How can I collect a users userid from an intranet aspx website using c#

I am building an internal website and I need to collect the userid of whoever browses to the site...they of course are logged into the network.

I am trying to adjust some code that works in Visual Basic to perform this task to c# but am havign difficulties getting it to work.

The Follwoing has been added to the web.config.

 <authorization>
        <!--  <allow users="[comma separated list of users]"
                   roles="[comma separated list of roles]"/>
            <deny  users="[comma separated list of users]"
                   roles="[comma separated list of roles]"/>
       -->
        <deny users="?" />
        <!-- Don't afford anonymous access -->
        <allow users="*" />
        <!-- But do allow all users (as long as they can authenticate against the domain). -->

      </authorization>



And here is teh code I am using in the code behind

using System;
using System.Data;
using System.Configuration;
using System.Collections;
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;
using System.Security.Principal;

namespace Intra
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            IPrincipal uid = HttpContext.Current.User;
            string id = uid.ToString();
            string lowerid = id.ToLower();
            Session["UID"] = lowerid;

            txtUserID.Text = lowerid;
        }

unfortunately the only thing this code colect is the following

system.security.principal.windowsprincipal

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