if(this.UserId>0)
{
DotNetNuke.Entities.Users.UserInfo USERINFO = DotNetNuke.Entities.Users.UserController.GetUserById(PortalId, this.UserId);
if (!USERINFO.IsInRole("Administrators"))
{
//do something
}}
else
{
string loginURL = string.Format("{0}://{1}/Default.aspx?ctl=login",
Request.Url.Scheme, Request.Url.Host);
Response.Redirect(loginURL);
}