private void billingToolStripMenuItem_Click(object sender, EventArgs e)
{
frmBilling frm = new frmBilling();
frm.Show();
this.Hide();
}
private void addBillingToolStripMenuItem_Click(object sender, EventArgs e)
{
this.IsMdiContainer = true;
frmBilling Form = new frmBilling();
Form.MdiParent = this;
Form.Show();
}