclick="btndel_Click" Text="删除" />
        <asp:Button ID="btncancel" runat="server" onclick="btncancel_Click" 
            Text="取消删除" />
       </td>
      </tr> 
    </table> 
    </form>
</body>
</html>
问题是当选中一行数据时，无法删除选中数据。