private void invoiceToolStripMenuItem_Click(object sender, EventArgs e)
 {
     frmInvoice invoice=new frmInvoice();
     invoice.Show();
 }
 private void addToolStripMenuItem_Click(object sender, EventArgs e)
 {
     Product selectProduct = GetSelectedCategory();
     frmInvoice frmInvoice = new frmInvoice(selectProduct);
     frmInvoice.FormClosed += new FormClosedEventHandler(frm2_FormClosed);
     frmInvoice.Show();
     Hide();
 }