The MDI form. The child form maximizes the size of the child form in the parent form. If the child form has been opened, the child form is displayed.

From: http://virusswb.cnblogs.com/

Using System;
Using System. Collections. Generic;
Using System. componentmodel;
Using System. Data;
Using System. drawing;
Using System. text;
Using System. Windows. forms;

Namespace Windowsapplication4
{
Public   Partial   Class Frmmain: Form
{
Public Frmmain ()
{
Initializecomponent ();
}

Private   Void Log on to toolstripmenuitem_click ( Object Sender, eventargs E)
{
Foreach (Form In Application. openforms)
{
If (Form. Name =   " Frmlogin " )
{
Form. Activate ();
Form. windowstate=Formwindowstate. maximized;
Return;
}
}

Frmlogin Login =   New Frmlogin ();

Login. mdiparent =   This ;
Login. windowstate = Formwindowstate. maximized;
Login. Show ();
}

Private   Void Log toolstripmenuitem_click ( Object Sender, eventargs E)
{
Foreach (Form In Application. openforms)
{
If (Form. Text =   " Frmloginfo " )
{
Form. Activate ();
Return;
}
}

Frmloginfo log =   New Frmloginfo ();
Log. mdiparent =   This ;
Log. windowstate = Formwindowstate. maximized;
Log. Show ();
}

Private   Void Frmmain_load ( Object Sender, eventargs E)
{< br> frmlogin login = New frmlogin ();
login. windowstate = formwindowstate. maximized;
login. mdiparent = This ;< br> login. show ();
}

}

}
The MDI form maximizes the size of the child form in the parent form. If the child form has already been opened, it is displayed and the form is not opened repeatedly. Of course, you can use the single-piece mode or the mutex Technology in multiple threads.