User.IsInRole() not working in MVC 4

Question

I know this question is asked many times. I tried all solutions but not getting it worked. I a using MVC4 and using forms authentication. I am not using membership but want to store roles in Forms Authentication. I am using below code to achieve my goal.

 FormsAuthentication.SetAuthCookie(result.ID.ToString(), false);
                GenericPrincipal gp = new GenericPrincipal(HttpContext.User.Identity, new string[] { "admin" });
                HttpContext.User = gp;

I am getting username and IsAuthenticated successfully using User.Identity.Name and User.Identity.IsAuthenticated but when I am trying to check role using code var role = User.IsInRole("admin"); it is returning false.

Please help.