private void openSubFormToolStripMenuItem_Click(object sender, EventArgs e)
{
if (a == null)
{
a = new sub_Form();
a.FormClosed += new FormClosedEventHandler(sub_Form_FormClosed);
a.MdiParent = this;
a.Show();
}
}
private void sub_Form_FormClosed(object sender, FormClosedEventArgs e)
{
a = null;
}