private void dataReturnSellStuBillFrmToolStripMenuItem_Click(object sender, EventArgs e) { if (FrmDataReturnSellStuBill.IsDisposed) { FrmDataReturnSellStuBill = new DataReturnSellStuBillFrm(); FrmDataReturnSellStuBill.MdiParent = this; FrmDataReturnSellStuBill.Show(); } else { if (FrmDataReturnSellStuBill.Visible) { FrmDataReturnSellStuBill.WindowState = FormWindowState.Normal; FrmDataReturnSellStuBill.Focus(); } else { FrmDataReturnSellStuBill.MdiParent = this; FrmDataReturnSellStuBill.Show(); } } }