Closing a form and then call another one in c# windows

Question

Hi to all,

                  I have a main form,in this i placed menustrip with some menuitems. When i clicked a menu item form will displayed.When I clicked another one the previous one should be closed.

 I used this below code

                           this.Close();

                    

                        AdminMainForm adminMainForm = new AdminMainForm();

                        adminMainForm.Show();

                       But this code not working for  my query.

                          when I use this.Hide() the Application is still running

Finally my query is when i enter and close the login form automativally it will displayed main form. please reply to me          

Regards

Nagaraju T