public void ChangeButtons()
 {
     if (courseRepository.CheckEnroll(int.Parse(user.UserId.ToString()), int.Parse(Request.QueryString["CourseId"])))
     {
         Enroll.Visible   = false;
         UnEnroll.Visible = true;
     }
     else
     {
         Enroll.Visible   = true;
         UnEnroll.Visible = false;
     }
 }