Sometimes you need to use Http handlers to respond to requests rather than classical asp.net pages. At that times if you want to use session objects you will see that HttpContext.Current.Session is null.
If you want to read session state from you http handler your http handler must implement System.Web.SessionState.IReadOnlySessionState interface. If you want to modify the session state you can implement System.Web.SessionState.IRequiresSessionState.
Sample:
public class Login : IHttpHandler, SessionState.IRequiresSessionState
Happy coding.
