void dat_FormClosed(object sender, FormClosedEventArgs e) { dat = null; }
private void deleteATrainingInformationToolStripMenuItem_Click(object sender, EventArgs e) { if (dat== null) { dat = new DeleteATraining(n); dat.MdiParent = this; dat.FormClosed += dat_FormClosed; dat.Show(); } else { dat.Activate(); } }