how to close mulitpule forms in .net with c#

You shouldn't keep your login page open. If your user logs in successfully you should open your main form and close the login form. Something like this (put this method in you login form and call it if the user successfully logs in):-

void ShowMainWindow()
{
    form mainForm = new MainForm();
    mainForm.Show();
    //this.Dispose();
    this.Close();
}

If you need to dispose of any resources used in the login form you should put the calls to dispose in the login forms Dispose method and call this from the above method. Then you can just close the main form when your user wants to exit the application.

Hope this helps.