Web Forms :: Authenticate Current Logged In User In Asp 3.5 - Error CS1061

I want to check that the user trying to log-in is authenticated and it belongs to a certain role. For this I have wrote the following code

protected void btnLoginButton_Click(object sender, EventArgs e)
{
/
if (this.User.Identity.IsAuthenticated && this.User.IsInRole("Admin"))
{
Response.Redirect("~/Admin/Default.aspx");
}
if (this.User.Identity.IsAuthenticated && this.User.IsInRole("Editor"))
{
Response.Redirect("~/Editor/Default.aspx");
}
if (this.User.Identity.IsAuthenticated && this.User.IsInRole("Moderator"))
{
Response.Redirect("~/Moderator/Default.aspx");
}
}

But when I ran this code it gave me this error error CS1061: 'Page' does not contain a definition for 'User' and no extension method 'User' accepting a first argument of type 'Page' could be found (are you missing a using directive or an assembly reference?) I have already added following directives to cs page-

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;

Can anybody tell what directie I should add for "this.User" <P.S> My application is web based and it is in ASP.Net 3.5(C#).