//Form1为父窗体,ChildForm为子窗体
//在Form1中写代码:
private static ChildForm childForm;private void menuItem2_Click(object sender, System.EventArgs e)
{
ChildForm childForm = GetChildForm();
childForm.Show();
}private ChildForm GetChildForm()
{
if( childForm == null || childForm.IsDisposed )
{
childForm = new ChildForm();
childForm.MdiParent = this;
}

return childForm;
}