data binding a aspx web forms dropdownlist to entity framework 6 database first model

Hi experts,

I saw this example on binding a web forms aspx dropdownlist to a Entity Framework 4 Database First model.

ADO.NET Entity Framework 4.0 : DataBinding with ASP.NET

At the bottom of that page you can download their example.

So i downloaded their example and changed the web config file to point to my sql server database and ran it and their example worked fine.

One thing I noticed though about that example though is that it is using Entity Framework 4.

So I'm re-creating this example from scratch.
I want to re-create that example using Visual Studio 2013, C#,  .Net Framework 4.5 and Entity Framework 6.0.

So this is what my project looks like.
All the code and naming conventions are the same as in their example.

my Visual Studio 2013 .Net Framework 4 Empty Web Forms Project
This is my code for my page


<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication1.WebForm1" %>

<!DOCTYPE html>

<html xmlns="">
<head runat="server">
    <form id="form1" runat="server">
        <asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
        <asp:GridView ID="GridView1" runat="server">

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

namespace WebApplication1
    public partial class WebForm1 : System.Web.UI.Page
        protected void Page_Load(object sender, EventArgs e)
            if (!Page.IsPostBack)
                using (var ctx = new NorthwindEntities())
                    DropDownList1.DataSource = ctx.Categories;

                    DropDownList1.DataTextField = "CategoryName";
                    DropDownList1.DataValueField = "CategoryID";


                    int cid = (int.Parse(DropDownList1.SelectedValue));

                    GridView1.DataSource = (from p in ctx.Products
                                            where p.CategoryID == cid
                                            select p).ToList();


        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
            using (var ctx1 = new NorthwindEntities())
                int cid = (int.Parse(DropDownList1.SelectedValue));

                GridView1.DataSource = (from p in ctx1.Products
                                        where p.CategoryID == cid
                                        select p).ToList();

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My Web.config looks like this:

<?xml version="1.0" encoding="utf-8"?>
  For more information on how to configure your ASP.NET application, please visit
    <!-- For more information on Entity Framework configuration, visit -->
    <section name="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework, Version=, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
    <compilation debug="true" targetFramework="4.5" />
    <httpRuntime targetFramework="4.5" />
    <add name="NorthwindEntities" connectionString="metadata=res://*/NW.csdl|res://*/NW.ssdl|res://*/NW.msl;provider=System.Data.SqlClient;provider connection string=&quot;data source=MyServerName;initial catalog=Northwind;integrated security=True;MultipleActiveResultSets=True;App=EntityFramework&quot;" providerName="System.Data.EntityClient" />
    <defaultConnectionFactory type="System.Data.Entity.Infrastructure.SqlConnectionFactory, EntityFramework" />
      <provider invariantName="System.Data.SqlClient" type="System.Data.Entity.SqlServer.SqlProviderServices, EntityFramework.SqlServer" />

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But when I run my page I get this error:

error I get when i run my page
Anyone know why I am getting this error? Anyone how I fix this error?
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Randy DownsOWNERCommented:
Maybe this will help. Basically you need you DataSource statements

If you are not interested in getting the reference to the binding list for yourself, and you are using databinding directly against a query as in the sample at the top of this post, you should know that WinForms will call IListSource.GetList() twice, causing the query to be executed on the server twice! The recommendation then is to bind to the results, rather than the query, using code similar to this:
grid.DataSource = someQuery.Execute(MergeOption.AppendOnly);

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In this case, since the resulting ObjectResult acts as a foward-only, enumerate-once cursor, its implementation of IListSource.GetList() is different from the one in the query: the call will consume the results (i.e. it will iterate through the whole results) and will cache the binding list in the ObjectResult instance. All subsequent calls to IListSource.GetList() will return the same binding list.

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