상속: INotifyPropertyChanging, INotifyPropertyChanged
 public static void Update(ExternalUser externalUserEntity, int userId)
 {
     var userToUpdate = DataContext.ExternalUsers.First(i => i.UserID == userId);
     userToUpdate.Name = externalUserEntity.Name;
     userToUpdate.Email = externalUserEntity.Email;
     DataContext.SubmitChanges();
 }
 public static ExternalUser Create(ExternalUser newExternalUser)
 {
     DataContext.ExternalUsers.InsertOnSubmit(newExternalUser);
     DataContext.SubmitChanges();
     return newExternalUser;
 }
 partial void UpdateExternalUser(ExternalUser instance);
 partial void DeleteExternalUser(ExternalUser instance);
 partial void InsertExternalUser(ExternalUser instance);