default.master: panel visible only certain roles.

hi,
i have a problem with my coding as this error below.
Compiler Error Message: CS0103: The name 'User' does not exist in the
current contextLine 15: protected void Page_Load(object sender, System.EventArgs e)
Line 16: {

Line 17: bool isAdmin = User.IsInRole("Administrator");

Line 18: panel2.Visible = isAdmin;

Line 19: 
this is my works.
page: Default.master


<script runat="server">



    protected void LoginStatus1_LoggingOut(object sender, LoginCancelEventArgs e)

    {

        FormsAuthentication.SignOut();

        Response.Redirect("login.aspx", true);

    }

   

    protected void Page_Load(object sender, Sy ...