Custom authentication problem

Hi All,
 
I m trying to implement custom authentication with reference to the url below
http://www.codeproject.com/KB/web-security/AspNetCustomAuth.aspx
 
Now I have created a property in my class like below.
 
private CustomIdentity _CurrentUser;
[Browsable(false)]
        [Description("Current user in CustomIdentity")]
        public virtual CustomIdentity CurrentUser
        {
            get { return _CurrentUser; }
        }
 
Now on the "OnInit" method of the page, I am trying to cast the HttpContext identity to the custom iden ...