Mdi formlarda child formaları 1 kere açmak için :
Form2 mChild;
private void optionsToolStripMenuItem_Click(object sender, EventArgs e) {
if (mChild == null) {
mChild = new Form2();
mChild.MdiParent = this;
mChild.FormClosed += new FormClosedEventHandler(mChild_FormClosed);
}
mChild.WindowState = FormWindowState.Normal;
mChild.Show();
}
void mChild_FormClosed(object sender, FormClosedEventArgs e) {
mChild = null;
}