check boxes are not checked in GridView on button submit

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ASP.NET

Gridview

i am trying to fetch images from a website by typing the website link in a text box and then click on fetch button, then i bind them with my gridview. That gridview contains images, checkboxes, a submit button out side the gridview. here is my code for grid view and code behind:

<asp:TextBox ID="txt_link" runat="server" CssClass="form-control"></asp:TextBox>
    <span class="input-group-btn">
        <asp:Button ID="btn_load" runat="server" Text="Load Values!" 
        CssClass="btn btn-info" onclick="btn_load_Click" />
   <asp:GridView ID="GridView1" CssClass="table table-responsive table-condensed" runat="server"
    AutoGenerateColumns="False" OnRowCommand="GridView1_RowCommand" OnRowDeleting="GridView1_RowDeleting"
    OnRowEditing="GridView1_RowEditing">
    <Columns>
      <asp:TemplateField>
            <HeaderTemplate>
                S No
            </HeaderTemplate>
            <ItemTemplate>
                <asp:Label ID="Label1" runat="server" Text='<%# Container.DataItemIndex + 1 %>'></asp:Label>
            </ItemTemplate>
            <ItemStyle HorizontalAlign="Left" CssClass="ItemSize" />
        </asp:TemplateField>
        <asp:TemplateField>
            <HeaderTemplate>
              img
            </HeaderTemplate>
            <ItemTemplate>
               <asp:Image ID="Image" runat="server" ImageUrl='<%# Container.DataItem %>' />
            </ItemTemplate>
            <ItemStyle HorizontalAlign="Left" CssClass="ItemSize" />
        </asp:TemplateField>
        <asp:TemplateField>
            <HeaderTemplate>
                Product Link
            </HeaderTemplate>
            <ItemTemplate>
             <asp:CheckBox ID="chebx" runat="server"/>

            </ItemTemplate>
        </asp:TemplateField>

    </Columns>
    </asp:GridView>

Code Behind:

protected void btn_load_Click(object sender, EventArgs e)
{
 HtmlWeb web = new HtmlWeb();
    HtmlAgilityPack.HtmlDocument doc = web.Load(txt_link.Text);
  var rateNode1 = from info in doc.DocumentNode.SelectNodes("//div[@class='ui-box ui-box-normal product-custom-desc']")
                       from linkee in info.SelectNodes("//img").Where(x=>x.Attributes.Contains("src"))
                       select new
                       {
                        linkee.Attributes["src"].Value
                       };
 List<String> images = new List<string>(rateNode1.Count());
  foreach (var a in rateNode1)
   {

       result = a.Value;

       images.Add(result.Substring(0,result.Length-10));

   }
   GridView1.DataSource = images;
   GridView1.DataBind();}

Up to here it worked fine. And for saving to local folder here is the code:

var folder = Server.MapPath("~/assets/" + txt_orig_id.Text);
    string filePath="";
    if (!Directory.Exists(folder))
    {
        Directory.CreateDirectory(folder);
         filePath = folder;
    }
    foreach (GridViewRow row in GridView1.Rows)
    {

            //CheckBox checkbox = row.FindControl("chebx") as CheckBox;
            CheckBox chk = (CheckBox)row.FindControl("chebx");
            Image img = row.FindControl("Image") as Image;
            if (chk.Checked)
            {
                Response.ContentType = ContentType;
                Response.AppendHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(filePath));
                Response.WriteFile(filePath);
                Response.End();

            }
            else {

            }

    }

but the main issue is it shows that none of the check box is checked. please tell me whats wrong in my code