The FormsAuthenticationModule class is activated when the following element is in the Web.config file.

<authentication mode="Forms" />

The FormsAuthenticationModule class constructs a GenericPrincipal object and stores it in the HTTP context. The GenericPrincipal object holds a reference to a FormsIdentity instance that represents the currently authenticated user. You should allow forms authentication to manage these tasks for you. If your applications have specific requirements, such as setting the User property to a custom class that implements the IPrincipal interface, your application should handle the PostAuthenticate event. The PostAuthenticate event occurs after the FormsAuthenticationModule has verified the forms authentication cookie and created the GenericPrincipal and FormsIdentity objects. Within this code, you can construct a custom IPrincipal object that wraps the FormsIdentity object, and then store it in the HttpContext. User property. 

Note:   If you do this, you will also need to set the IPrincipal reference on the Thread.CurrentPrincipal property to ensure that the HttpContext object and the thread point to the same authentication information.