Error checking type for HttpContext.Current.User.Identity

This code was converted from VB.NET and I'm getting the following errors:

System.Web.HttpContext.Current' is a 'property' but is used like a 'type'
and
Cannot implicitly convert type 'System.Security.Principal.IIdentity' to 'System.Web.Security.FormsIdentity'. An explicit conversion exists (are you missing a cast?)

public void doAuthentication()
    {
        if (HttpContext.Current.User != null)
        {
            if (HttpContext.Current.User.Identity.IsAuthenticated)
            {
                if (typeof (HttpContext.Current.User.Identity) is FormsIdentity)
                {
                    FormsIdentity id = HttpContext.Current.User.Identity;
                    FormsAuthenticationTicket ticket = id.Ticket;
                    string strUserData = ticket.UserData;
                    string[] strRoles = strUserData.Split(',');
                    HttpContext.Current.User = new GenericPrincipal(id, strRoles);
                }
            }
        }
    }

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