How to use List as attribute that saves in Session?

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C#

ASP.NET

http://csharptuning.blogspot.com/2007/05/uploading-multiple-files.html

I'm trying to build a webform that can upload any number of files and send as email attachment. I use the guide linked above. Howvever, when i try to build the code I get "Using the generic type 'System.Collections.Generic.List<T>' requires '1' type arguments" I'm not sure how to fix this. The guide wants this attribute to save a list of fileupload controls to Session["AddedControls"]

protected List AddedControls
{
get
{
if (Session["AddedControls"] == null)
Session["AddedControls"] = new List();
return (List)Session["AddedControls"];
}
set
{
Session["AddedControls"] = value;
}
}

full code
file_upload2.aspx.cs

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Collections.Generic;
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;

public partial class file_upload2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
        }
        
    }
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
}
    
    protected void Page_PreInit(object sender, EventArgs e)
{
foreach (Control ctrl in AddedControls)
{
divFileUpload.Controls.Add(ctrl);
}
}

protected void btnUpload_Click(object sender, EventArgs e)
{
for (int i = 0; i < Request.Files.Count; i++)
{
HttpPostedFile file = (HttpPostedFile)Request.Files[i];
if (file.ContentLength > 0)
{
try
{
file.SaveAs(Server.MapPath("../") + "\\tmp\\" + file.FileName.Substring(file.FileName.LastIndexOf("\\") + 1));
}
catch (Exception ex)
{
Response.Write("" + ex.Message + "");
continue;
}
}
}
}
 
protected List AddedControls
{
get
{
if (Session["AddedControls"] == null)
Session["AddedControls"] = new List();
return (List)Session["AddedControls"];
}
set
{
Session["AddedControls"] = value;
}
}

    protected void lnkAddMore_Click(object sender, EventArgs e)
{
FileUpload fileUpload = new FileUpload();
Literal lt = new Literal();
lt.Text = "<br/>";
divFileUpload.Controls.Add(fileUpload);
divFileUpload.Controls.Add(lt);
AddedControls.Add(fileUpload);
AddedControls.Add(lt);
}

}

file_upload2.aspx

<%@ Page Language="C#"  AutoEventWireup="true" CodeFile="file_upload2.aspx.cs" Inherits="file_upload2" Title="Fileupload" %>
                                                            
<div>FileName: 
<asp:FileUpload ID="FileUpload1" runat="server" /> 
<asp:LinkButton ID="lnkAddMore" Text='Add More...' runat="server" OnClick="lnkAddMore_Click" /></asp:LinkButton> 
<asp:Button ID="btnUpload" runat="server" Text="Upload" OnClick="btnUpload_Click" /><br /> 
<div runat="server" id="divFileUpload" /> </div></div>