Question

What namespace is it from, cause it shows me that User doesnt exist in the current context!

Here are my list of namespaces. Which one should i add:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Web.Configuration;
using System.Web.Security;

Another question, is that the way to check that the person who logged in is an administrator?

 if (Roles.IsUserInRole(User.Identity.Name, "Administrators"))
      {

Answer 1

You need the following namespace:

using System.Security.Principal;

The easiest way to check for roles is:

User.IsInRole("Administractor");